|
LC Oscillator Questions
Pictures available in ABSE
The top trace (yellow) is taken between C4 and R2. The bottom trace (cyan) is taken at the base of the transistor. There is a switchercad file, but the simulation will show allot of distortion that really isn't present in the prototype circuit, because of lots of circuit capactance I suspect. R1 was something I was playing with to try and tame the voltage across L1/C3 being applied to the base. Hello all, I was tinkering with this LC oscillator (Colpitts/Clapp) this weekend. I arrived at the values of C1 and C2 empirically after starting with a crystal oscillator circuit. The values in the original circuit created a horrid waveform that looked allot like the simulation. After much tinkering around and simulating, I come to the conclusion that getting a perfect waveform is nearly impossible, especially with big swing. It seems that the transistor likes to take a bite out of the right half of the peak of the wave. What is the secret to beautiful waveforms? Do I need another LC resonator on the output to fix it up? I mean, I'm getting a pretty nice wave now, but there is still some distortion that you can just see at the top of the peaks on the yellow trace. How do you control the peak voltages of an LC resonattor without mangling the waveform? The waveform at the junction of L1/C3 is of course quite beautiful, how do I get it from there to the output? ;-) I realize that I will need a buffer stage(s) before I can make any real use of the signal, but I want the input to the buffer to be as perfect as possible. Thanks :-) |
LC Oscillator Questions
On Mar 19, 9:23 am, "Anthony Fremont" wrote:
Pictures available in ABSE The top trace (yellow) is taken between C4 and R2. The bottom trace (cyan) is taken at the base of the transistor. There is a switchercad file, but the simulation will show allot of distortion that really isn't present in the prototype circuit, because of lots of circuit capactance I suspect. R1 was something I was playing with to try and tame the voltage across L1/C3 being applied to the base. Hello all, I was tinkering with this LC oscillator (Colpitts/Clapp) this weekend. I arrived at the values of C1 and C2 empirically after starting with a crystal oscillator circuit. The values in the original circuit created a horrid waveform that looked allot like the simulation. After much tinkering around and simulating, I come to the conclusion that getting a perfect waveform is nearly impossible, especially with big swing. It seems that the transistor likes to take a bite out of the right half of the peak of the wave. What is the secret to beautiful waveforms? Do I need another LC resonator on the output to fix it up? I mean, I'm getting a pretty nice wave now, but there is still some distortion that you can just see at the top of the peaks on the yellow trace. How do you control the peak voltages of an LC resonattor without mangling the waveform? The waveform at the junction of L1/C3 is of course quite beautiful, how do I get it from there to the output? ;-) I realize that I will need a buffer stage(s) before I can make any real use of the signal, but I want the input to the buffer to be as perfect as possible. Thanks :-) The waveform in a high Q tank that's lightly coupled to the amplifier should be very nearly sinusoidal. If in addition, the amplifier remains linear and represents a constant impedance over the whole cycle of the waveform, then the waveforms should everywhere be sinusoidal. If the amplifier+tank has barely enough loop gain to sustain oscillation, then clipping will be minimal, but it's also possible to detect the level and control the gain of the amplifier. You could, for example, use a light bulb like HP did in their original audio oscillator. Beware, though, that best oscillator performance in other regards may not be achieved the same way you achieve lowest harmonic distortion. Be careful that you optimize the right things for your application. In the work I do, I need to measure distortion, and the generators I use don't have low enough distortion in their outputs to be directly useful. The distortion levels in the "raw" outputs are generally about -40 to -50dBc. I use filters to make things better, and can get to -140dBc distortion levels fairly easily. If it's low harmonic distortion you want, I'd suggest that it may be better to just put a filter on the output of the oscillator that has only moderately low harmonic output, and not worry so much about that aspect of oscillator performance. Filters work well when the oscillator frequency range is about 1.5:1 or less. Much more than that and you'd need to switch in different filters depending on the oscillator frequency. Cheers, Tom |
LC Oscillator Questions
Hello Anthony,
1. Please set the following option to sitch off data reduction/compression in the result file.. ..options plotwinsize=0 2. You have to set a small maximum timestep in the .TRAN line too. Maybe a value of 0.01*Period of oscillation if you hunt for very low distortion. Can you send me your file (.asc-file and model-file?) to check it? Best regards, Helmut "Anthony Fremont" schrieb im Newsbeitrag ... Pictures available in ABSE The top trace (yellow) is taken between C4 and R2. The bottom trace (cyan) is taken at the base of the transistor. There is a switchercad file, but the simulation will show allot of distortion that really isn't present in the prototype circuit, because of lots of circuit capactance I suspect. R1 was something I was playing with to try and tame the voltage across L1/C3 being applied to the base. Hello all, I was tinkering with this LC oscillator (Colpitts/Clapp) this weekend. I arrived at the values of C1 and C2 empirically after starting with a crystal oscillator circuit. The values in the original circuit created a horrid waveform that looked allot like the simulation. After much tinkering around and simulating, I come to the conclusion that getting a perfect waveform is nearly impossible, especially with big swing. It seems that the transistor likes to take a bite out of the right half of the peak of the wave. What is the secret to beautiful waveforms? Do I need another LC resonator on the output to fix it up? I mean, I'm getting a pretty nice wave now, but there is still some distortion that you can just see at the top of the peaks on the yellow trace. How do you control the peak voltages of an LC resonattor without mangling the waveform? The waveform at the junction of L1/C3 is of course quite beautiful, how do I get it from there to the output? ;-) I realize that I will need a buffer stage(s) before I can make any real use of the signal, but I want the input to the buffer to be as perfect as possible. Thanks :-) |
LC Oscillator Questions
Anthony Fremont wrote:
Pictures available in ABSE The top trace (yellow) is taken between C4 and R2. The bottom trace (cyan) is taken at the base of the transistor. There is a switchercad file, but the simulation will show allot of distortion that really isn't present in the prototype circuit, because of lots of circuit capactance I suspect. R1 was something I was playing with to try and tame the voltage across L1/C3 being applied to the base. Hello all, I was tinkering with this LC oscillator (Colpitts/Clapp) this weekend. I arrived at the values of C1 and C2 empirically after starting with a crystal oscillator circuit. The values in the original circuit created a horrid waveform that looked allot like the simulation. After much tinkering around and simulating, I come to the conclusion that getting a perfect waveform is nearly impossible, especially with big swing. It seems that the transistor likes to take a bite out of the right half of the peak of the wave. What is the secret to beautiful waveforms? Do I need another LC resonator on the output to fix it up? I mean, I'm getting a pretty nice wave now, but there is still some distortion that you can just see at the top of the peaks on the yellow trace. How do you control the peak voltages of an LC resonattor without mangling the waveform? The waveform at the junction of L1/C3 is of course quite beautiful, how do I get it from there to the output? ;-) I realize that I will need a buffer stage(s) before I can make any real use of the signal, but I want the input to the buffer to be as perfect as possible. Thanks :-) The secret to a beautiful waveform is -- you usually don't need it straight from the oscillator. There are a lot of things that you want out of an LC oscillator. Low phase noise, frequency stability, consistently strong oscillation, pure tone, etc. Of these, the only two that you can't clean up later in the following amplifier chain is low phase noise and frequency stability. Concentrate on those, & don't sweat the nice waveform. Frequency stability and phase noise performance are often achieved by intentionally designing the amplifier so the active element operates in class C, without ever going into voltage saturation. This keeps it's drain (or collector) impedance high, yet delivers a large voltage swing to the gate (or base) to keep phase noise low. It also gives you a more or less consistent standing voltage in the tank, which helps the design of the following buffer stages. If you absolutely positively must tap the World's Most Beautiful Sine Wave off of the oscillator section, consider a parallel-tuned tank that's loosely coupled to the active element. Then loosely couple your output tap to that -- it's your best chance. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html |
LC Oscillator Questions
K7ITM wrote:
On Mar 19, 9:23 am, "Anthony Fremont" wrote: The waveform in a high Q tank that's lightly coupled to the amplifier should be very nearly sinusoidal. If in addition, the amplifier remains linear and represents a constant impedance over the whole cycle of the waveform, then the waveforms should everywhere be sinusoidal. If the amplifier+tank has barely enough loop gain to sustain oscillation, then clipping will be minimal, but it's also possible to detect the level and control the gain of the amplifier. You could, for example, use a light bulb like HP did in their original audio oscillator. Beware, though, that best oscillator performance in other regards may not be achieved the same way you achieve lowest harmonic distortion. Be careful that you optimize the right things for your application. After reading the other replies, it seems aparent that the shape of the signal from the first stage is not that critical, it is stability and phase noise that are most important. So, I should put things back where there is clipping to be sure that the oscillator oscillates and then clean up the signal in subsequent stages. In the work I do, I need to measure distortion, and the generators I use don't have low enough distortion in their outputs to be directly useful. The distortion levels in the "raw" outputs are generally about -40 to -50dBc. I use filters to make things better, and can get to -140dBc distortion levels fairly easily. If it's low harmonic distortion you want, I'd suggest that it may be better to just put a filter on the output of the oscillator that has only moderately low harmonic output, and not worry so much about that aspect of oscillator performance. Filters work well when the oscillator frequency range is about 1.5:1 or less. Much more than that and you'd need to switch in different filters depending on the oscillator frequency. Thanks. :-) |
LC Oscillator Questions
Helmut Sennewald wrote:
Hello Anthony, 1. Please set the following option to sitch off data reduction/compression in the result file.. .options plotwinsize=0 2. You have to set a small maximum timestep in the .TRAN line too. Maybe a value of 0.01*Period of oscillation if you hunt for very low distortion. Can you send me your file (.asc-file and model-file?) to check it? In alt.binaries.schematics.electronic I have posted the schematic, the asc-file and an oscilloscope screen shot from an actual circuit. Here is the asc-file contents: Version 4 SHEET 1 880 708 WIRE -704 -96 -784 -96 WIRE -400 -96 -704 -96 WIRE -224 -96 -400 -96 WIRE -704 -16 -704 -96 WIRE -400 -16 -400 -96 WIRE -224 32 -224 -96 WIRE -544 80 -592 80 WIRE -400 80 -400 64 WIRE -400 80 -464 80 WIRE -288 80 -400 80 WIRE -592 128 -592 80 WIRE -400 144 -400 80 WIRE -784 160 -784 -96 WIRE -400 240 -400 208 WIRE -224 240 -224 128 WIRE -224 240 -400 240 WIRE -80 240 -224 240 WIRE 48 240 -16 240 WIRE -784 272 -784 240 WIRE -704 272 -704 48 WIRE -592 272 -592 208 WIRE -400 272 -400 240 WIRE -224 288 -224 240 WIRE -592 384 -592 336 WIRE -400 384 -400 336 WIRE -400 384 -592 384 WIRE -224 384 -224 368 WIRE -224 384 -400 384 WIRE -400 448 -400 384 FLAG -784 272 0 FLAG -400 448 0 FLAG -704 272 0 FLAG 48 320 0 SYMBOL voltage -784 144 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 5.8 SYMBOL res -416 -32 R0 SYMATTR InstName R3 SYMATTR Value 100k SYMBOL npn -288 32 R0 SYMATTR InstName Q3 SYMATTR Value 2N3904 SYMBOL cap -416 144 R0 SYMATTR InstName C1 SYMATTR Value .01µ SYMBOL res -240 272 R0 SYMATTR InstName R7 SYMATTR Value 1k SYMBOL cap -416 272 R0 SYMATTR InstName C2 SYMATTR Value 500p SYMBOL ind -608 112 R0 SYMATTR InstName L1 SYMATTR Value 20µ SYMATTR SpiceLine Rser=.1 SYMBOL cap -608 272 R0 SYMATTR InstName C3 SYMATTR Value 200p SYMBOL cap -16 224 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C4 SYMATTR Value 270p SYMBOL res -448 64 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R1 SYMATTR Value .001 SYMBOL cap -720 -16 R0 SYMATTR InstName C5 SYMATTR Value 10µ SYMBOL res 32 224 R0 SYMATTR InstName R2 SYMATTR Value 10000k TEXT -792 360 Left 0 !.tran 50uS Thank you for your time. Best regards, Helmut "Anthony Fremont" schrieb im Newsbeitrag ... Pictures available in ABSE The top trace (yellow) is taken between C4 and R2. The bottom trace (cyan) is taken at the base of the transistor. There is a switchercad file, but the simulation will show allot of distortion that really isn't present in the prototype circuit, because of lots of circuit capactance I suspect. R1 was something I was playing with to try and tame the voltage across L1/C3 being applied to the base. Hello all, I was tinkering with this LC oscillator (Colpitts/Clapp) this weekend. I arrived at the values of C1 and C2 empirically after starting with a crystal oscillator circuit. The values in the original circuit created a horrid waveform that looked allot like the simulation. After much tinkering around and simulating, I come to the conclusion that getting a perfect waveform is nearly impossible, especially with big swing. It seems that the transistor likes to take a bite out of the right half of the peak of the wave. What is the secret to beautiful waveforms? Do I need another LC resonator on the output to fix it up? I mean, I'm getting a pretty nice wave now, but there is still some distortion that you can just see at the top of the peaks on the yellow trace. How do you control the peak voltages of an LC resonattor without mangling the waveform? The waveform at the junction of L1/C3 is of course quite beautiful, how do I get it from there to the output? ;-) I realize that I will need a buffer stage(s) before I can make any real use of the signal, but I want the input to the buffer to be as perfect as possible. Thanks :-) |
LC Oscillator Questions
"Anthony Fremont" schrieb im Newsbeitrag
... Helmut Sennewald wrote: Hello Anthony, 1. Please set the following option to sitch off data reduction/compression in the result file.. .options plotwinsize=0 2. You have to set a small maximum timestep in the .TRAN line too. Maybe a value of 0.01*Period of oscillation if you hunt for very low distortion. Can you send me your file (.asc-file and model-file?) to check it? In alt.binaries.schematics.electronic I have posted the schematic, the asc-file and an oscilloscope screen shot from an actual circuit. Here is the asc-file contents: Hello Anthony, The large capacitance of C1 (10nF) has caused an interrupted oscillation. Please change its value to 1000p and the oscillator will work as expected. I have also added MEASURE-commands to measure the frequency. View - SPICE Error Log Another method is using the FFT in the waveform viewer. Best regards, Helmut Save as "osc1.asc". Version 4 SHEET 1 880 708 WIRE -688 -96 -784 -96 WIRE -576 -96 -688 -96 WIRE -304 -96 -576 -96 WIRE -784 -64 -784 -96 WIRE -688 -64 -688 -96 WIRE -576 -16 -576 -96 WIRE -304 32 -304 -96 WIRE -784 48 -784 16 WIRE -688 48 -688 0 WIRE -576 80 -576 64 WIRE -480 80 -576 80 WIRE -432 80 -480 80 WIRE -368 80 -432 80 WIRE -576 128 -576 80 WIRE -432 144 -432 80 WIRE -576 240 -576 208 WIRE -432 240 -432 208 WIRE -304 240 -304 128 WIRE -304 240 -432 240 WIRE -240 240 -304 240 WIRE -160 240 -240 240 WIRE -64 240 -96 240 WIRE -32 240 -64 240 WIRE -576 272 -576 240 WIRE -432 272 -432 240 WIRE -32 272 -32 240 WIRE -304 288 -304 240 WIRE -32 368 -32 352 WIRE -576 384 -576 336 WIRE -432 384 -432 336 WIRE -432 384 -576 384 WIRE -304 384 -304 368 WIRE -304 384 -432 384 WIRE -432 416 -432 384 FLAG -784 48 0 FLAG -432 416 0 FLAG -688 48 0 FLAG -32 368 0 FLAG -64 240 out FLAG -240 240 e FLAG -480 80 b FLAG -576 240 lc SYMBOL voltage -784 -80 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 5.8 SYMBOL res -592 -32 R0 SYMATTR InstName R3 SYMATTR Value 100k SYMBOL npn -368 32 R0 SYMATTR InstName Q3 SYMATTR Value 2N3904 SYMBOL cap -448 144 R0 SYMATTR InstName C1 SYMATTR Value 1000p SYMBOL res -320 272 R0 SYMATTR InstName R7 SYMATTR Value 1k SYMBOL cap -448 272 R0 SYMATTR InstName C2 SYMATTR Value 500p SYMBOL ind -592 112 R0 WINDOW 39 36 108 Left 0 SYMATTR InstName L1 SYMATTR Value 20µ SYMATTR SpiceLine Rser=.1 SYMBOL cap -592 272 R0 SYMATTR InstName C3 SYMATTR Value 200p SYMBOL cap -96 224 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C4 SYMATTR Value 270p SYMBOL cap -704 -64 R0 SYMATTR InstName C5 SYMATTR Value 10µ SYMBOL res -48 256 R0 SYMATTR InstName R2 SYMATTR Value 100k TEXT -824 -152 Left 0 !.tran 0 200uS 0 4n TEXT -824 -184 Left 0 !.options plotwinsize=0 TEXT -816 472 Left 0 !.measure tran t1 FIND time WHEN V(out)=0 TD=90u RISE=1 TEXT -816 504 Left 0 !.measure tran t2 FIND time WHEN V(out)=0 TD=90u RISE=101 TEXT -816 536 Left 0 !.measure tran f0 PARAM 100/(t2-t1) TEXT -816 576 Left 0 ;View - SPICE Error Log \nfor the measured frequency TEXT -520 -184 Left 0 ;C1 changed to 1000p! |
LC Oscillator Questions
Tim Wescott wrote:
The secret to a beautiful waveform is -- you usually don't need it straight from the oscillator. Okay.. There are a lot of things that you want out of an LC oscillator. Low phase noise, frequency stability, consistently strong oscillation, pure tone, etc. Of these, the only two that you can't clean up later in the following amplifier chain is low phase noise and frequency stability. Concentrate on those, & don't sweat the nice waveform. Okay, that certainly explains why all the sample circuits I find don't expend any great effort at creaing a nice sine wave, and none at explaining why. What you say certainly makes sense, especially if there are no really negative consequences of having the oscillator make a "less than perfectly shaped" wave. Frequency stability and phase noise performance are often achieved by intentionally designing the amplifier so the active element operates in class C, without ever going into voltage saturation. This keeps it's drain (or collector) impedance high, yet delivers a large voltage swing to the gate (or base) to keep phase noise low. It also gives you a more or less consistent standing voltage in the tank, which helps the design of the following buffer stages. If you absolutely positively must tap the World's Most Beautiful Sine Wave off of the oscillator section, consider a parallel-tuned tank that's loosely coupled to the active element. Then loosely couple your output tap to that -- it's your best chance. Ok, thanks for the information. :-) I did allot of googling but found nothing that explained it like this. I was thinking of building a little single conversion superhet WWV receiver for 10MHz, if I continue with that I'll just concentrate on cleaning it up in another stage. Some material I read suggested keeping Xl of L1 at ~300Ohms, the series Xc (C3) at ~200Ohms and Xc of C1/C2 at 45Ohms. Do you have any thoughts on that? Right now I have way too much inductance for 3.5MHz by that theory, and judging from other circuits I've seen. 10uH seems to be the going thing for around 4MHz? |
LC Oscillator Questions
"Anthony Fremont" wrote in message ... K7ITM wrote: On Mar 19, 9:23 am, "Anthony Fremont" wrote: The waveform in a high Q tank that's lightly coupled to the amplifier should be very nearly sinusoidal. If in addition, the amplifier remains linear and represents a constant impedance over the whole cycle of the waveform, then the waveforms should everywhere be sinusoidal. If the amplifier+tank has barely enough loop gain to sustain oscillation, then clipping will be minimal, but it's also possible to detect the level and control the gain of the amplifier. You could, for example, use a light bulb like HP did in their original audio oscillator. Beware, though, that best oscillator performance in other regards may not be achieved the same way you achieve lowest harmonic distortion. Be careful that you optimize the right things for your application. After reading the other replies, it seems aparent that the shape of the signal from the first stage is not that critical, it is stability and phase noise that are most important. So, I should put things back where there is clipping to be sure that the oscillator oscillates and then clean up the signal in subsequent stages. I have never seen clipping. These things are supposed to limit in cutoff, not saturation. As the signal build up, the conduction angle gets smaller and smaller until the device runs out of gain. That is another way of saying that the DC value of the gate voltage gets more negative the bigger the amplitude. This works out automatically with a JFET. You need about 10K - 100K DC resistance from gate to ground. Using a bipolar transistor is not a good idea. Tam |
LC Oscillator Questions
Anthony Fremont wrote:
Tim Wescott wrote: The secret to a beautiful waveform is -- you usually don't need it straight from the oscillator. Okay.. There are a lot of things that you want out of an LC oscillator. Low phase noise, frequency stability, consistently strong oscillation, pure tone, etc. Of these, the only two that you can't clean up later in the following amplifier chain is low phase noise and frequency stability. Concentrate on those, & don't sweat the nice waveform. Okay, that certainly explains why all the sample circuits I find don't expend any great effort at creaing a nice sine wave, and none at explaining why. What you say certainly makes sense, especially if there are no really negative consequences of having the oscillator make a "less than perfectly shaped" wave. Frequency stability and phase noise performance are often achieved by intentionally designing the amplifier so the active element operates in class C, without ever going into voltage saturation. This keeps it's drain (or collector) impedance high, yet delivers a large voltage swing to the gate (or base) to keep phase noise low. It also gives you a more or less consistent standing voltage in the tank, which helps the design of the following buffer stages. If you absolutely positively must tap the World's Most Beautiful Sine Wave off of the oscillator section, consider a parallel-tuned tank that's loosely coupled to the active element. Then loosely couple your output tap to that -- it's your best chance. Ok, thanks for the information. :-) I did allot of googling but found nothing that explained it like this. I was thinking of building a little single conversion superhet WWV receiver for 10MHz, if I continue with that I'll just concentrate on cleaning it up in another stage. Some material I read suggested keeping Xl of L1 at ~300Ohms, the series Xc (C3) at ~200Ohms and Xc of C1/C2 at 45Ohms. Do you have any thoughts on that? Right now I have way too much inductance for 3.5MHz by that theory, and judging from other circuits I've seen. 10uH seems to be the going thing for around 4MHz? That sounds more or less right. With a Clapp oscillator the main tank is isolated by the series cap, so more of the energy is kept in the coil and C3, and less of it shows up in C1, C2, and the transistor. If you're driving a balanced mixer you want to have an LO signal that doesn't have much even-harmonic (2nd, 4th, etc.) energy in it, but for a casual receiver that's the least of your worries. Since you're operating at a fixed frequency it may be a good idea to just feed the oscillator output into a single-tuned resonant circuit to clean it up, then send it on to the mixer. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html |
LC Oscillator Questions
Tam/WB2TT wrote:
"Anthony Fremont" wrote in message I have never seen clipping. These things are supposed to limit in cutoff, not saturation. As the signal build up, the conduction angle gets smaller and smaller until the device runs out of gain. That is another way of saying that the DC value of the gate voltage gets more negative the bigger the amplitude. This works out automatically with a JFET. You need about 10K - 100K DC resistance from gate to ground. Using a bipolar transistor is not a good idea. I was wondering about the load that a bipolar would present. I will see if I can find a JFET in my junk pile, thank you. :-) |
LC Oscillator Questions
Anthony Fremont wrote:
Pictures available in ABSE The top trace (yellow) is taken between C4 and R2. The bottom trace (cyan) is taken at the base of the transistor. There is a switchercad file, but the simulation will show allot of distortion that really isn't present in the prototype circuit, because of lots of circuit capactance I suspect. R1 was something I was playing with to try and tame the voltage across L1/C3 being applied to the base. Hello all, I was tinkering with this LC oscillator (Colpitts/Clapp) this weekend. I arrived at the values of C1 and C2 empirically after starting with a crystal oscillator circuit. The values in the original circuit created a horrid waveform that looked allot like the simulation. After much tinkering around and simulating, I come to the conclusion that getting a perfect waveform is nearly impossible, especially with big swing. It seems that the transistor likes to take a bite out of the right half of the peak of the wave. What is the secret to beautiful waveforms? Do I need another LC resonator on the output to fix it up? I mean, I'm getting a pretty nice wave now, but there is still some distortion that you can just see at the top of the peaks on the yellow trace. How do you control the peak voltages of an LC resonattor without mangling the waveform? The waveform at the junction of L1/C3 is of course quite beautiful, how do I get it from there to the output? ;-) I realize that I will need a buffer stage(s) before I can make any real use of the signal, but I want the input to the buffer to be as perfect as possible. Thanks :-) In some LC oscillators, the amplitude of the oscillation is controlled by a feedback loop. For example, a rectifier can be used to create a DC voltage proportional to the oscillation amplitude on the LC tank, and then an op-amp can be used to compare the rectifier output signal to a reference voltage. The output from the op-amp can be filtered and then used to control the current in the oscillator core. It is difficult to do all of this in a way that keeps the phase noise low, but given the right simulation tools (e.g. SpectreRF which is rather expensive), good results can be obtained. In particular, a well-defined oscillation amplitude can help to keep the KVCO well controlled, which is useful in PLLs. Chris |
LC Oscillator Questions
Helmut Sennewald wrote:
"Anthony Fremont" schrieb im Newsbeitrag ... Helmut Sennewald wrote: Hello Anthony, 1. Please set the following option to sitch off data reduction/compression in the result file.. .options plotwinsize=0 2. You have to set a small maximum timestep in the .TRAN line too. Maybe a value of 0.01*Period of oscillation if you hunt for very low distortion. Can you send me your file (.asc-file and model-file?) to check it? In alt.binaries.schematics.electronic I have posted the schematic, the asc-file and an oscilloscope screen shot from an actual circuit. Here is the asc-file contents: Hello Anthony, The large capacitance of C1 (10nF) has caused an interrupted oscillation. Please change its value to 1000p and the oscillator will work as expected. I have also added MEASURE-commands to measure the frequency. View - SPICE Error Log Another method is using the FFT in the waveform viewer. Best regards, Helmut Save as "osc1.asc". Version 4 SHEET 1 880 708 WIRE -688 -96 -784 -96 WIRE -576 -96 -688 -96 WIRE -304 -96 -576 -96 WIRE -784 -64 -784 -96 WIRE -688 -64 -688 -96 WIRE -576 -16 -576 -96 WIRE -304 32 -304 -96 WIRE -784 48 -784 16 WIRE -688 48 -688 0 WIRE -576 80 -576 64 WIRE -480 80 -576 80 WIRE -432 80 -480 80 WIRE -368 80 -432 80 WIRE -576 128 -576 80 WIRE -432 144 -432 80 WIRE -576 240 -576 208 WIRE -432 240 -432 208 WIRE -304 240 -304 128 WIRE -304 240 -432 240 WIRE -240 240 -304 240 WIRE -160 240 -240 240 WIRE -64 240 -96 240 WIRE -32 240 -64 240 WIRE -576 272 -576 240 WIRE -432 272 -432 240 WIRE -32 272 -32 240 WIRE -304 288 -304 240 WIRE -32 368 -32 352 WIRE -576 384 -576 336 WIRE -432 384 -432 336 WIRE -432 384 -576 384 WIRE -304 384 -304 368 WIRE -304 384 -432 384 WIRE -432 416 -432 384 FLAG -784 48 0 FLAG -432 416 0 FLAG -688 48 0 FLAG -32 368 0 FLAG -64 240 out FLAG -240 240 e FLAG -480 80 b FLAG -576 240 lc SYMBOL voltage -784 -80 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 5.8 SYMBOL res -592 -32 R0 SYMATTR InstName R3 SYMATTR Value 100k SYMBOL npn -368 32 R0 SYMATTR InstName Q3 SYMATTR Value 2N3904 SYMBOL cap -448 144 R0 SYMATTR InstName C1 SYMATTR Value 1000p SYMBOL res -320 272 R0 SYMATTR InstName R7 SYMATTR Value 1k SYMBOL cap -448 272 R0 SYMATTR InstName C2 SYMATTR Value 500p SYMBOL ind -592 112 R0 WINDOW 39 36 108 Left 0 SYMATTR InstName L1 SYMATTR Value 20µ SYMATTR SpiceLine Rser=.1 SYMBOL cap -592 272 R0 SYMATTR InstName C3 SYMATTR Value 200p SYMBOL cap -96 224 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C4 SYMATTR Value 270p SYMBOL cap -704 -64 R0 SYMATTR InstName C5 SYMATTR Value 10µ SYMBOL res -48 256 R0 SYMATTR InstName R2 SYMATTR Value 100k TEXT -824 -152 Left 0 !.tran 0 200uS 0 4n TEXT -824 -184 Left 0 !.options plotwinsize=0 TEXT -816 472 Left 0 !.measure tran t1 FIND time WHEN V(out)=0 TD=90u RISE=1 TEXT -816 504 Left 0 !.measure tran t2 FIND time WHEN V(out)=0 TD=90u RISE=101 TEXT -816 536 Left 0 !.measure tran f0 PARAM 100/(t2-t1) TEXT -816 576 Left 0 ;View - SPICE Error Log \nfor the measured frequency TEXT -520 -184 Left 0 ;C1 changed to 1000p! Thank you very much. :-) I have now switched to using an MPF102 JFET instead of the bipolar and much less capacitance for C1 (now 470pF). I only get a 2V peak to peak signal out now, but it's quite nice looking. |
LC Oscillator Questions
Tim Wescott wrote:
Anthony Fremont wrote: Some material I read suggested keeping Xl of L1 at ~300Ohms, the series Xc (C3) at ~200Ohms and Xc of C1/C2 at 45Ohms. Do you have any thoughts on that? Right now I have way too much inductance for 3.5MHz by that theory, and judging from other circuits I've seen. 10uH seems to be the going thing for around 4MHz? That sounds more or less right. With a Clapp oscillator the main tank is isolated by the series cap, so more of the energy is kept in the coil and C3, and less of it shows up in C1, C2, and the transistor. If you're driving a balanced mixer you want to have an LO signal that doesn't have much even-harmonic (2nd, 4th, etc.) energy in it, but for a casual receiver that's the least of your worries. Since you're operating at a fixed frequency it may be a good idea to just feed the oscillator output into a single-tuned resonant circuit to clean it up, then send it on to the mixer. Ok, I've now put in an MPF102 and changed R3 to a pull-down. I lowered C1 to 470pF and I get a nifty 2V p-p sine wave on the output. It really tamed the tank circuit voltage down as well. Which brings up a question, with the tank now completely DC blocked from Vcc and Vss, where does it get it's energy. I assume that it must come thru the gate. How does that happen? :-? My circuit is much like Figure 1 here, without the diode though: http://www.electronics-tutorials.com...scillators.htm |
LC Oscillator Questions
"Anthony Fremont" wrote in message ... Pictures available in ABSE The top trace (yellow) is taken between C4 and R2. The bottom trace (cyan) is taken at the base of the transistor. There is a switchercad file, but the simulation will show allot of distortion that really isn't present in the prototype circuit, because of lots of circuit capactance I suspect. R1 was something I was playing with to try and tame the voltage across L1/C3 being applied to the base. Hello all, I was tinkering with this LC oscillator (Colpitts/Clapp) this weekend. I arrived at the values of C1 and C2 empirically after starting with a crystal oscillator circuit. The values in the original circuit created a horrid waveform that looked allot like the simulation. After much tinkering around and simulating, I come to the conclusion that getting a perfect waveform is nearly impossible, especially with big swing. It seems that the transistor likes to take a bite out of the right half of the peak of the wave. What is the secret to beautiful waveforms? Do I need another LC resonator on the output to fix it up? I mean, I'm getting a pretty nice wave now, but there is still some distortion that you can just see at the top of the peaks on the yellow trace. How do you control the peak voltages of an LC resonattor without mangling the waveform? The waveform at the junction of L1/C3 is of course quite beautiful, how do I get it from there to the output? ;-) I realize that I will need a buffer stage(s) before I can make any real use of the signal, but I want the input to the buffer to be as perfect as possible. Thanks :-) The prettiest waveforms come from balanced oscillators. Distortion then turns up as 3rd 5th 7th etc harmonics which are far less ugly than the 2nd 3rd 4th 5th etc generated by the single ended types. Balanced ALC is also easier and more effective. My own experience says that 'prettier' is better. Those oscillators offering gross distorted outputs also seem to suffer badly in other areas and gross distortion always causes problems further down the line. Procuring good quality is a classic black art, one aspect is to allow the LC just an occasional vague glimpse of the maintaining amplifier. Another is to cause limiting by use of an amp having a gentle gain change (eg Fet v bipolar) and the other is ALC. (Or all three together). Failing that, there is always the cop-out of an output filter :) john -- Posted via a free Usenet account from http://www.teranews.com |
LC Oscillator Questions
Anthony Fremont wrote:
Tim Wescott wrote: Anthony Fremont wrote: Some material I read suggested keeping Xl of L1 at ~300Ohms, the series Xc (C3) at ~200Ohms and Xc of C1/C2 at 45Ohms. Do you have any thoughts on that? Right now I have way too much inductance for 3.5MHz by that theory, and judging from other circuits I've seen. 10uH seems to be the going thing for around 4MHz? That sounds more or less right. With a Clapp oscillator the main tank is isolated by the series cap, so more of the energy is kept in the coil and C3, and less of it shows up in C1, C2, and the transistor. If you're driving a balanced mixer you want to have an LO signal that doesn't have much even-harmonic (2nd, 4th, etc.) energy in it, but for a casual receiver that's the least of your worries. Since you're operating at a fixed frequency it may be a good idea to just feed the oscillator output into a single-tuned resonant circuit to clean it up, then send it on to the mixer. Ok, I've now put in an MPF102 and changed R3 to a pull-down. I lowered C1 to 470pF and I get a nifty 2V p-p sine wave on the output. It really tamed the tank circuit voltage down as well. Which brings up a question, with the tank now completely DC blocked from Vcc and Vss, where does it get it's energy. I assume that it must come thru the gate. How does that happen? :-? My circuit is much like Figure 1 here, without the diode though: http://www.electronics-tutorials.com...scillators.htm It comes from the source, through the coupling capacitors -- Cfb-a and Cfb-b in your link. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html |
LC Oscillator Questions
Tam/WB2TT wrote:
I have never seen clipping. These things are supposed to limit in cutoff, not saturation. As the signal build up, the conduction angle gets smaller and smaller until the device runs out of gain. That is another way of saying that the DC value of the gate voltage gets more negative the bigger the amplitude. This works out automatically with a JFET. You need about 10K - 100K DC resistance from gate to ground. Using a bipolar transistor is not a good idea. I have now changed it to an MPF102 that I've had laying around for many years. It works great, thanks. :-) |
LC Oscillator Questions
"Anthony Fremont" wrote in message ... Tam/WB2TT wrote: I have never seen clipping. These things are supposed to limit in cutoff, not saturation. As the signal build up, the conduction angle gets smaller and smaller until the device runs out of gain. That is another way of saying that the DC value of the gate voltage gets more negative the bigger the amplitude. This works out automatically with a JFET. You need about 10K - 100K DC resistance from gate to ground. Using a bipolar transistor is not a good idea. I have now changed it to an MPF102 that I've had laying around for many years. It works great, thanks. :-) Glad it worked out. By the way the feedback path is through the capacitive network between source and gate. That would be more obvious in the configuration that uses a tapped inductor, but works the same way. Leaving out the diode was the right thing to do; it just adds to the noise. I don't know what kind of stability and linearity you need, but if that is important, do not use the common type of ceramic capacitors that are meant for bypassing. They are lossy, and their value varies with applied voltage. Use mica, NPO ceramic, or Mylar and similar for larger values. Tam |
LC Oscillator Questions
On Mon, 19 Mar 2007 16:25:26 -0500, "Anthony Fremont"
wrote: Tam/WB2TT wrote: "Anthony Fremont" wrote in message I have never seen clipping. These things are supposed to limit in cutoff, not saturation. As the signal build up, the conduction angle gets smaller and smaller until the device runs out of gain. That is another way of saying that the DC value of the gate voltage gets more negative the bigger the amplitude. This works out automatically with a JFET. You need about 10K - 100K DC resistance from gate to ground. Using a bipolar transistor is not a good idea. I was wondering about the load that a bipolar would present. I will see if I can find a JFET in my junk pile, thank you. :-) Without going to the purity levels that Tom requires, I've always found that bipolars can be used to produce a fairly reasonable "visibly sinusoidal" (see note) waveform. Follow the oscillator with an amplifier stage which drives a limiter/clipper, and use that to control a gain element in the oscillator. It's like the incandescent non-linearity arrangement except the oscillator stage waveform remains fairly clean. (Note: Harmonic distortion not readily discernible on a CRO) |
LC Oscillator Questions
Tam/WB2TT wrote:
"Anthony Fremont" wrote in message I have now changed it to an MPF102 that I've had laying around for many years. It works great, thanks. :-) Glad it worked out. By the way the feedback path is through the capacitive network between source and gate. That would be more obvious in the configuration that uses a tapped inductor, but works I had to play around with this quite a bit to get it running near 10MHz. It seemed to be more picky about the cap ratios than their actual values. the same way. Leaving out the diode was the right thing to do; it just adds to the noise. I don't know what kind of stability and I figured that it would cause horrid clipping the way it was installed. I thought maybe the designer intended it to be the other way around to protect the transistor from reverse voltages on the gate, hmm.... always something to keep you guessing. ;-) linearity you need, but if that is important, do not use the common type of ceramic capacitors that are meant for bypassing. They are lossy, and their value varies with applied voltage. Use mica, NPO ceramic, or Mylar and similar for larger values. I don't really "need" anything in particular, it's just an exercise to try and learn something. I just used the ceramic caps because they were handy. I didn't have a pile of NPO caps laying around. ;-) I d Time to add a buffer now since the scope probes load it down so badly now. I'm getting 1.1Vpp into the apparent 5M load of two scope probes. It's probably pulled a ton off frequency as well. Let's see.....(removes one probe).....yep, 90KHz rise in frequency after taking one probe off. |
LC Oscillator Questions
On Mar 19, 12:23 pm, "Anthony Fremont" wrote:
Pictures available in ABSE The top trace (yellow) is taken between C4 and R2. The bottom trace (cyan) is taken at the base of the transistor. There is a switchercad file, but the simulation will show allot of distortion that really isn't present in the prototype circuit, because of lots of circuit capactance I suspect. R1 was something I was playing with to try and tame the voltage across L1/C3 being applied to the base. Hello all, I was tinkering with this LC oscillator (Colpitts/Clapp) this weekend. I arrived at the values of C1 and C2 empirically after starting with a crystal oscillator circuit. The values in the original circuit created a horrid waveform that looked allot like the simulation. After much tinkering around and simulating, I come to the conclusion that getting a perfect waveform is nearly impossible, especially with big swing. It seems that the transistor likes to take a bite out of the right half of the peak of the wave. What is the secret to beautiful waveforms? Do I need another LC resonator on the output to fix it up? I mean, I'm getting a pretty nice wave now, but there is still some distortion that you can just see at the top of the peaks on the yellow trace. How do you control the peak voltages of an LC resonattor without mangling the waveform? The waveform at the junction of L1/C3 is of course quite beautiful, how do I get it from there to the output? ;-) Oscillators have to have gain greater than one at the frequency of oscillation. When turned on, the amplitude builds up until something in the circuit cuts back the gain. In simple oscillators, that "something that cuts back the gain" is almost always the active device saturating and distorting its output. The higher your gain, the more reliable the oscillator starting up, but also the higher the distortion. If you take the output not from the output of the active device, but from a lightly-coupled tank, then you'll see something much more like the sine wave you were expecting. This is what you see at the L1/C3 junction. But still you'll get lower distortion there if the active device isn't driven so far into saturation/distortion. And by definition you cannot suck much power out of the L1/C3 junction without decreasing the Q of the tank and making distortion there too. You can add a few more active devices and not only buffer things but also put a fairly linear AGC in the loop. This still has distortion, but this is done intentionally in a rectifier to derive the AGC control voltage, which is then filtered. The intentional distortion does not have to appear in the output! Clever use of devices can make the AGC loop quite beautiful. Look at the Wien Bridge or Meacham Bridge oscillators that use a light bulb in the bridge to not only be the loop-control device but also do filtering (thermal time constant of the filament). Tim. |
LC Oscillator Questions
john jardine wrote:
The prettiest waveforms come from balanced oscillators. Distortion then turns up as 3rd 5th 7th etc harmonics which are far less ugly than the 2nd 3rd 4th 5th etc generated by the single ended types. Balanced ALC is also easier and more effective. The "conversation" that L1 and C3 sure looks nice on the scope. :-) My own experience says that 'prettier' is better. Those oscillators offering gross distorted outputs also seem to suffer badly in other areas and gross distortion always causes problems further down the line. Procuring good quality is a classic black art, one aspect is to allow the LC just an occasional vague glimpse of the maintaining amplifier. Another is to cause limiting by use of an amp having a gentle gain change (eg Fet v bipolar) and the other is ALC. (Or all three together). Well there sure isn't much talk about it out there. Material I find is like, "here's a schematic, pick a coil and cap and your done. No one seems to care what the result looks like. Seems like you can make a reasonably decent wave _and_ still have the oscillator start reliably. Failing that, there is always the cop-out of an output filter :) Seems to be the way people like to do it. ;-) john |
LC Oscillator Questions
Tim Shoppa wrote:
Oscillators have to have gain greater than one at the frequency of oscillation. When turned on, the amplitude builds up until something in the circuit cuts back the gain. In simple oscillators, that "something that cuts back the gain" is almost always the active device saturating and distorting its output. The higher your gain, the more reliable the oscillator starting up, but also the higher the distortion. If you take the output not from the output of the active device, but from a lightly-coupled tank, then you'll see something much more like the sine wave you were expecting. This is what you see at the L1/C3 junction. But still you'll get lower distortion there if the active device isn't driven so far into saturation/distortion. And by definition you cannot suck much power out of the L1/C3 junction without decreasing the Q of the tank and making distortion there too. It seams reasonable that if I can look at the junction with a scope and the wave looks good, I should be able to tap it with a secondary JFET without destroying it. Yet I see no examples of that being done. I guess it's just easier to accomplish the waveform repair by using a tank on the output of the oscillator and not loading down the primary tank circuit. You can add a few more active devices and not only buffer things but also put a fairly linear AGC in the loop. This still has distortion, but this is done intentionally in a rectifier to derive the AGC control voltage, which is then filtered. The intentional distortion does not have to appear in the output! This sounds like what Chris Jones was talking about. Do you have a link so I could check it out? Clever use of devices can make the AGC loop quite beautiful. Look at the Wien Bridge or Meacham Bridge oscillators that use a light bulb in the bridge to not only be the loop-control device but also do filtering (thermal time constant of the filament). Clever stuff. :-) |
LC Oscillator Questions
In message , Anthony Fremont
writes [snip] What is the secret to beautiful waveforms? Do I need another LC resonator on the output to fix it up? I mean, I'm getting a pretty nice wave now, but there is still some distortion that you can just see at the top of the peaks on the yellow trace. Symmetry is a nice word to use where LC resonance is concerned. Symmetry for the drive and clipping mechanisms. Have a look at a long-tail transistor pair with cross-coupled collector-base feedback resistors, and collectors driving a centre-tapped LC resonant circuit. -- Tony Williams |
LC Oscillator Questions
On Mar 20, 7:43 pm, "Anthony Fremont" wrote:
You can add a few more active devices and not only buffer things but also put a fairly linear AGC in the loop. This still has distortion, but this is done intentionally in a rectifier to derive the AGC control voltage, which is then filtered. The intentional distortion does not have to appear in the output! This sounds like what Chris Jones was talking about. Do you have a link so I could check it out? A "classic" oscillator with AGC is the Sulzer Oscillator. Very nice pics and schematics at http://leapsecond.com/museum/sul25-1/ Clever use of devices can make the AGC loop quite beautiful. Look at the Wien Bridge or Meacham Bridge oscillators that use a light bulb in the bridge to not only be the loop-control device but also do filtering (thermal time constant of the filament). Clever stuff. :-) For really nitty-gritty stuff about low-distortion oscillators, see Jim Williams' examples in Linear Technologies appnote AN-43. (Go to http://www.linear.com/ and do a search for "AN43" without the hyphen). Most of the examples there are bridges, and distortions in the sub-0.1% category are achieved AND THEN IMPROVED UPON BY FACTORS OF HUNDREDS! Also google "Meacham Bridge" and "Wien Bridge". Tim. |
LC Oscillator Questions
On Mar 20, 7:43 pm, "Anthony Fremont" wrote:
This sounds like what Chris Jones was talking about. Do you have a link so I could check it out? In addition to the low-noise-low-distortion-high-stability URL's I pointed you towards in my other followup, recent ARRL Handbooks have some really clever low-noise VFO circuits using a multitude of approaches, including explicit AGC circuitry. I have been slowly working my way through the cookbook examples and every example has its merits. In typical ham use, for better or worse, stability and reliability to start-up are often the most important criteria. What you are complaining about when you see a distorted output, is something that is actually a design goal of oscillators that are followed by multipliers. One very common method over the years of decoupling the frequency- determining tuned circuits from other frequencies generated in a radio is to run the oscillator grid tank at half the output frequency and depend on distorition to make the desired output frequency. In the simplest case a balanced or push-pull oscillator is a "No-No" because you WANT the second harmonic. The electron-coupled oscillator that was in the 50's/60's/early 70's handbooks is a classic design. Tim. |
LC Oscillator Questions
On Mar 20, 4:32 pm, "Anthony Fremont" wrote:
Well there sure isn't much talk about it out there. Material I find is like, "here's a schematic, pick a coil and cap and your done. No one seems to care what the result looks like. Seems like you can make a reasonably decent wave _and_ still have the oscillator start reliably. Some people worry a whole lot about it. What they worry about, typically, is phase noise first and stability second. Lack of harmonics in the waveform are generally lower on the list. That's because filtering out harmonics is relatively easy, compared with cleaning up phase noise and stability problems. You can find lots of articles on minimizing phase noise, but there's also quite a bit of trade-secret sorts of knowledge whose owners aren't particularly interested in sharing, understandably. It's far from trivial to get the phase noise and spurious performance you'll find in the good commercial signal generators. Good as they are, though, I know of none that's good enough to be used without harmonic filtering to do low-level harmonic distortion measurements. Cheers, Tom |
| All times are GMT +1. The time now is 07:16 PM. |
Powered by vBulletin® Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
RadioBanter.com