|
Varactor tuning diodes.
Yes, the RF generally does cause nearly instantaneous voltage changes,
and therefore capacitance changes, across the diode. Spice should handle this properly. You need to lower that RF voltage! In addition, the average capacitance will depend on the amplitude of the RF because of the nonlinear nature of the capacitance versus voltage, so it's important to keep the RF level reasonably constant, for stability. It's fairly common in covering a wide range, especially if you also want low phase noise, to break it up into subranges which might be selected with switching or PIN diodes. Of course, it's also critical to keep the control voltage "quiet" because at 10MHz/volt or so, a millivolt will give you 10kHz of frequency deviation. If you look up other uses for varactor diodes, you'll find that the characteristic you are asking about is used to make efficient frequency multipliers, and even mixers and amplifiers. (Search for "parametric amplifier.") Cheers, Tom In message , James French wrote: Hello, I have a general question regarding tuning diodes: Does the oscillation voltage alter their capacitance, or is capacitance a slow changing value - like resistance and PIN diodes? The reason I ask is that I am spicing (simulation) a VCO I wish to build (wide range 140-240MHz) and get an oscillation voltage of 20 volts peak (maybe the Q is too high). This high oscillation voltage would easily swamp the tuning voltage, 3-15 volts, and forward bias the diode itself. If anyone wishs to reply directly they must remove the "xxx" from the return address or write to: james dot fenech at nec dot com dot au Thanks, James. |
Hi All,
thanks for the many responses. My use of this VCO if for a homebrew spectrum analyser, 0-100MHz with a 140MHz first IF. This sort of rules out doing it in ranges - maybe? I'm only an RF beginer, and spicing the circuit gives me "some" confidence that it may work, but I understand that spice may be quite a bit off the mark with parasitics and all. The idea of reducing the RF voltage across the part is a good one, but how do I do it? I am actually using two in series (back to back) but 20 volts peak seems a bit much. I have the ARRL handbook (1997 or so) but this doesn't have much in the way of theory. Is there any other reference anyone can recommend? I could just build it and see if it works, but I wouldn't be able to test much except measure the operating frequency. James. "Tom Bruhns" wrote in message m... Yes, the RF generally does cause nearly instantaneous voltage changes, and therefore capacitance changes, across the diode. Spice should handle this properly. You need to lower that RF voltage! In addition, the average capacitance will depend on the amplitude of the RF because of the nonlinear nature of the capacitance versus voltage, so it's important to keep the RF level reasonably constant, for stability. It's fairly common in covering a wide range, especially if you also want low phase noise, to break it up into subranges which might be selected with switching or PIN diodes. Of course, it's also critical to keep the control voltage "quiet" because at 10MHz/volt or so, a millivolt will give you 10kHz of frequency deviation. If you look up other uses for varactor diodes, you'll find that the characteristic you are asking about is used to make efficient frequency multipliers, and even mixers and amplifiers. (Search for "parametric amplifier.") Cheers, Tom In message , James French wrote: Hello, I have a general question regarding tuning diodes: Does the oscillation voltage alter their capacitance, or is capacitance a slow changing value - like resistance and PIN diodes? The reason I ask is that I am spicing (simulation) a VCO I wish to build (wide range 140-240MHz) and get an oscillation voltage of 20 volts peak (maybe the Q is too high). This high oscillation voltage would easily swamp the tuning voltage, 3-15 volts, and forward bias the diode itself. If anyone wishs to reply directly they must remove the "xxx" from the return address or write to: james dot fenech at nec dot com dot au Thanks, James. |
Hi All,
thanks for the many responses. My use of this VCO if for a homebrew spectrum analyser, 0-100MHz with a 140MHz first IF. This sort of rules out doing it in ranges - maybe? I'm only an RF beginer, and spicing the circuit gives me "some" confidence that it may work, but I understand that spice may be quite a bit off the mark with parasitics and all. The idea of reducing the RF voltage across the part is a good one, but how do I do it? I am actually using two in series (back to back) but 20 volts peak seems a bit much. I have the ARRL handbook (1997 or so) but this doesn't have much in the way of theory. Is there any other reference anyone can recommend? I could just build it and see if it works, but I wouldn't be able to test much except measure the operating frequency. James. "Tom Bruhns" wrote in message m... Yes, the RF generally does cause nearly instantaneous voltage changes, and therefore capacitance changes, across the diode. Spice should handle this properly. You need to lower that RF voltage! In addition, the average capacitance will depend on the amplitude of the RF because of the nonlinear nature of the capacitance versus voltage, so it's important to keep the RF level reasonably constant, for stability. It's fairly common in covering a wide range, especially if you also want low phase noise, to break it up into subranges which might be selected with switching or PIN diodes. Of course, it's also critical to keep the control voltage "quiet" because at 10MHz/volt or so, a millivolt will give you 10kHz of frequency deviation. If you look up other uses for varactor diodes, you'll find that the characteristic you are asking about is used to make efficient frequency multipliers, and even mixers and amplifiers. (Search for "parametric amplifier.") Cheers, Tom In message , James French wrote: Hello, I have a general question regarding tuning diodes: Does the oscillation voltage alter their capacitance, or is capacitance a slow changing value - like resistance and PIN diodes? The reason I ask is that I am spicing (simulation) a VCO I wish to build (wide range 140-240MHz) and get an oscillation voltage of 20 volts peak (maybe the Q is too high). This high oscillation voltage would easily swamp the tuning voltage, 3-15 volts, and forward bias the diode itself. If anyone wishs to reply directly they must remove the "xxx" from the return address or write to: james dot fenech at nec dot com dot au Thanks, James. |
I assume your VCO has to tune 140-240MHz?? Email me directly and I'll give
you a circuit that will easily do the job. ) Joe W3JDR "James Fenech" wrote in message ... Hi All, thanks for the many responses. My use of this VCO if for a homebrew spectrum analyser, 0-100MHz with a 140MHz first IF. This sort of rules out doing it in ranges - maybe? I'm only an RF beginer, and spicing the circuit gives me "some" confidence that it may work, but I understand that spice may be quite a bit off the mark with parasitics and all. The idea of reducing the RF voltage across the part is a good one, but how do I do it? I am actually using two in series (back to back) but 20 volts peak seems a bit much. I have the ARRL handbook (1997 or so) but this doesn't have much in the way of theory. Is there any other reference anyone can recommend? I could just build it and see if it works, but I wouldn't be able to test much except measure the operating frequency. James. "Tom Bruhns" wrote in message m... Yes, the RF generally does cause nearly instantaneous voltage changes, and therefore capacitance changes, across the diode. Spice should handle this properly. You need to lower that RF voltage! In addition, the average capacitance will depend on the amplitude of the RF because of the nonlinear nature of the capacitance versus voltage, so it's important to keep the RF level reasonably constant, for stability. It's fairly common in covering a wide range, especially if you also want low phase noise, to break it up into subranges which might be selected with switching or PIN diodes. Of course, it's also critical to keep the control voltage "quiet" because at 10MHz/volt or so, a millivolt will give you 10kHz of frequency deviation. If you look up other uses for varactor diodes, you'll find that the characteristic you are asking about is used to make efficient frequency multipliers, and even mixers and amplifiers. (Search for "parametric amplifier.") Cheers, Tom In message , James French wrote: Hello, I have a general question regarding tuning diodes: Does the oscillation voltage alter their capacitance, or is capacitance a slow changing value - like resistance and PIN diodes? The reason I ask is that I am spicing (simulation) a VCO I wish to build (wide range 140-240MHz) and get an oscillation voltage of 20 volts peak (maybe the Q is too high). This high oscillation voltage would easily swamp the tuning voltage, 3-15 volts, and forward bias the diode itself. If anyone wishs to reply directly they must remove the "xxx" from the return address or write to: james dot fenech at nec dot com dot au Thanks, James. |
I assume your VCO has to tune 140-240MHz?? Email me directly and I'll give
you a circuit that will easily do the job. ) Joe W3JDR "James Fenech" wrote in message ... Hi All, thanks for the many responses. My use of this VCO if for a homebrew spectrum analyser, 0-100MHz with a 140MHz first IF. This sort of rules out doing it in ranges - maybe? I'm only an RF beginer, and spicing the circuit gives me "some" confidence that it may work, but I understand that spice may be quite a bit off the mark with parasitics and all. The idea of reducing the RF voltage across the part is a good one, but how do I do it? I am actually using two in series (back to back) but 20 volts peak seems a bit much. I have the ARRL handbook (1997 or so) but this doesn't have much in the way of theory. Is there any other reference anyone can recommend? I could just build it and see if it works, but I wouldn't be able to test much except measure the operating frequency. James. "Tom Bruhns" wrote in message m... Yes, the RF generally does cause nearly instantaneous voltage changes, and therefore capacitance changes, across the diode. Spice should handle this properly. You need to lower that RF voltage! In addition, the average capacitance will depend on the amplitude of the RF because of the nonlinear nature of the capacitance versus voltage, so it's important to keep the RF level reasonably constant, for stability. It's fairly common in covering a wide range, especially if you also want low phase noise, to break it up into subranges which might be selected with switching or PIN diodes. Of course, it's also critical to keep the control voltage "quiet" because at 10MHz/volt or so, a millivolt will give you 10kHz of frequency deviation. If you look up other uses for varactor diodes, you'll find that the characteristic you are asking about is used to make efficient frequency multipliers, and even mixers and amplifiers. (Search for "parametric amplifier.") Cheers, Tom In message , James French wrote: Hello, I have a general question regarding tuning diodes: Does the oscillation voltage alter their capacitance, or is capacitance a slow changing value - like resistance and PIN diodes? The reason I ask is that I am spicing (simulation) a VCO I wish to build (wide range 140-240MHz) and get an oscillation voltage of 20 volts peak (maybe the Q is too high). This high oscillation voltage would easily swamp the tuning voltage, 3-15 volts, and forward bias the diode itself. If anyone wishs to reply directly they must remove the "xxx" from the return address or write to: james dot fenech at nec dot com dot au Thanks, James. |
I highly recommend _Experimental Methods in RF Design_, by Hayward,
Cambell, and Larkin. It's published by the ARRL and available from them and numerous other sources. Besides theory and a lot of real, practical, tested circuits and projects, it includes simple test equipment you can build yourself. Roy Lewallen, W7EL James Fenech wrote: . . . I have the ARRL handbook (1997 or so) but this doesn't have much in the way of theory. Is there any other reference anyone can recommend? . . . |
I highly recommend _Experimental Methods in RF Design_, by Hayward,
Cambell, and Larkin. It's published by the ARRL and available from them and numerous other sources. Besides theory and a lot of real, practical, tested circuits and projects, it includes simple test equipment you can build yourself. Roy Lewallen, W7EL James Fenech wrote: . . . I have the ARRL handbook (1997 or so) but this doesn't have much in the way of theory. Is there any other reference anyone can recommend? . . . |
Hi Roy,
thanks for the suggestion. I am considering buying this book. Can I ask how much theory, and how deep does it go? I am an engineer (digital electornics and software background) and actually like some theory to help me understand what I am doing. I already have some "real" test equipment, 50MHz CRO, signal generator, multimeter, etc. So "simple" test equipment may not be too much of an improvement. I've looked over the internet, and some books that I have, but found no real example circuit on wideband VCOs. The only "sort of close enough" circuit I found is at: http://www.newwaveinstruments.com/resources/rf_microwave_resources/sections/oscillator_vco_theory_design_circuit.htm#Voltage%2 0Controled%20Oscillator%20(VCO)%20Circuits The second one down - Colpitts. Are there any such examples in this book? Thanks, James. "Roy Lewallen" wrote in message ... I highly recommend _Experimental Methods in RF Design_, by Hayward, Cambell, and Larkin. It's published by the ARRL and available from them and numerous other sources. Besides theory and a lot of real, practical, tested circuits and projects, it includes simple test equipment you can build yourself. Roy Lewallen, W7EL James Fenech wrote: . . . I have the ARRL handbook (1997 or so) but this doesn't have much in the way of theory. Is there any other reference anyone can recommend? . . . |
Hi Roy,
thanks for the suggestion. I am considering buying this book. Can I ask how much theory, and how deep does it go? I am an engineer (digital electornics and software background) and actually like some theory to help me understand what I am doing. I already have some "real" test equipment, 50MHz CRO, signal generator, multimeter, etc. So "simple" test equipment may not be too much of an improvement. I've looked over the internet, and some books that I have, but found no real example circuit on wideband VCOs. The only "sort of close enough" circuit I found is at: http://www.newwaveinstruments.com/resources/rf_microwave_resources/sections/oscillator_vco_theory_design_circuit.htm#Voltage%2 0Controled%20Oscillator%20(VCO)%20Circuits The second one down - Colpitts. Are there any such examples in this book? Thanks, James. "Roy Lewallen" wrote in message ... I highly recommend _Experimental Methods in RF Design_, by Hayward, Cambell, and Larkin. It's published by the ARRL and available from them and numerous other sources. Besides theory and a lot of real, practical, tested circuits and projects, it includes simple test equipment you can build yourself. Roy Lewallen, W7EL James Fenech wrote: . . . I have the ARRL handbook (1997 or so) but this doesn't have much in the way of theory. Is there any other reference anyone can recommend? . . . |
You're climbing a steep hill here James. It is the % change in frequency
that is the problem. Professional spec analysers use a 2 GHz LO with a 2:1 range (to scan 0-2GHz), but I believe that is a magnetically tuned YIG resonator. If you could get the frequency higher, so the sweep range is narrower, then mix to the desired freq with a balanced mixer...that would be an easier VCO design. Steve K;9;d;c;i The punctuation is my feeble attempt at spam-bot blocking. "James Fenech" wrote in message ... Hi Roy, thanks for the suggestion. I am considering buying this book. Can I ask how much theory, and how deep does it go? I am an engineer (digital electornics and software background) and actually like some theory to help me understand what I am doing. I already have some "real" test equipment, 50MHz CRO, signal generator, multimeter, etc. So "simple" test equipment may not be too much of an improvement. I've looked over the internet, and some books that I have, but found no real example circuit on wideband VCOs. The only "sort of close enough" circuit I found is at: http://www.newwaveinstruments.com/resources/rf_microwave_resources/sections/oscillator_vco_theory_design_circuit.htm#Voltage%2 0Controled%20Oscillator%20(VCO)%20Circuits The second one down - Colpitts. Are there any such examples in this book? Thanks, James. "Roy Lewallen" wrote in message ... I highly recommend _Experimental Methods in RF Design_, by Hayward, Cambell, and Larkin. It's published by the ARRL and available from them and numerous other sources. Besides theory and a lot of real, practical, tested circuits and projects, it includes simple test equipment you can build yourself. Roy Lewallen, W7EL James Fenech wrote: . . . I have the ARRL handbook (1997 or so) but this doesn't have much in the way of theory. Is there any other reference anyone can recommend? . . . |
You're climbing a steep hill here James. It is the % change in frequency
that is the problem. Professional spec analysers use a 2 GHz LO with a 2:1 range (to scan 0-2GHz), but I believe that is a magnetically tuned YIG resonator. If you could get the frequency higher, so the sweep range is narrower, then mix to the desired freq with a balanced mixer...that would be an easier VCO design. Steve K;9;d;c;i The punctuation is my feeble attempt at spam-bot blocking. "James Fenech" wrote in message ... Hi Roy, thanks for the suggestion. I am considering buying this book. Can I ask how much theory, and how deep does it go? I am an engineer (digital electornics and software background) and actually like some theory to help me understand what I am doing. I already have some "real" test equipment, 50MHz CRO, signal generator, multimeter, etc. So "simple" test equipment may not be too much of an improvement. I've looked over the internet, and some books that I have, but found no real example circuit on wideband VCOs. The only "sort of close enough" circuit I found is at: http://www.newwaveinstruments.com/resources/rf_microwave_resources/sections/oscillator_vco_theory_design_circuit.htm#Voltage%2 0Controled%20Oscillator%20(VCO)%20Circuits The second one down - Colpitts. Are there any such examples in this book? Thanks, James. "Roy Lewallen" wrote in message ... I highly recommend _Experimental Methods in RF Design_, by Hayward, Cambell, and Larkin. It's published by the ARRL and available from them and numerous other sources. Besides theory and a lot of real, practical, tested circuits and projects, it includes simple test equipment you can build yourself. Roy Lewallen, W7EL James Fenech wrote: . . . I have the ARRL handbook (1997 or so) but this doesn't have much in the way of theory. Is there any other reference anyone can recommend? . . . |
2:1 tuning range with varactor tuning is very doable in the frequency range
he's considering. I've done 3:1 at lower frequencies. No steep hill at all. Joe W3JDR "Steve Nosko" wrote in message ... You're climbing a steep hill here James. It is the % change in frequency that is the problem. Professional spec analysers use a 2 GHz LO with a 2:1 range (to scan 0-2GHz), but I believe that is a magnetically tuned YIG resonator. If you could get the frequency higher, so the sweep range is narrower, then mix to the desired freq with a balanced mixer...that would be an easier VCO design. Steve K;9;d;c;i The punctuation is my feeble attempt at spam-bot blocking. "James Fenech" wrote in message ... Hi Roy, thanks for the suggestion. I am considering buying this book. Can I ask how much theory, and how deep does it go? I am an engineer (digital electornics and software background) and actually like some theory to help me understand what I am doing. I already have some "real" test equipment, 50MHz CRO, signal generator, multimeter, etc. So "simple" test equipment may not be too much of an improvement. I've looked over the internet, and some books that I have, but found no real example circuit on wideband VCOs. The only "sort of close enough" circuit I found is at: http://www.newwaveinstruments.com/resources/rf_microwave_resources/sections/oscillator_vco_theory_design_circuit.htm#Voltage%2 0Controled%20Oscillator%20(VCO)%20Circuits The second one down - Colpitts. Are there any such examples in this book? Thanks, James. "Roy Lewallen" wrote in message ... I highly recommend _Experimental Methods in RF Design_, by Hayward, Cambell, and Larkin. It's published by the ARRL and available from them and numerous other sources. Besides theory and a lot of real, practical, tested circuits and projects, it includes simple test equipment you can build yourself. Roy Lewallen, W7EL James Fenech wrote: . . . I have the ARRL handbook (1997 or so) but this doesn't have much in the way of theory. Is there any other reference anyone can recommend? . . . |
2:1 tuning range with varactor tuning is very doable in the frequency range
he's considering. I've done 3:1 at lower frequencies. No steep hill at all. Joe W3JDR "Steve Nosko" wrote in message ... You're climbing a steep hill here James. It is the % change in frequency that is the problem. Professional spec analysers use a 2 GHz LO with a 2:1 range (to scan 0-2GHz), but I believe that is a magnetically tuned YIG resonator. If you could get the frequency higher, so the sweep range is narrower, then mix to the desired freq with a balanced mixer...that would be an easier VCO design. Steve K;9;d;c;i The punctuation is my feeble attempt at spam-bot blocking. "James Fenech" wrote in message ... Hi Roy, thanks for the suggestion. I am considering buying this book. Can I ask how much theory, and how deep does it go? I am an engineer (digital electornics and software background) and actually like some theory to help me understand what I am doing. I already have some "real" test equipment, 50MHz CRO, signal generator, multimeter, etc. So "simple" test equipment may not be too much of an improvement. I've looked over the internet, and some books that I have, but found no real example circuit on wideband VCOs. The only "sort of close enough" circuit I found is at: http://www.newwaveinstruments.com/resources/rf_microwave_resources/sections/oscillator_vco_theory_design_circuit.htm#Voltage%2 0Controled%20Oscillator%20(VCO)%20Circuits The second one down - Colpitts. Are there any such examples in this book? Thanks, James. "Roy Lewallen" wrote in message ... I highly recommend _Experimental Methods in RF Design_, by Hayward, Cambell, and Larkin. It's published by the ARRL and available from them and numerous other sources. Besides theory and a lot of real, practical, tested circuits and projects, it includes simple test equipment you can build yourself. Roy Lewallen, W7EL James Fenech wrote: . . . I have the ARRL handbook (1997 or so) but this doesn't have much in the way of theory. Is there any other reference anyone can recommend? . . . |
Thanks Joe,
I wouldn't have thought it would be too difficult. Minicircuits sells parts that have a 2:1 range (not the right range though), and I only want 1.7:1 or so. Interesting idea about downmixing, but I really don't want another mixer and oscillator in this thing. Acording to my pspice results I can get the range, but I'm a little concerned about the high voltage swing on the varactors causing very poor phase noise. James. "W3JDR" wrote in message ... 2:1 tuning range with varactor tuning is very doable in the frequency range he's considering. I've done 3:1 at lower frequencies. No steep hill at all. Joe W3JDR "Steve Nosko" wrote in message ... You're climbing a steep hill here James. It is the % change in frequency that is the problem. Professional spec analysers use a 2 GHz LO with a 2:1 range (to scan 0-2GHz), but I believe that is a magnetically tuned YIG resonator. If you could get the frequency higher, so the sweep range is narrower, then mix to the desired freq with a balanced mixer...that would be an easier VCO design. Steve K;9;d;c;i The punctuation is my feeble attempt at spam-bot blocking. "James Fenech" wrote in message ... Hi Roy, thanks for the suggestion. I am considering buying this book. Can I ask how much theory, and how deep does it go? I am an engineer (digital electornics and software background) and actually like some theory to help me understand what I am doing. I already have some "real" test equipment, 50MHz CRO, signal generator, multimeter, etc. So "simple" test equipment may not be too much of an improvement. I've looked over the internet, and some books that I have, but found no real example circuit on wideband VCOs. The only "sort of close enough" circuit I found is at: http://www.newwaveinstruments.com/resources/rf_microwave_resources/sections/oscillator_vco_theory_design_circuit.htm#Voltage%2 0Controled%20Oscillator%20(VCO)%20Circuits The second one down - Colpitts. Are there any such examples in this book? Thanks, James. "Roy Lewallen" wrote in message ... I highly recommend _Experimental Methods in RF Design_, by Hayward, Cambell, and Larkin. It's published by the ARRL and available from them and numerous other sources. Besides theory and a lot of real, practical, tested circuits and projects, it includes simple test equipment you can build yourself. Roy Lewallen, W7EL James Fenech wrote: . . . I have the ARRL handbook (1997 or so) but this doesn't have much in the way of theory. Is there any other reference anyone can recommend? . . . |
Thanks Joe,
I wouldn't have thought it would be too difficult. Minicircuits sells parts that have a 2:1 range (not the right range though), and I only want 1.7:1 or so. Interesting idea about downmixing, but I really don't want another mixer and oscillator in this thing. Acording to my pspice results I can get the range, but I'm a little concerned about the high voltage swing on the varactors causing very poor phase noise. James. "W3JDR" wrote in message ... 2:1 tuning range with varactor tuning is very doable in the frequency range he's considering. I've done 3:1 at lower frequencies. No steep hill at all. Joe W3JDR "Steve Nosko" wrote in message ... You're climbing a steep hill here James. It is the % change in frequency that is the problem. Professional spec analysers use a 2 GHz LO with a 2:1 range (to scan 0-2GHz), but I believe that is a magnetically tuned YIG resonator. If you could get the frequency higher, so the sweep range is narrower, then mix to the desired freq with a balanced mixer...that would be an easier VCO design. Steve K;9;d;c;i The punctuation is my feeble attempt at spam-bot blocking. "James Fenech" wrote in message ... Hi Roy, thanks for the suggestion. I am considering buying this book. Can I ask how much theory, and how deep does it go? I am an engineer (digital electornics and software background) and actually like some theory to help me understand what I am doing. I already have some "real" test equipment, 50MHz CRO, signal generator, multimeter, etc. So "simple" test equipment may not be too much of an improvement. I've looked over the internet, and some books that I have, but found no real example circuit on wideband VCOs. The only "sort of close enough" circuit I found is at: http://www.newwaveinstruments.com/resources/rf_microwave_resources/sections/oscillator_vco_theory_design_circuit.htm#Voltage%2 0Controled%20Oscillator%20(VCO)%20Circuits The second one down - Colpitts. Are there any such examples in this book? Thanks, James. "Roy Lewallen" wrote in message ... I highly recommend _Experimental Methods in RF Design_, by Hayward, Cambell, and Larkin. It's published by the ARRL and available from them and numerous other sources. Besides theory and a lot of real, practical, tested circuits and projects, it includes simple test equipment you can build yourself. Roy Lewallen, W7EL James Fenech wrote: . . . I have the ARRL handbook (1997 or so) but this doesn't have much in the way of theory. Is there any other reference anyone can recommend? . . . |
James
In general, very wide tuning range and very good phase noise are sort of antithetical. If you use the circuit I sent you, the phase noise will be quite good for your simple spectrum analyzer application (unless you're planning to have a very high resolution/very low bandwidth mode). Don't get too hung up on your Spice modeling. It can keep you from trying some things that are known to work. In general, if I can build it faster than I can model it, I'll just go right for building it. I use modeling to get another perspective, but not to get the whole picture. Joe "James Fenech" wrote in message ... Thanks Joe, I wouldn't have thought it would be too difficult. Minicircuits sells parts that have a 2:1 range (not the right range though), and I only want 1.7:1 or so. Interesting idea about downmixing, but I really don't want another mixer and oscillator in this thing. Acording to my pspice results I can get the range, but I'm a little concerned about the high voltage swing on the varactors causing very poor phase noise. James. "W3JDR" wrote in message ... 2:1 tuning range with varactor tuning is very doable in the frequency range he's considering. I've done 3:1 at lower frequencies. No steep hill at all. Joe W3JDR "Steve Nosko" wrote in message ... You're climbing a steep hill here James. It is the % change in frequency that is the problem. Professional spec analysers use a 2 GHz LO with a 2:1 range (to scan 0-2GHz), but I believe that is a magnetically tuned YIG resonator. If you could get the frequency higher, so the sweep range is narrower, then mix to the desired freq with a balanced mixer...that would be an easier VCO design. Steve K;9;d;c;i The punctuation is my feeble attempt at spam-bot blocking. "James Fenech" wrote in message ... Hi Roy, thanks for the suggestion. I am considering buying this book. Can I ask how much theory, and how deep does it go? I am an engineer (digital electornics and software background) and actually like some theory to help me understand what I am doing. I already have some "real" test equipment, 50MHz CRO, signal generator, multimeter, etc. So "simple" test equipment may not be too much of an improvement. I've looked over the internet, and some books that I have, but found no real example circuit on wideband VCOs. The only "sort of close enough" circuit I found is at: http://www.newwaveinstruments.com/resources/rf_microwave_resources/sections/oscillator_vco_theory_design_circuit.htm#Voltage%2 0Controled%20Oscillator%20(VCO)%20Circuits The second one down - Colpitts. Are there any such examples in this book? Thanks, James. "Roy Lewallen" wrote in message ... I highly recommend _Experimental Methods in RF Design_, by Hayward, Cambell, and Larkin. It's published by the ARRL and available from them and numerous other sources. Besides theory and a lot of real, practical, tested circuits and projects, it includes simple test equipment you can build yourself. Roy Lewallen, W7EL James Fenech wrote: . . . I have the ARRL handbook (1997 or so) but this doesn't have much in the way of theory. Is there any other reference anyone can recommend? . . . |
James
In general, very wide tuning range and very good phase noise are sort of antithetical. If you use the circuit I sent you, the phase noise will be quite good for your simple spectrum analyzer application (unless you're planning to have a very high resolution/very low bandwidth mode). Don't get too hung up on your Spice modeling. It can keep you from trying some things that are known to work. In general, if I can build it faster than I can model it, I'll just go right for building it. I use modeling to get another perspective, but not to get the whole picture. Joe "James Fenech" wrote in message ... Thanks Joe, I wouldn't have thought it would be too difficult. Minicircuits sells parts that have a 2:1 range (not the right range though), and I only want 1.7:1 or so. Interesting idea about downmixing, but I really don't want another mixer and oscillator in this thing. Acording to my pspice results I can get the range, but I'm a little concerned about the high voltage swing on the varactors causing very poor phase noise. James. "W3JDR" wrote in message ... 2:1 tuning range with varactor tuning is very doable in the frequency range he's considering. I've done 3:1 at lower frequencies. No steep hill at all. Joe W3JDR "Steve Nosko" wrote in message ... You're climbing a steep hill here James. It is the % change in frequency that is the problem. Professional spec analysers use a 2 GHz LO with a 2:1 range (to scan 0-2GHz), but I believe that is a magnetically tuned YIG resonator. If you could get the frequency higher, so the sweep range is narrower, then mix to the desired freq with a balanced mixer...that would be an easier VCO design. Steve K;9;d;c;i The punctuation is my feeble attempt at spam-bot blocking. "James Fenech" wrote in message ... Hi Roy, thanks for the suggestion. I am considering buying this book. Can I ask how much theory, and how deep does it go? I am an engineer (digital electornics and software background) and actually like some theory to help me understand what I am doing. I already have some "real" test equipment, 50MHz CRO, signal generator, multimeter, etc. So "simple" test equipment may not be too much of an improvement. I've looked over the internet, and some books that I have, but found no real example circuit on wideband VCOs. The only "sort of close enough" circuit I found is at: http://www.newwaveinstruments.com/resources/rf_microwave_resources/sections/oscillator_vco_theory_design_circuit.htm#Voltage%2 0Controled%20Oscillator%20(VCO)%20Circuits The second one down - Colpitts. Are there any such examples in this book? Thanks, James. "Roy Lewallen" wrote in message ... I highly recommend _Experimental Methods in RF Design_, by Hayward, Cambell, and Larkin. It's published by the ARRL and available from them and numerous other sources. Besides theory and a lot of real, practical, tested circuits and projects, it includes simple test equipment you can build yourself. Roy Lewallen, W7EL James Fenech wrote: . . . I have the ARRL handbook (1997 or so) but this doesn't have much in the way of theory. Is there any other reference anyone can recommend? . . . |
Seems to me I read a note about W7ZOI starting to build a variation of his
original spectrum analyzer which will cover up to 200 MHz. http://users.easystreet.com/w7zoi/vhfbpf.html He expects to use a mini-circuit VCO. POS-535 VCO from MiniCircuits will sweep from 260 to 460 MHz Given a first IF of 260 MHz, it all works nicely. Personally, I'd be inclined to do something like this, using the minicircuits part, since they tend to be fairly linear tuning MHz/VOLT which simplifies the sweep circuitry a lot. I have built broadband tuning range VCOs before, and I know it can be done, but in a spectrum analyzer, the performance is usually limited by the first VCO so using a good performance VCO makes sense to me. Jim N6BIU |
Seems to me I read a note about W7ZOI starting to build a variation of his
original spectrum analyzer which will cover up to 200 MHz. http://users.easystreet.com/w7zoi/vhfbpf.html He expects to use a mini-circuit VCO. POS-535 VCO from MiniCircuits will sweep from 260 to 460 MHz Given a first IF of 260 MHz, it all works nicely. Personally, I'd be inclined to do something like this, using the minicircuits part, since they tend to be fairly linear tuning MHz/VOLT which simplifies the sweep circuitry a lot. I have built broadband tuning range VCOs before, and I know it can be done, but in a spectrum analyzer, the performance is usually limited by the first VCO so using a good performance VCO makes sense to me. Jim N6BIU |
_Experimental Methods_ is oriented toward the advanced amateur, so you
won't find the depth and level of math you would in a college text or a specialized text on one of the many topics covered in that book. However, there's a lot more depth than you'll find in something like the _ARRL Handbook_. Another thing is that the authors are all experienced and professional engineers who've spent years doing RF design, and all know a very great deal about the covered topics. In my opinion, only someone with a very deep and basic understanding of the fundamentals can accurately explain theory in an intuitive and easy to understand way. The authors have that understanding. I have no doubt you'll be very pleased with the book. Re test equipment: how about devices for measuring RF power, impedance, frequency, inductance, capacitance, Q, spectra, noise figure, and crystal characteristics? I don't believe there's an example of a wideband VCO. The authors concentrate a lot on keeping phase noise low, so use other methods to achieve wideband operation than with a wideband VCO. If you're looking for a more theoretical treatment of oscillators and other RF circuits, I recommend Hayward's _Introduction to Radio Frequency Design_, now also published by the ARRL. (You might find a used first edition, published by Wiley as I recall.) After reading the chapter on oscillators, you'll know enough to get a good start at least at designing your own VCO. There are a lot of practical tips and observations about oscillators in _Oscillator Design and Computer Simulation_ by Randall Rhea (Noble publishing, ISBN1-884932-30-4), although in my opinion it doesn't convey as fundamental understanding of oscillator operation as Hayward's books. Roy Lewallen, W7EL James Fenech wrote: Hi Roy, thanks for the suggestion. I am considering buying this book. Can I ask how much theory, and how deep does it go? I am an engineer (digital electornics and software background) and actually like some theory to help me understand what I am doing. I already have some "real" test equipment, 50MHz CRO, signal generator, multimeter, etc. So "simple" test equipment may not be too much of an improvement. I've looked over the internet, and some books that I have, but found no real example circuit on wideband VCOs. The only "sort of close enough" circuit I found is at: http://www.newwaveinstruments.com/resources/rf_microwave_resources/sections/oscillator_vco_theory_design_circuit.htm#Voltage%2 0Controled%20Oscillator%20(VCO)%20Circuits The second one down - Colpitts. Are there any such examples in this book? Thanks, James. "Roy Lewallen" wrote in message ... I highly recommend _Experimental Methods in RF Design_, by Hayward, Cambell, and Larkin. It's published by the ARRL and available from them and numerous other sources. Besides theory and a lot of real, practical, tested circuits and projects, it includes simple test equipment you can build yourself. Roy Lewallen, W7EL James Fenech wrote: . . . I have the ARRL handbook (1997 or so) but this doesn't have much in the way of theory. Is there any other reference anyone can recommend? . . . |
_Experimental Methods_ is oriented toward the advanced amateur, so you
won't find the depth and level of math you would in a college text or a specialized text on one of the many topics covered in that book. However, there's a lot more depth than you'll find in something like the _ARRL Handbook_. Another thing is that the authors are all experienced and professional engineers who've spent years doing RF design, and all know a very great deal about the covered topics. In my opinion, only someone with a very deep and basic understanding of the fundamentals can accurately explain theory in an intuitive and easy to understand way. The authors have that understanding. I have no doubt you'll be very pleased with the book. Re test equipment: how about devices for measuring RF power, impedance, frequency, inductance, capacitance, Q, spectra, noise figure, and crystal characteristics? I don't believe there's an example of a wideband VCO. The authors concentrate a lot on keeping phase noise low, so use other methods to achieve wideband operation than with a wideband VCO. If you're looking for a more theoretical treatment of oscillators and other RF circuits, I recommend Hayward's _Introduction to Radio Frequency Design_, now also published by the ARRL. (You might find a used first edition, published by Wiley as I recall.) After reading the chapter on oscillators, you'll know enough to get a good start at least at designing your own VCO. There are a lot of practical tips and observations about oscillators in _Oscillator Design and Computer Simulation_ by Randall Rhea (Noble publishing, ISBN1-884932-30-4), although in my opinion it doesn't convey as fundamental understanding of oscillator operation as Hayward's books. Roy Lewallen, W7EL James Fenech wrote: Hi Roy, thanks for the suggestion. I am considering buying this book. Can I ask how much theory, and how deep does it go? I am an engineer (digital electornics and software background) and actually like some theory to help me understand what I am doing. I already have some "real" test equipment, 50MHz CRO, signal generator, multimeter, etc. So "simple" test equipment may not be too much of an improvement. I've looked over the internet, and some books that I have, but found no real example circuit on wideband VCOs. The only "sort of close enough" circuit I found is at: http://www.newwaveinstruments.com/resources/rf_microwave_resources/sections/oscillator_vco_theory_design_circuit.htm#Voltage%2 0Controled%20Oscillator%20(VCO)%20Circuits The second one down - Colpitts. Are there any such examples in this book? Thanks, James. "Roy Lewallen" wrote in message ... I highly recommend _Experimental Methods in RF Design_, by Hayward, Cambell, and Larkin. It's published by the ARRL and available from them and numerous other sources. Besides theory and a lot of real, practical, tested circuits and projects, it includes simple test equipment you can build yourself. Roy Lewallen, W7EL James Fenech wrote: . . . I have the ARRL handbook (1997 or so) but this doesn't have much in the way of theory. Is there any other reference anyone can recommend? . . . |
Thanks for the feedback Roy, I've now got the book on order. Unfortunately
I've now blown my book budget and will need to wait a while before ordering anything else. James. "Roy Lewallen" wrote in message ... _Experimental Methods_ is oriented toward the advanced amateur, so you won't find the depth and level of math you would in a college text or a specialized text on one of the many topics covered in that book. However, there's a lot more depth than you'll find in something like the _ARRL Handbook_. Another thing is that the authors are all experienced and professional engineers who've spent years doing RF design, and all know a very great deal about the covered topics. In my opinion, only someone with a very deep and basic understanding of the fundamentals can accurately explain theory in an intuitive and easy to understand way. The authors have that understanding. I have no doubt you'll be very pleased with the book. Re test equipment: how about devices for measuring RF power, impedance, frequency, inductance, capacitance, Q, spectra, noise figure, and crystal characteristics? I don't believe there's an example of a wideband VCO. The authors concentrate a lot on keeping phase noise low, so use other methods to achieve wideband operation than with a wideband VCO. If you're looking for a more theoretical treatment of oscillators and other RF circuits, I recommend Hayward's _Introduction to Radio Frequency Design_, now also published by the ARRL. (You might find a used first edition, published by Wiley as I recall.) After reading the chapter on oscillators, you'll know enough to get a good start at least at designing your own VCO. There are a lot of practical tips and observations about oscillators in _Oscillator Design and Computer Simulation_ by Randall Rhea (Noble publishing, ISBN1-884932-30-4), although in my opinion it doesn't convey as fundamental understanding of oscillator operation as Hayward's books. Roy Lewallen, W7EL James Fenech wrote: Hi Roy, thanks for the suggestion. I am considering buying this book. Can I ask how much theory, and how deep does it go? I am an engineer (digital electornics and software background) and actually like some theory to help me understand what I am doing. I already have some "real" test equipment, 50MHz CRO, signal generator, multimeter, etc. So "simple" test equipment may not be too much of an improvement. I've looked over the internet, and some books that I have, but found no real example circuit on wideband VCOs. The only "sort of close enough" circuit I found is at: http://www.newwaveinstruments.com/resources/rf_microwave_resources/sections/oscillator_vco_theory_design_circuit.htm#Voltage%2 0Controled%20Oscillator%20(VCO)%20Circuits The second one down - Colpitts. Are there any such examples in this book? Thanks, James. "Roy Lewallen" wrote in message ... I highly recommend _Experimental Methods in RF Design_, by Hayward, Cambell, and Larkin. It's published by the ARRL and available from them and numerous other sources. Besides theory and a lot of real, practical, tested circuits and projects, it includes simple test equipment you can build yourself. Roy Lewallen, W7EL James Fenech wrote: . . . I have the ARRL handbook (1997 or so) but this doesn't have much in the way of theory. Is there any other reference anyone can recommend? . . . |
Thanks for the feedback Roy, I've now got the book on order. Unfortunately
I've now blown my book budget and will need to wait a while before ordering anything else. James. "Roy Lewallen" wrote in message ... _Experimental Methods_ is oriented toward the advanced amateur, so you won't find the depth and level of math you would in a college text or a specialized text on one of the many topics covered in that book. However, there's a lot more depth than you'll find in something like the _ARRL Handbook_. Another thing is that the authors are all experienced and professional engineers who've spent years doing RF design, and all know a very great deal about the covered topics. In my opinion, only someone with a very deep and basic understanding of the fundamentals can accurately explain theory in an intuitive and easy to understand way. The authors have that understanding. I have no doubt you'll be very pleased with the book. Re test equipment: how about devices for measuring RF power, impedance, frequency, inductance, capacitance, Q, spectra, noise figure, and crystal characteristics? I don't believe there's an example of a wideband VCO. The authors concentrate a lot on keeping phase noise low, so use other methods to achieve wideband operation than with a wideband VCO. If you're looking for a more theoretical treatment of oscillators and other RF circuits, I recommend Hayward's _Introduction to Radio Frequency Design_, now also published by the ARRL. (You might find a used first edition, published by Wiley as I recall.) After reading the chapter on oscillators, you'll know enough to get a good start at least at designing your own VCO. There are a lot of practical tips and observations about oscillators in _Oscillator Design and Computer Simulation_ by Randall Rhea (Noble publishing, ISBN1-884932-30-4), although in my opinion it doesn't convey as fundamental understanding of oscillator operation as Hayward's books. Roy Lewallen, W7EL James Fenech wrote: Hi Roy, thanks for the suggestion. I am considering buying this book. Can I ask how much theory, and how deep does it go? I am an engineer (digital electornics and software background) and actually like some theory to help me understand what I am doing. I already have some "real" test equipment, 50MHz CRO, signal generator, multimeter, etc. So "simple" test equipment may not be too much of an improvement. I've looked over the internet, and some books that I have, but found no real example circuit on wideband VCOs. The only "sort of close enough" circuit I found is at: http://www.newwaveinstruments.com/resources/rf_microwave_resources/sections/oscillator_vco_theory_design_circuit.htm#Voltage%2 0Controled%20Oscillator%20(VCO)%20Circuits The second one down - Colpitts. Are there any such examples in this book? Thanks, James. "Roy Lewallen" wrote in message ... I highly recommend _Experimental Methods in RF Design_, by Hayward, Cambell, and Larkin. It's published by the ARRL and available from them and numerous other sources. Besides theory and a lot of real, practical, tested circuits and projects, it includes simple test equipment you can build yourself. Roy Lewallen, W7EL James Fenech wrote: . . . I have the ARRL handbook (1997 or so) but this doesn't have much in the way of theory. Is there any other reference anyone can recommend? . . . |
Hans Summers wrote:
See my spectrum analyser: http://www.hanssummers.com/electroni...yser/index.htm which has 145MHz 1st IF and covers right up to (and through!) the 1st IF, using an SA602 VCO covering 145MHz to something like 300MHz with no problem and apparently excellent linearity. The varicap is BB105. Dynamic range is limited to something like 70dB by the choice of front end (SA602) but it's a fun and useful project that is quite easy to build. If anyone wants the circuit diagram and full construction details let me know. I very much enjoyed seeing all the photos of your construction and would like to receive the schematic and details. Thanks. -- John Popelish |
Hans Summers wrote:
See my spectrum analyser: http://www.hanssummers.com/electroni...yser/index.htm which has 145MHz 1st IF and covers right up to (and through!) the 1st IF, using an SA602 VCO covering 145MHz to something like 300MHz with no problem and apparently excellent linearity. The varicap is BB105. Dynamic range is limited to something like 70dB by the choice of front end (SA602) but it's a fun and useful project that is quite easy to build. If anyone wants the circuit diagram and full construction details let me know. I very much enjoyed seeing all the photos of your construction and would like to receive the schematic and details. Thanks. -- John Popelish |
My comments are colored by experience desigining quite low noise, relitavely
narrow bandwidth VCOs, so all the other comments are very valid. I will note that there are two aspects of noise to consider, that I can think of off hand. One is that: 1) Too low of an absolute DC voltage on the varactors will make for a noisy VCO and 2) A wide tuning range means that the varactor is relatively tightly coupled into the oscillator which means that control line noise/spurs you'll have to watch control line noise & spurs. 3) This also means the Varactors have more effect on the oscillator Q and therefore can also mean increased noise. OK nobody expects the Spanish inquisition. If you can determine the actual noise performance and keep it in mind that the analyzer has a noise limit, you'll be ok. Been away from it too long and can't seem to recall why we used to use two, back-to-back varactors...brain is full...seems like it was to reduce the voltage across the varactor. 'guards, Steve K;9;D:C:I "W3JDR" wrote in message ... 2:1 tuning range with varactor tuning is very doable in the frequency range he's considering. I've done 3:1 at lower frequencies. No steep hill at all. Joe W3JDR "Steve Nosko" wrote in message ... You're climbing a steep hill here James. It is the % change in frequency that is the problem. Professional spec analysers use a 2 GHz LO with a 2:1 range (to scan 0-2GHz), but I believe that is a magnetically tuned YIG resonator. If you could get the frequency higher, so the sweep range is narrower, then mix to the desired freq with a balanced mixer...that would be an easier VCO design. Steve K;9;d;c;i The punctuation is my feeble attempt at spam-bot blocking. "James Fenech" wrote in message ... Hi Roy, thanks for the suggestion. I am considering buying this book. Can I ask how much theory, and how deep does it go? I am an engineer (digital electornics and software background) and actually like some theory to help me understand what I am doing. I already have some "real" test equipment, 50MHz CRO, signal generator, multimeter, etc. So "simple" test equipment may not be too much of an improvement. I've looked over the internet, and some books that I have, but found no real example circuit on wideband VCOs. The only "sort of close enough" circuit I found is at: http://www.newwaveinstruments.com/resources/rf_microwave_resources/sections/oscillator_vco_theory_design_circuit.htm#Voltage%2 0Controled%20Oscillator%20(VCO)%20Circuits The second one down - Colpitts. Are there any such examples in this book? Thanks, James. "Roy Lewallen" wrote in message ... I highly recommend _Experimental Methods in RF Design_, by Hayward, Cambell, and Larkin. It's published by the ARRL and available from them and numerous other sources. Besides theory and a lot of real, practical, tested circuits and projects, it includes simple test equipment you can build yourself. Roy Lewallen, W7EL James Fenech wrote: . . . I have the ARRL handbook (1997 or so) but this doesn't have much in the way of theory. Is there any other reference anyone can recommend? . . . |
My comments are colored by experience desigining quite low noise, relitavely
narrow bandwidth VCOs, so all the other comments are very valid. I will note that there are two aspects of noise to consider, that I can think of off hand. One is that: 1) Too low of an absolute DC voltage on the varactors will make for a noisy VCO and 2) A wide tuning range means that the varactor is relatively tightly coupled into the oscillator which means that control line noise/spurs you'll have to watch control line noise & spurs. 3) This also means the Varactors have more effect on the oscillator Q and therefore can also mean increased noise. OK nobody expects the Spanish inquisition. If you can determine the actual noise performance and keep it in mind that the analyzer has a noise limit, you'll be ok. Been away from it too long and can't seem to recall why we used to use two, back-to-back varactors...brain is full...seems like it was to reduce the voltage across the varactor. 'guards, Steve K;9;D:C:I "W3JDR" wrote in message ... 2:1 tuning range with varactor tuning is very doable in the frequency range he's considering. I've done 3:1 at lower frequencies. No steep hill at all. Joe W3JDR "Steve Nosko" wrote in message ... You're climbing a steep hill here James. It is the % change in frequency that is the problem. Professional spec analysers use a 2 GHz LO with a 2:1 range (to scan 0-2GHz), but I believe that is a magnetically tuned YIG resonator. If you could get the frequency higher, so the sweep range is narrower, then mix to the desired freq with a balanced mixer...that would be an easier VCO design. Steve K;9;d;c;i The punctuation is my feeble attempt at spam-bot blocking. "James Fenech" wrote in message ... Hi Roy, thanks for the suggestion. I am considering buying this book. Can I ask how much theory, and how deep does it go? I am an engineer (digital electornics and software background) and actually like some theory to help me understand what I am doing. I already have some "real" test equipment, 50MHz CRO, signal generator, multimeter, etc. So "simple" test equipment may not be too much of an improvement. I've looked over the internet, and some books that I have, but found no real example circuit on wideband VCOs. The only "sort of close enough" circuit I found is at: http://www.newwaveinstruments.com/resources/rf_microwave_resources/sections/oscillator_vco_theory_design_circuit.htm#Voltage%2 0Controled%20Oscillator%20(VCO)%20Circuits The second one down - Colpitts. Are there any such examples in this book? Thanks, James. "Roy Lewallen" wrote in message ... I highly recommend _Experimental Methods in RF Design_, by Hayward, Cambell, and Larkin. It's published by the ARRL and available from them and numerous other sources. Besides theory and a lot of real, practical, tested circuits and projects, it includes simple test equipment you can build yourself. Roy Lewallen, W7EL James Fenech wrote: . . . I have the ARRL handbook (1997 or so) but this doesn't have much in the way of theory. Is there any other reference anyone can recommend? . . . |
Steve,
I think the main reason for back-to-back diodes is to prevent rectification of the RF. Rectification can cause several bad things, including pushing DC current back out the tuning voltage line, instability, and increased phase noise. Having said that, I tried back-to-back diodes a couple times and I don't ever recall ending up with it in the final design, so it must not have added all that much value. On the negative side, it halves the capacitance. One technique to improve the phase noise in wide-band VCO's was shown by Ulrich Rhode. He uses several diodes in parallel in order to decrease the RF current in each diode, reportedly lowering losses and improving noise. I never tried it myself. Joe W3JDR "Steve Nosko" wrote in message ... My comments are colored by experience desigining quite low noise, relitavely narrow bandwidth VCOs, so all the other comments are very valid. I will note that there are two aspects of noise to consider, that I can think of off hand. One is that: 1) Too low of an absolute DC voltage on the varactors will make for a noisy VCO and 2) A wide tuning range means that the varactor is relatively tightly coupled into the oscillator which means that control line noise/spurs you'll have to watch control line noise & spurs. 3) This also means the Varactors have more effect on the oscillator Q and therefore can also mean increased noise. OK nobody expects the Spanish inquisition. If you can determine the actual noise performance and keep it in mind that the analyzer has a noise limit, you'll be ok. Been away from it too long and can't seem to recall why we used to use two, back-to-back varactors...brain is full...seems like it was to reduce the voltage across the varactor. 'guards, Steve K;9;D:C:I "W3JDR" wrote in message ... 2:1 tuning range with varactor tuning is very doable in the frequency range he's considering. I've done 3:1 at lower frequencies. No steep hill at all. Joe W3JDR "Steve Nosko" wrote in message ... You're climbing a steep hill here James. It is the % change in frequency that is the problem. Professional spec analysers use a 2 GHz LO with a 2:1 range (to scan 0-2GHz), but I believe that is a magnetically tuned YIG resonator. If you could get the frequency higher, so the sweep range is narrower, then mix to the desired freq with a balanced mixer...that would be an easier VCO design. Steve K;9;d;c;i The punctuation is my feeble attempt at spam-bot blocking. "James Fenech" wrote in message ... Hi Roy, thanks for the suggestion. I am considering buying this book. Can I ask how much theory, and how deep does it go? I am an engineer (digital electornics and software background) and actually like some theory to help me understand what I am doing. I already have some "real" test equipment, 50MHz CRO, signal generator, multimeter, etc. So "simple" test equipment may not be too much of an improvement. I've looked over the internet, and some books that I have, but found no real example circuit on wideband VCOs. The only "sort of close enough" circuit I found is at: http://www.newwaveinstruments.com/resources/rf_microwave_resources/sections/oscillator_vco_theory_design_circuit.htm#Voltage%2 0Controled%20Oscillator%20(VCO)%20Circuits The second one down - Colpitts. Are there any such examples in this book? Thanks, James. "Roy Lewallen" wrote in message ... I highly recommend _Experimental Methods in RF Design_, by Hayward, Cambell, and Larkin. It's published by the ARRL and available from them and numerous other sources. Besides theory and a lot of real, practical, tested circuits and projects, it includes simple test equipment you can build yourself. Roy Lewallen, W7EL James Fenech wrote: . . . I have the ARRL handbook (1997 or so) but this doesn't have much in the way of theory. Is there any other reference anyone can recommend? . . . |
Steve,
I think the main reason for back-to-back diodes is to prevent rectification of the RF. Rectification can cause several bad things, including pushing DC current back out the tuning voltage line, instability, and increased phase noise. Having said that, I tried back-to-back diodes a couple times and I don't ever recall ending up with it in the final design, so it must not have added all that much value. On the negative side, it halves the capacitance. One technique to improve the phase noise in wide-band VCO's was shown by Ulrich Rhode. He uses several diodes in parallel in order to decrease the RF current in each diode, reportedly lowering losses and improving noise. I never tried it myself. Joe W3JDR "Steve Nosko" wrote in message ... My comments are colored by experience desigining quite low noise, relitavely narrow bandwidth VCOs, so all the other comments are very valid. I will note that there are two aspects of noise to consider, that I can think of off hand. One is that: 1) Too low of an absolute DC voltage on the varactors will make for a noisy VCO and 2) A wide tuning range means that the varactor is relatively tightly coupled into the oscillator which means that control line noise/spurs you'll have to watch control line noise & spurs. 3) This also means the Varactors have more effect on the oscillator Q and therefore can also mean increased noise. OK nobody expects the Spanish inquisition. If you can determine the actual noise performance and keep it in mind that the analyzer has a noise limit, you'll be ok. Been away from it too long and can't seem to recall why we used to use two, back-to-back varactors...brain is full...seems like it was to reduce the voltage across the varactor. 'guards, Steve K;9;D:C:I "W3JDR" wrote in message ... 2:1 tuning range with varactor tuning is very doable in the frequency range he's considering. I've done 3:1 at lower frequencies. No steep hill at all. Joe W3JDR "Steve Nosko" wrote in message ... You're climbing a steep hill here James. It is the % change in frequency that is the problem. Professional spec analysers use a 2 GHz LO with a 2:1 range (to scan 0-2GHz), but I believe that is a magnetically tuned YIG resonator. If you could get the frequency higher, so the sweep range is narrower, then mix to the desired freq with a balanced mixer...that would be an easier VCO design. Steve K;9;d;c;i The punctuation is my feeble attempt at spam-bot blocking. "James Fenech" wrote in message ... Hi Roy, thanks for the suggestion. I am considering buying this book. Can I ask how much theory, and how deep does it go? I am an engineer (digital electornics and software background) and actually like some theory to help me understand what I am doing. I already have some "real" test equipment, 50MHz CRO, signal generator, multimeter, etc. So "simple" test equipment may not be too much of an improvement. I've looked over the internet, and some books that I have, but found no real example circuit on wideband VCOs. The only "sort of close enough" circuit I found is at: http://www.newwaveinstruments.com/resources/rf_microwave_resources/sections/oscillator_vco_theory_design_circuit.htm#Voltage%2 0Controled%20Oscillator%20(VCO)%20Circuits The second one down - Colpitts. Are there any such examples in this book? Thanks, James. "Roy Lewallen" wrote in message ... I highly recommend _Experimental Methods in RF Design_, by Hayward, Cambell, and Larkin. It's published by the ARRL and available from them and numerous other sources. Besides theory and a lot of real, practical, tested circuits and projects, it includes simple test equipment you can build yourself. Roy Lewallen, W7EL James Fenech wrote: . . . I have the ARRL handbook (1997 or so) but this doesn't have much in the way of theory. Is there any other reference anyone can recommend? . . . |
One technique to improve the phase noise in wide-band VCO's was shown by
Ulrich Rhode. He uses several diodes in parallel in order to decrease the RF current in each diode, reportedly lowering losses and improving noise. I never tried it myself. If the diodes are the limiting factor, it can help. The second diode seemed to do the most good, the third, a little more, but any more didn't help (in my designs). The way the varactor is made will affect the noise performance. Ask the manufacturers which process yields their best phase-noise performance. Most of them are very helpful. As I recall, planar, epitaxial construction, with thermal compression bonded leads gave my best results. Hyperabrupt diodes gave the worst. But things change rapidly these days, so ask. 73, John - K6QQ |
One technique to improve the phase noise in wide-band VCO's was shown by
Ulrich Rhode. He uses several diodes in parallel in order to decrease the RF current in each diode, reportedly lowering losses and improving noise. I never tried it myself. If the diodes are the limiting factor, it can help. The second diode seemed to do the most good, the third, a little more, but any more didn't help (in my designs). The way the varactor is made will affect the noise performance. Ask the manufacturers which process yields their best phase-noise performance. Most of them are very helpful. As I recall, planar, epitaxial construction, with thermal compression bonded leads gave my best results. Hyperabrupt diodes gave the worst. But things change rapidly these days, so ask. 73, John - K6QQ |
On Fri, 9 Jan 2004 16:31:56 -0800, "John Moriarity"
wrote: One technique to improve the phase noise in wide-band VCO's was shown by Ulrich Rhode. He uses several diodes in parallel in order to decrease the RF current in each diode, reportedly lowering losses and improving noise. I never tried it myself. Or is it simply that when using multiple varactors in parallel, the amount of capacitance needed from each varactor is reduced, hence a higher tuning voltage must be used with the same inductance ? With a high tuning voltage, the capacitance/voltage ratio is smaller, hence the capacitance difference would be smaller on the separate RF half cycles. This would reduce the phase noise. If the diodes are the limiting factor, it can help. The second diode seemed to do the most good, the third, a little more, but any more didn't help (in my designs). Your observations would support my theory, since adding further varactors would only increase the tuning voltage slightly with the same inductance. Paul OH3LWR |
On Fri, 9 Jan 2004 16:31:56 -0800, "John Moriarity"
wrote: One technique to improve the phase noise in wide-band VCO's was shown by Ulrich Rhode. He uses several diodes in parallel in order to decrease the RF current in each diode, reportedly lowering losses and improving noise. I never tried it myself. Or is it simply that when using multiple varactors in parallel, the amount of capacitance needed from each varactor is reduced, hence a higher tuning voltage must be used with the same inductance ? With a high tuning voltage, the capacitance/voltage ratio is smaller, hence the capacitance difference would be smaller on the separate RF half cycles. This would reduce the phase noise. If the diodes are the limiting factor, it can help. The second diode seemed to do the most good, the third, a little more, but any more didn't help (in my designs). Your observations would support my theory, since adding further varactors would only increase the tuning voltage slightly with the same inductance. Paul OH3LWR |
In article , "W3JDR"
writes: Steve, I think the main reason for back-to-back diodes is to prevent rectification of the RF. Rectification can cause several bad things, including pushing DC current back out the tuning voltage line, instability, and increased phase noise. Having said that, I tried back-to-back diodes a couple times and I don't ever recall ending up with it in the final design, so it must not have added all that much value. On the negative side, it halves the capacitance. One technique to improve the phase noise in wide-band VCO's was shown by Ulrich Rhode. He uses several diodes in parallel in order to decrease the RF current in each diode, reportedly lowering losses and improving noise. I never tried it myself. Variable-capacitance diodes exhibit that phenomenon only in reverse-DC-bias connections. In forward-DC-bias connection they behave generally like ordinary diodes, conducting very little current until about 0.6 V across the junction. A single variable-capacitance diode across an RF circuit will conduct - and thus have an effect on the RF tuned circuit - when the combination of DC tuning bias and RF voltages are above the forward-conduction breakpoint. The purpose of "back-to-back" connection is to keep the (now two) variable-capacitance diodes always in reverse-conduction...the RF voltage (peak-to-peak) is not supposed to exceed either the break- down voltage of the diodes or cause either of them to be forward- biased during any part of the RF cycle. With no forward conduction, the variable capacitance diodes remain just that - variable capacitances. When forward conduction occurs, it adds more non-linearity to the RF circuit and tends to decrease the action of the variable capacitance. When used in low-level RF stages of a receiver input, the RF voltage hardly ever exceeds 1 V peak-to-peak and thus the variable capacitance diode never goes into forward conduction. Single diodes can be used there without doubling-up. Len Anderson retired (from regular hours) electronic engineer person |
In article , "W3JDR"
writes: Steve, I think the main reason for back-to-back diodes is to prevent rectification of the RF. Rectification can cause several bad things, including pushing DC current back out the tuning voltage line, instability, and increased phase noise. Having said that, I tried back-to-back diodes a couple times and I don't ever recall ending up with it in the final design, so it must not have added all that much value. On the negative side, it halves the capacitance. One technique to improve the phase noise in wide-band VCO's was shown by Ulrich Rhode. He uses several diodes in parallel in order to decrease the RF current in each diode, reportedly lowering losses and improving noise. I never tried it myself. Variable-capacitance diodes exhibit that phenomenon only in reverse-DC-bias connections. In forward-DC-bias connection they behave generally like ordinary diodes, conducting very little current until about 0.6 V across the junction. A single variable-capacitance diode across an RF circuit will conduct - and thus have an effect on the RF tuned circuit - when the combination of DC tuning bias and RF voltages are above the forward-conduction breakpoint. The purpose of "back-to-back" connection is to keep the (now two) variable-capacitance diodes always in reverse-conduction...the RF voltage (peak-to-peak) is not supposed to exceed either the break- down voltage of the diodes or cause either of them to be forward- biased during any part of the RF cycle. With no forward conduction, the variable capacitance diodes remain just that - variable capacitances. When forward conduction occurs, it adds more non-linearity to the RF circuit and tends to decrease the action of the variable capacitance. When used in low-level RF stages of a receiver input, the RF voltage hardly ever exceeds 1 V peak-to-peak and thus the variable capacitance diode never goes into forward conduction. Single diodes can be used there without doubling-up. Len Anderson retired (from regular hours) electronic engineer person |
On Sat, 10 Jan 2004 12:50:46 +0200, Paul Keinanen
wrote: On Fri, 9 Jan 2004 16:31:56 -0800, "John Moriarity" wrote: One technique to improve the phase noise in wide-band VCO's was shown by Ulrich Rhode. He uses several diodes in parallel in order to decrease the RF current in each diode, reportedly lowering losses and improving noise. I never tried it myself. Or is it simply that when using multiple varactors in parallel, the amount of capacitance needed from each varactor is reduced, hence a higher tuning voltage must be used with the same inductance ? With a high tuning voltage, the capacitance/voltage ratio is smaller, hence the capacitance difference would be smaller on the separate RF half cycles. This would reduce the phase noise. If the diodes are the limiting factor, it can help. The second diode seemed to do the most good, the third, a little more, but any more didn't help (in my designs). Your observations would support my theory, since adding further varactors would only increase the tuning voltage slightly with the same inductance. Paul OH3LWR suppose the anti-paralell configuration could improve it further like http://home.online.no/~la8ak/images/1et27.gif , first got the idea from high level PIN switch in Electronics design, but have not seen the application elsewhere JM http://home.online.no/~la8ak/L2.htm -- Amount of SPAM is so large that MailWasher must delete 99% of the incoming mails Cannot check every email manually. Please use intelligent title for email. Mails without titles or using just "hi" is deleted |
On Sat, 10 Jan 2004 12:50:46 +0200, Paul Keinanen
wrote: On Fri, 9 Jan 2004 16:31:56 -0800, "John Moriarity" wrote: One technique to improve the phase noise in wide-band VCO's was shown by Ulrich Rhode. He uses several diodes in parallel in order to decrease the RF current in each diode, reportedly lowering losses and improving noise. I never tried it myself. Or is it simply that when using multiple varactors in parallel, the amount of capacitance needed from each varactor is reduced, hence a higher tuning voltage must be used with the same inductance ? With a high tuning voltage, the capacitance/voltage ratio is smaller, hence the capacitance difference would be smaller on the separate RF half cycles. This would reduce the phase noise. If the diodes are the limiting factor, it can help. The second diode seemed to do the most good, the third, a little more, but any more didn't help (in my designs). Your observations would support my theory, since adding further varactors would only increase the tuning voltage slightly with the same inductance. Paul OH3LWR suppose the anti-paralell configuration could improve it further like http://home.online.no/~la8ak/images/1et27.gif , first got the idea from high level PIN switch in Electronics design, but have not seen the application elsewhere JM http://home.online.no/~la8ak/L2.htm -- Amount of SPAM is so large that MailWasher must delete 99% of the incoming mails Cannot check every email manually. Please use intelligent title for email. Mails without titles or using just "hi" is deleted |
Ok, many of you have suggested rather than just simulation I should actually
build something. Joe (W3JDR) has kindly donated a circuit diagram which I started off with. Best way to get going quickly is dead bug style, soldered straight to the copper side of a PCB. I have never used this style of construction as it looked flimsy, but it looked like a good way to go just for prototyping. Well, the circuit went together fast and the finished product is very steady, so I am now a convert to this construction method! I may need a new iron though, as the basic low power model I have struggled to heat up the large area of board. Firstly I built the circuit (almost) as directed, slight modifications were made to suit parts I had. Fixed capacitors were used as this allowed evaluation without worrying about the diodes performance. The oscillation voltage across the tank was about 1 vp-p measured with an older 50MHz CRO and a 1:1 probe. Oscillation seemed stable and I liked the result. Question: I used the ARRL equation for a cylindrical inductor and seemed way off (50%). Should I expect the copper sheet to affect the inductance, and how. I then replaced the fixed capacitors with BB909 (3-30pF) diodes. The result wasn't so good. Oscillation voltage was less (lower Q tank maybe) and the stablility seemed poor (again poor Q). I will attempt more measurements tonight. James. |
Ok, many of you have suggested rather than just simulation I should actually
build something. Joe (W3JDR) has kindly donated a circuit diagram which I started off with. Best way to get going quickly is dead bug style, soldered straight to the copper side of a PCB. I have never used this style of construction as it looked flimsy, but it looked like a good way to go just for prototyping. Well, the circuit went together fast and the finished product is very steady, so I am now a convert to this construction method! I may need a new iron though, as the basic low power model I have struggled to heat up the large area of board. Firstly I built the circuit (almost) as directed, slight modifications were made to suit parts I had. Fixed capacitors were used as this allowed evaluation without worrying about the diodes performance. The oscillation voltage across the tank was about 1 vp-p measured with an older 50MHz CRO and a 1:1 probe. Oscillation seemed stable and I liked the result. Question: I used the ARRL equation for a cylindrical inductor and seemed way off (50%). Should I expect the copper sheet to affect the inductance, and how. I then replaced the fixed capacitors with BB909 (3-30pF) diodes. The result wasn't so good. Oscillation voltage was less (lower Q tank maybe) and the stablility seemed poor (again poor Q). I will attempt more measurements tonight. James. |
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