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Oscillating 3rd overtone XTAL at fundamental?
Hi.
I have spent part of the weekend trying to resonate al old CB XTAL at its fundamental frequency. The XTAL is labelled 27.125 MHz, with a fundamental of about 9.041 MHz, which falls into 18m HAM band when multiplied by two. I assume 27MHz XTALs are 3rd overtone. Since the circuitry is/will be digital, the oscillator is one gate of a 74HC14. 470ohm resistor from gate output to the parallel of 1Mohm//XTAL. 10pF and 33pF (from the junkbox) capacitors to ground on each side of the XTAL. No oscillation (I have no oscilloscope, I use my HF receiver to troubleshoot oscillators at known frequencies). If I replace the XTAL with a 10.000 MHz rock the oscillation is loud and clear. I have never built something with an overtone XTAL. I know that I need an output resonating circuit if I want to extract the 3rd harmonic. Do I need the same if I want the fundamental? Are overtone XTALs "harder" to resonate? Are those old CB XTALs 3rd overtone? I assume my 27MHz XTAL works. :-) Thanks in advance for all suggestions, Paolo IK1ZYW |
PaoloC ) writes:
Hi. I have spent part of the weekend trying to resonate al old CB XTAL at its fundamental frequency. The XTAL is labelled 27.125 MHz, with a fundamental of about 9.041 MHz, which falls into 18m HAM band when multiplied by two. I assume 27MHz XTALs are 3rd overtone. Since the circuitry is/will be digital, the oscillator is one gate of a 74HC14. 470ohm resistor from gate output to the parallel of 1Mohm//XTAL. 10pF and 33pF (from the junkbox) capacitors to ground on each side of the XTAL. No oscillation (I have no oscilloscope, I use my HF receiver to troubleshoot oscillators at known frequencies). If I replace the XTAL with a 10.000 MHz rock the oscillation is loud and clear. I have never built something with an overtone XTAL. I know that I need an output resonating circuit if I want to extract the 3rd harmonic. Do I need the same if I want the fundamental? Are overtone XTALs "harder" to resonate? Are those old CB XTALs 3rd overtone? I assume my 27MHz XTAL works. :-) Thanks in advance for all suggestions, Paolo IK1ZYW Could the crystal be a receive crystal? CB crystals tended to show the channel number or the frequency of the channel, and so if you simply looked at the marked frequency, it would not tell you if it's for transmit or receive. Though the ones I've seen did mark them with "R" or "T". The point is that if it's a receive crystal, it wouldn't be 1/3 of 27.125 but 27.125-IF and then divided by three. I'd say there shouldn't be any problem getting an overtone crystal to oscillate on the fundamental; if there's any problem it's getting an overtone crystal to oscillate on its overtone frequency. Of course, they usually oscillate at a slightly different frequency, so maybe you need to tune around a bit. Or maybe the circuit values are wrong for that crystal or frequency. Or the crystal is indeed dead. Michael VE2BVW |
PaoloC ) writes:
Hi. I have spent part of the weekend trying to resonate al old CB XTAL at its fundamental frequency. The XTAL is labelled 27.125 MHz, with a fundamental of about 9.041 MHz, which falls into 18m HAM band when multiplied by two. I assume 27MHz XTALs are 3rd overtone. Since the circuitry is/will be digital, the oscillator is one gate of a 74HC14. 470ohm resistor from gate output to the parallel of 1Mohm//XTAL. 10pF and 33pF (from the junkbox) capacitors to ground on each side of the XTAL. No oscillation (I have no oscilloscope, I use my HF receiver to troubleshoot oscillators at known frequencies). If I replace the XTAL with a 10.000 MHz rock the oscillation is loud and clear. I have never built something with an overtone XTAL. I know that I need an output resonating circuit if I want to extract the 3rd harmonic. Do I need the same if I want the fundamental? Are overtone XTALs "harder" to resonate? Are those old CB XTALs 3rd overtone? I assume my 27MHz XTAL works. :-) Thanks in advance for all suggestions, Paolo IK1ZYW Could the crystal be a receive crystal? CB crystals tended to show the channel number or the frequency of the channel, and so if you simply looked at the marked frequency, it would not tell you if it's for transmit or receive. Though the ones I've seen did mark them with "R" or "T". The point is that if it's a receive crystal, it wouldn't be 1/3 of 27.125 but 27.125-IF and then divided by three. I'd say there shouldn't be any problem getting an overtone crystal to oscillate on the fundamental; if there's any problem it's getting an overtone crystal to oscillate on its overtone frequency. Of course, they usually oscillate at a slightly different frequency, so maybe you need to tune around a bit. Or maybe the circuit values are wrong for that crystal or frequency. Or the crystal is indeed dead. Michael VE2BVW |
PaoloC wrote in message ...
Hi. I have spent part of the weekend trying to resonate al old CB XTAL at its fundamental frequency. need the same if I want the fundamental? Are overtone XTALs "harder" to resonate? Are those old CB XTALs 3rd overtone? I assume my 27MHz XTAL works. :-) Thanks in advance for all suggestions, Paolo IK1ZYW Yup They are 3rd overtone. Try a little series inductance with the rock, and maybe a few pf of capacitance in parallel. I seem to remember these being cut for a 25 pf load versus the more common 330 or 32 pf load of fundamental rocks, but the rock may also be defective (I've seen quite a few) Good luck, Mike. |
PaoloC wrote in message ...
Hi. I have spent part of the weekend trying to resonate al old CB XTAL at its fundamental frequency. need the same if I want the fundamental? Are overtone XTALs "harder" to resonate? Are those old CB XTALs 3rd overtone? I assume my 27MHz XTAL works. :-) Thanks in advance for all suggestions, Paolo IK1ZYW Yup They are 3rd overtone. Try a little series inductance with the rock, and maybe a few pf of capacitance in parallel. I seem to remember these being cut for a 25 pf load versus the more common 330 or 32 pf load of fundamental rocks, but the rock may also be defective (I've seen quite a few) Good luck, Mike. |
Yup, they are 3rd. Try a little series inductance or parallel
capacitance. A few pf of cap. may do the trick. Basically these were usually cut for 25 pf load versus the usual 32 or so of fundamental rocks. BTW the fundamental will not be at exactly one third of the overtone freq. (I don't remember why) You may very well have a bad xtal too, it happens quite often. Good Luck, Mike |
Yup, they are 3rd. Try a little series inductance or parallel
capacitance. A few pf of cap. may do the trick. Basically these were usually cut for 25 pf load versus the usual 32 or so of fundamental rocks. BTW the fundamental will not be at exactly one third of the overtone freq. (I don't remember why) You may very well have a bad xtal too, it happens quite often. Good Luck, Mike |
Can't quite tell from your description what your circuit is, but if I recall
correctly back when i was doing hardware design a crystal oscillator using logic gates required two gates. I used to use three---two for the oscillator and the third for a buffer. The two inverters where tied together with a capacitor between them. Then the crystal was placed from the input of the first gate to the output of the second. The output of the second also went to the buffer stage. !-----------------crystal------------| Like this: - gate one---capacitor---gate two---gate three---circuit output As I recall, there was also a resistor from the input of gate one to ground, and from the output of gate two to ground. I do not recall the capacitor or resistor values, but when properly built this circuit never failed to oscillate with any crystal within the range of the gates (fundamental mode only). Jim N8EE "PaoloC" wrote in message ... Hi. I have spent part of the weekend trying to resonate al old CB XTAL at its fundamental frequency. The XTAL is labelled 27.125 MHz, with a fundamental of about 9.041 MHz, which falls into 18m HAM band when multiplied by two. I assume 27MHz XTALs are 3rd overtone. Since the circuitry is/will be digital, the oscillator is one gate of a 74HC14. 470ohm resistor from gate output to the parallel of 1Mohm//XTAL. 10pF and 33pF (from the junkbox) capacitors to ground on each side of the XTAL. No oscillation (I have no oscilloscope, I use my HF receiver to troubleshoot oscillators at known frequencies). If I replace the XTAL with a 10.000 MHz rock the oscillation is loud and clear. I have never built something with an overtone XTAL. I know that I need an output resonating circuit if I want to extract the 3rd harmonic. Do I need the same if I want the fundamental? Are overtone XTALs "harder" to resonate? Are those old CB XTALs 3rd overtone? I assume my 27MHz XTAL works. :-) Thanks in advance for all suggestions, Paolo IK1ZYW |
Can't quite tell from your description what your circuit is, but if I recall
correctly back when i was doing hardware design a crystal oscillator using logic gates required two gates. I used to use three---two for the oscillator and the third for a buffer. The two inverters where tied together with a capacitor between them. Then the crystal was placed from the input of the first gate to the output of the second. The output of the second also went to the buffer stage. !-----------------crystal------------| Like this: - gate one---capacitor---gate two---gate three---circuit output As I recall, there was also a resistor from the input of gate one to ground, and from the output of gate two to ground. I do not recall the capacitor or resistor values, but when properly built this circuit never failed to oscillate with any crystal within the range of the gates (fundamental mode only). Jim N8EE "PaoloC" wrote in message ... Hi. I have spent part of the weekend trying to resonate al old CB XTAL at its fundamental frequency. The XTAL is labelled 27.125 MHz, with a fundamental of about 9.041 MHz, which falls into 18m HAM band when multiplied by two. I assume 27MHz XTALs are 3rd overtone. Since the circuitry is/will be digital, the oscillator is one gate of a 74HC14. 470ohm resistor from gate output to the parallel of 1Mohm//XTAL. 10pF and 33pF (from the junkbox) capacitors to ground on each side of the XTAL. No oscillation (I have no oscilloscope, I use my HF receiver to troubleshoot oscillators at known frequencies). If I replace the XTAL with a 10.000 MHz rock the oscillation is loud and clear. I have never built something with an overtone XTAL. I know that I need an output resonating circuit if I want to extract the 3rd harmonic. Do I need the same if I want the fundamental? Are overtone XTALs "harder" to resonate? Are those old CB XTALs 3rd overtone? I assume my 27MHz XTAL works. :-) Thanks in advance for all suggestions, Paolo IK1ZYW |
"JLB" ) writes:
Can't quite tell from your description what your circuit is, but if I recall correctly back when i was doing hardware design a crystal oscillator using logic gates required two gates. I used to use three---two for the oscillator and the third for a buffer. The two inverters where tied together with a capacitor between them. Then the crystal was placed from the input of the first gate to the output of the second. The output of the second also went to the buffer stage. !-----------------crystal------------| Like this: - gate one---capacitor---gate two---gate three---circuit output As I recall, there was also a resistor from the input of gate one to ground, and from the output of gate two to ground. I do not recall the capacitor or resistor values, but when properly built this circuit never failed to oscillate with any crystal within the range of the gates (fundamental mode only). Jim N8EE That was a standard oscillator with TTL. I used a 7400, and 470ohm resistors from input to output of each gate. No coupling capacitor was needed, though there was a low value cap in series with the crystal for load capacitance. (The resistors linearized the gates.) There were obviously variations on the theme, and your suggestion was one of them. And as you say, these tended to oscillate with most crystals. I had one built up as a module, so I'd have a general purpose oscillator read, to check crystals or as a signal source. But, for some reason, when they moved to CMOS oscillators, the norm became one gate, with a feedback resistor and a pair of capacitors, one from input to ground, and the other from output to ground. Michael VE2BVW "PaoloC" wrote in message ... Hi. I have spent part of the weekend trying to resonate al old CB XTAL at its fundamental frequency. The XTAL is labelled 27.125 MHz, with a fundamental of about 9.041 MHz, which falls into 18m HAM band when multiplied by two. I assume 27MHz XTALs are 3rd overtone. Since the circuitry is/will be digital, the oscillator is one gate of a 74HC14. 470ohm resistor from gate output to the parallel of 1Mohm//XTAL. 10pF and 33pF (from the junkbox) capacitors to ground on each side of the XTAL. No oscillation (I have no oscilloscope, I use my HF receiver to troubleshoot oscillators at known frequencies). If I replace the XTAL with a 10.000 MHz rock the oscillation is loud and clear. I have never built something with an overtone XTAL. I know that I need an output resonating circuit if I want to extract the 3rd harmonic. Do I need the same if I want the fundamental? Are overtone XTALs "harder" to resonate? Are those old CB XTALs 3rd overtone? I assume my 27MHz XTAL works. :-) Thanks in advance for all suggestions, Paolo IK1ZYW |
"JLB" ) writes:
Can't quite tell from your description what your circuit is, but if I recall correctly back when i was doing hardware design a crystal oscillator using logic gates required two gates. I used to use three---two for the oscillator and the third for a buffer. The two inverters where tied together with a capacitor between them. Then the crystal was placed from the input of the first gate to the output of the second. The output of the second also went to the buffer stage. !-----------------crystal------------| Like this: - gate one---capacitor---gate two---gate three---circuit output As I recall, there was also a resistor from the input of gate one to ground, and from the output of gate two to ground. I do not recall the capacitor or resistor values, but when properly built this circuit never failed to oscillate with any crystal within the range of the gates (fundamental mode only). Jim N8EE That was a standard oscillator with TTL. I used a 7400, and 470ohm resistors from input to output of each gate. No coupling capacitor was needed, though there was a low value cap in series with the crystal for load capacitance. (The resistors linearized the gates.) There were obviously variations on the theme, and your suggestion was one of them. And as you say, these tended to oscillate with most crystals. I had one built up as a module, so I'd have a general purpose oscillator read, to check crystals or as a signal source. But, for some reason, when they moved to CMOS oscillators, the norm became one gate, with a feedback resistor and a pair of capacitors, one from input to ground, and the other from output to ground. Michael VE2BVW "PaoloC" wrote in message ... Hi. I have spent part of the weekend trying to resonate al old CB XTAL at its fundamental frequency. The XTAL is labelled 27.125 MHz, with a fundamental of about 9.041 MHz, which falls into 18m HAM band when multiplied by two. I assume 27MHz XTALs are 3rd overtone. Since the circuitry is/will be digital, the oscillator is one gate of a 74HC14. 470ohm resistor from gate output to the parallel of 1Mohm//XTAL. 10pF and 33pF (from the junkbox) capacitors to ground on each side of the XTAL. No oscillation (I have no oscilloscope, I use my HF receiver to troubleshoot oscillators at known frequencies). If I replace the XTAL with a 10.000 MHz rock the oscillation is loud and clear. I have never built something with an overtone XTAL. I know that I need an output resonating circuit if I want to extract the 3rd harmonic. Do I need the same if I want the fundamental? Are overtone XTALs "harder" to resonate? Are those old CB XTALs 3rd overtone? I assume my 27MHz XTAL works. :-) Thanks in advance for all suggestions, Paolo IK1ZYW |
"Michael Black" wrote in message ... But, for some reason, when they moved to CMOS oscillators, the norm became one gate, with a feedback resistor and a pair of capacitors, one from input to ground, and the other from output to ground. Oh, yes. That rang a bell. CMOS logic is easier to get to work as an analog device than TTL. At least for the good ol' RCA 4000 series. I am not familiar with the newer 74HC00 series to judge its performance. I would suspect that the 'bias' resistors would have to be carefully chosen and that it would be sensitive to both the crystal frequency and the ambient temperature. When I was doing circuit desgin everyone most were still using TTL. The 74LS00 series was the hot item at the time. Just from a logical viewpoint (pun not intended, by the way) I still think he should try using two gates in a ring circuit. Jim N8EE |
"Michael Black" wrote in message ... But, for some reason, when they moved to CMOS oscillators, the norm became one gate, with a feedback resistor and a pair of capacitors, one from input to ground, and the other from output to ground. Oh, yes. That rang a bell. CMOS logic is easier to get to work as an analog device than TTL. At least for the good ol' RCA 4000 series. I am not familiar with the newer 74HC00 series to judge its performance. I would suspect that the 'bias' resistors would have to be carefully chosen and that it would be sensitive to both the crystal frequency and the ambient temperature. When I was doing circuit desgin everyone most were still using TTL. The 74LS00 series was the hot item at the time. Just from a logical viewpoint (pun not intended, by the way) I still think he should try using two gates in a ring circuit. Jim N8EE |
"Michael Black" wrote in message ... PaoloC ) writes: Hi. I have spent part of the weekend trying to resonate al old CB XTAL at its fundamental frequency. The XTAL is labelled 27.125 MHz, with a fundamental of about 9.041 MHz, which falls into 18m HAM band when multiplied by two. I assume 27MHz XTALs are 3rd overtone.... Could the crystal be a receive crystal? CB crystals tended to show the channel number or the frequency of the channel, and so if you simply looked at the marked frequency, it would not tell you if it's for transmit or receive. Though the ones I've seen did mark them with "R" or "T". The point is that if it's a receive crystal, it wouldn't be 1/3 of 27.125 but 27.125-IF and then divided by three. IFs were frequently 455KHz I have used fundamental crystals on their overtones and overtone crystals on the fundamental. They are not exactly 1/3 or 1/5 ratio due to parasitics in the crystal. All crystals have the fund and OT responses, but the responses are optimized for the intended use - by changing some of the construction. (I'm talking AT cuts) Overtone crystals *tend* to be used in the SERIES resonant mode, though not always. Fundamental crystals tend to be used mostly in the parallel resonant mode. This will also cause additinal frequency error when using the marked frequency. The parallel freq is higher than the series (if I recall correctly) If I recall, the two gate oscillator is a series resonant oscillator and the one gate is parallel. I would use a transistor Colpits oscillator myself. The digital gate oscillators can run the crystal at a higher drive than it should causing more crystal heating than desired (more drift w/time). If you are going to use a OT xtal as a fund., I'd pick a parallel type osc like the Colpits (I think it has another name when it has a crystal rather than a coil). I never did like the digital gate oscillators. They tend to be a *bruit force* oscillator. Because of the Rs of the crystal, crystals of some frequencies are easier to over drive than others. Unfortunately, I don't remember which is which. The fact that your 10mMHz xtal works is a good start. The other one probably will, but since it was made for the third OT, there may be enough difference to keep it from oscillating without some circuit change. If you have no scope, measure the current drain. There may be a change when the xtal is inserted if it is oscillating. Also, you could try measuring some voltage through a large resistor or choke to see changes when the xtal is inserted. -- Steve N, K,9;d, c. i My email has no u's. |
"Michael Black" wrote in message ... PaoloC ) writes: Hi. I have spent part of the weekend trying to resonate al old CB XTAL at its fundamental frequency. The XTAL is labelled 27.125 MHz, with a fundamental of about 9.041 MHz, which falls into 18m HAM band when multiplied by two. I assume 27MHz XTALs are 3rd overtone.... Could the crystal be a receive crystal? CB crystals tended to show the channel number or the frequency of the channel, and so if you simply looked at the marked frequency, it would not tell you if it's for transmit or receive. Though the ones I've seen did mark them with "R" or "T". The point is that if it's a receive crystal, it wouldn't be 1/3 of 27.125 but 27.125-IF and then divided by three. IFs were frequently 455KHz I have used fundamental crystals on their overtones and overtone crystals on the fundamental. They are not exactly 1/3 or 1/5 ratio due to parasitics in the crystal. All crystals have the fund and OT responses, but the responses are optimized for the intended use - by changing some of the construction. (I'm talking AT cuts) Overtone crystals *tend* to be used in the SERIES resonant mode, though not always. Fundamental crystals tend to be used mostly in the parallel resonant mode. This will also cause additinal frequency error when using the marked frequency. The parallel freq is higher than the series (if I recall correctly) If I recall, the two gate oscillator is a series resonant oscillator and the one gate is parallel. I would use a transistor Colpits oscillator myself. The digital gate oscillators can run the crystal at a higher drive than it should causing more crystal heating than desired (more drift w/time). If you are going to use a OT xtal as a fund., I'd pick a parallel type osc like the Colpits (I think it has another name when it has a crystal rather than a coil). I never did like the digital gate oscillators. They tend to be a *bruit force* oscillator. Because of the Rs of the crystal, crystals of some frequencies are easier to over drive than others. Unfortunately, I don't remember which is which. The fact that your 10mMHz xtal works is a good start. The other one probably will, but since it was made for the third OT, there may be enough difference to keep it from oscillating without some circuit change. If you have no scope, measure the current drain. There may be a change when the xtal is inserted if it is oscillating. Also, you could try measuring some voltage through a large resistor or choke to see changes when the xtal is inserted. -- Steve N, K,9;d, c. i My email has no u's. |
Hi,
thanks to all those who replied and discussed this topic, providing me with a lot of good suggestions. I will try a different ex-CB rock, just in case the one I picked is broken. I actually tried a "Colpitts" oscillator with it and had no success either. 73, Paolo IK1ZYW |
Hi,
thanks to all those who replied and discussed this topic, providing me with a lot of good suggestions. I will try a different ex-CB rock, just in case the one I picked is broken. I actually tried a "Colpitts" oscillator with it and had no success either. 73, Paolo IK1ZYW |
Get something working with a good crystal first.
A Google search (Colpitts Crystal oscillator circuits) turned up lots of gate oscillators. This has the type of transistor oscillators I am used to using: http://hem.passagen.se/communication/txo.html There are many other references at: http://users.telenet.be/educypedia/e...osciltypes.htm I don't like this circuit: http://www.electronics-tutorials.com...scillators.htm |
Get something working with a good crystal first.
A Google search (Colpitts Crystal oscillator circuits) turned up lots of gate oscillators. This has the type of transistor oscillators I am used to using: http://hem.passagen.se/communication/txo.html There are many other references at: http://users.telenet.be/educypedia/e...osciltypes.htm I don't like this circuit: http://www.electronics-tutorials.com...scillators.htm |
I dunn-o. I had great success with this cricuit. It's used in all the older Motorola channel elements. Just about any rock will fly at the fundamental unless its really bad. You can play with the ratio of the two feedback caps (Base-to-emitter and emitter-to-ground) Base-to-emitter cap decrease to get more feedback. I even modeled one of these on P-Spice. Really neat to see it oscillate. like Figure 7: http://www.northcountryradio.com/PDFs/column007.pdf -- Steve N, K,9;d, c. i My email has no u's. "Steve Nosko" wrote in message ... Get something working with a good crystal first. A Google search (Colpitts Crystal oscillator circuits) turned up lots of gate oscillators. This has the type of transistor oscillators I am used to using: http://hem.passagen.se/communication/txo.html There are many other references at: http://users.telenet.be/educypedia/e...osciltypes.htm I don't like this circuit: http://www.electronics-tutorials.com...scillators.htm |
I dunn-o. I had great success with this cricuit. It's used in all the older Motorola channel elements. Just about any rock will fly at the fundamental unless its really bad. You can play with the ratio of the two feedback caps (Base-to-emitter and emitter-to-ground) Base-to-emitter cap decrease to get more feedback. I even modeled one of these on P-Spice. Really neat to see it oscillate. like Figure 7: http://www.northcountryradio.com/PDFs/column007.pdf -- Steve N, K,9;d, c. i My email has no u's. "Steve Nosko" wrote in message ... Get something working with a good crystal first. A Google search (Colpitts Crystal oscillator circuits) turned up lots of gate oscillators. This has the type of transistor oscillators I am used to using: http://hem.passagen.se/communication/txo.html There are many other references at: http://users.telenet.be/educypedia/e...osciltypes.htm I don't like this circuit: http://www.electronics-tutorials.com...scillators.htm |
In message , Steve Nosko
writes "Michael Black" wrote in message ... PaoloC ) writes: Hi. I have spent part of the weekend trying to resonate al old CB XTAL at its fundamental frequency. The XTAL is labelled 27.125 MHz, with a fundamental of about 9.041 MHz, which falls into 18m HAM band when multiplied by two. I assume 27MHz XTALs are 3rd overtone.... Could the crystal be a receive crystal? CB crystals tended to show the channel number or the frequency of the channel, and so if you simply looked at the marked frequency, it would not tell you if it's for transmit or receive. Though the ones I've seen did mark them with "R" or "T". The point is that if it's a receive crystal, it wouldn't be 1/3 of 27.125 but 27.125-IF and then divided by three. IFs were frequently 455KHz I have used fundamental crystals on their overtones and overtone crystals on the fundamental. They are not exactly 1/3 or 1/5 ratio due to parasitics in the crystal. All crystals have the fund and OT responses, but the responses are optimized for the intended use - by changing some of the construction. (I'm talking AT cuts) Overtone crystals *tend* to be used in the SERIES resonant mode, though not always. Fundamental crystals tend to be used mostly in the parallel resonant mode. This will also cause additinal frequency error when using the marked frequency. The parallel freq is higher than the series (if I recall correctly) If I recall, the two gate oscillator is a series resonant oscillator and the one gate is parallel. I would use a transistor Colpits oscillator myself. The digital gate oscillators can run the crystal at a higher drive than it should causing more crystal heating than desired (more drift w/time). If you are going to use a OT xtal as a fund., I'd pick a parallel type osc like the Colpits (I think it has another name when it has a crystal rather than a coil). I never did like the digital gate oscillators. They tend to be a *bruit force* oscillator. Because of the Rs of the crystal, crystals of some frequencies are easier to over drive than others. Unfortunately, I don't remember which is which. The fact that your 10mMHz xtal works is a good start. The other one probably will, but since it was made for the third OT, there may be enough difference to keep it from oscillating without some circuit change. If you have no scope, measure the current drain. There may be a change when the xtal is inserted if it is oscillating. Also, you could try measuring some voltage through a large resistor or choke to see changes when the xtal is inserted. Regarding 'series' and 'parallel' modes, ALL crystals actually resonate in a series mode. The simplistic equivalent circuit of a crystal is a series L-C tuned circuit with a capacitor across the whole thing. The series C (call it C1) is relatively very small (compared with the parallel C, C2) and the L is very large (ie the L/C1 ratio is large). The parallel C2 largely consists of the physical capacitance caused by the plating on each side of the actual crystal. In some circuits the value of C2 is deliberately increased by adding actual parallel capacitance (see later). Looking at the equivalent circuit, you will see that the series C1, the parallel C2 and the inductance are all in a loop, ie the three form another series-tuned circuit. As C1 and C2 are in series, the straight-through 'series' resonance of L and C1 must be at al lower frequency than the 'parallel' resonance of L + C1 + C2 (although the two resonances are always very close together). In the 'normal' series-resonance mode (L with C1), an RF current is simply fed through the crystal. This will be the case when it is connected between the pins of an IC and there are no other 'tuning' elements. Adding parallel C should have no effect on the series-resonance, but it does bring the 'series' and 'parallel' resonances closer together. This tends to force the oscillation frequency lower. The 'parallel' mode tends to be more applicable where impedances are higher (eg in a Colpitts circuit). As C2 is much greater than C1, the oscillation frequency does not change much if C2 is varied. However, it does change. Most crystals intended to operate in this 'parallel' mode specify their frequency with a 30pF shunt capacitor. Such crystals are usually used at the lower frequencies (up to a few MHz). At the higher frequencies, the 'series' mode is almost always used. As C1 is very small, in the 'series' mode (L and C1) adding capacity in series with the crystal has very little effect on the frequency unless the extra capacitor is itself very small (and this tends to kill the oscillation). Finally, 'overtone' crystals should always be usable on the lower overtones (essentially the odd harmonic frequencies of the fundamental), and may (even if some TLC is required) operate on some of the higher overtones. I think I have this right! Cheers, Ian. -- |
In message , Steve Nosko
writes "Michael Black" wrote in message ... PaoloC ) writes: Hi. I have spent part of the weekend trying to resonate al old CB XTAL at its fundamental frequency. The XTAL is labelled 27.125 MHz, with a fundamental of about 9.041 MHz, which falls into 18m HAM band when multiplied by two. I assume 27MHz XTALs are 3rd overtone.... Could the crystal be a receive crystal? CB crystals tended to show the channel number or the frequency of the channel, and so if you simply looked at the marked frequency, it would not tell you if it's for transmit or receive. Though the ones I've seen did mark them with "R" or "T". The point is that if it's a receive crystal, it wouldn't be 1/3 of 27.125 but 27.125-IF and then divided by three. IFs were frequently 455KHz I have used fundamental crystals on their overtones and overtone crystals on the fundamental. They are not exactly 1/3 or 1/5 ratio due to parasitics in the crystal. All crystals have the fund and OT responses, but the responses are optimized for the intended use - by changing some of the construction. (I'm talking AT cuts) Overtone crystals *tend* to be used in the SERIES resonant mode, though not always. Fundamental crystals tend to be used mostly in the parallel resonant mode. This will also cause additinal frequency error when using the marked frequency. The parallel freq is higher than the series (if I recall correctly) If I recall, the two gate oscillator is a series resonant oscillator and the one gate is parallel. I would use a transistor Colpits oscillator myself. The digital gate oscillators can run the crystal at a higher drive than it should causing more crystal heating than desired (more drift w/time). If you are going to use a OT xtal as a fund., I'd pick a parallel type osc like the Colpits (I think it has another name when it has a crystal rather than a coil). I never did like the digital gate oscillators. They tend to be a *bruit force* oscillator. Because of the Rs of the crystal, crystals of some frequencies are easier to over drive than others. Unfortunately, I don't remember which is which. The fact that your 10mMHz xtal works is a good start. The other one probably will, but since it was made for the third OT, there may be enough difference to keep it from oscillating without some circuit change. If you have no scope, measure the current drain. There may be a change when the xtal is inserted if it is oscillating. Also, you could try measuring some voltage through a large resistor or choke to see changes when the xtal is inserted. Regarding 'series' and 'parallel' modes, ALL crystals actually resonate in a series mode. The simplistic equivalent circuit of a crystal is a series L-C tuned circuit with a capacitor across the whole thing. The series C (call it C1) is relatively very small (compared with the parallel C, C2) and the L is very large (ie the L/C1 ratio is large). The parallel C2 largely consists of the physical capacitance caused by the plating on each side of the actual crystal. In some circuits the value of C2 is deliberately increased by adding actual parallel capacitance (see later). Looking at the equivalent circuit, you will see that the series C1, the parallel C2 and the inductance are all in a loop, ie the three form another series-tuned circuit. As C1 and C2 are in series, the straight-through 'series' resonance of L and C1 must be at al lower frequency than the 'parallel' resonance of L + C1 + C2 (although the two resonances are always very close together). In the 'normal' series-resonance mode (L with C1), an RF current is simply fed through the crystal. This will be the case when it is connected between the pins of an IC and there are no other 'tuning' elements. Adding parallel C should have no effect on the series-resonance, but it does bring the 'series' and 'parallel' resonances closer together. This tends to force the oscillation frequency lower. The 'parallel' mode tends to be more applicable where impedances are higher (eg in a Colpitts circuit). As C2 is much greater than C1, the oscillation frequency does not change much if C2 is varied. However, it does change. Most crystals intended to operate in this 'parallel' mode specify their frequency with a 30pF shunt capacitor. Such crystals are usually used at the lower frequencies (up to a few MHz). At the higher frequencies, the 'series' mode is almost always used. As C1 is very small, in the 'series' mode (L and C1) adding capacity in series with the crystal has very little effect on the frequency unless the extra capacitor is itself very small (and this tends to kill the oscillation). Finally, 'overtone' crystals should always be usable on the lower overtones (essentially the odd harmonic frequencies of the fundamental), and may (even if some TLC is required) operate on some of the higher overtones. I think I have this right! Cheers, Ian. -- |
Steve Nosko wrote:
"Michael Black" wrote in message ... PaoloC ) writes: Hi. I have spent part of the weekend trying to resonate al old CB XTAL at its fundamental frequency. The XTAL is labelled 27.125 MHz, with a fundamental of about 9.041 MHz, which falls into 18m HAM band when multiplied by two. I assume 27MHz XTALs are 3rd overtone.... Could the crystal be a receive crystal? CB crystals tended to show the channel number or the frequency of the channel, and so if you simply looked at the marked frequency, it would not tell you if it's for transmit or receive. Though the ones I've seen did mark them with "R" or "T". The point is that if it's a receive crystal, it wouldn't be 1/3 of 27.125 but 27.125-IF and then divided by three. IFs were frequently 455KHz But not always! Many cb sets used an if frequency of 1600khz plus or minus a few 100khz. I have some old Lafayette if cans at 1650khz that came out of an old cb set. Some also used double conversion with a first if anywhere from 1.5mhz to 13mhz and a down conversion to 455khz. So that receive rock could be anywhere in frequency. |
Steve Nosko wrote:
"Michael Black" wrote in message ... PaoloC ) writes: Hi. I have spent part of the weekend trying to resonate al old CB XTAL at its fundamental frequency. The XTAL is labelled 27.125 MHz, with a fundamental of about 9.041 MHz, which falls into 18m HAM band when multiplied by two. I assume 27MHz XTALs are 3rd overtone.... Could the crystal be a receive crystal? CB crystals tended to show the channel number or the frequency of the channel, and so if you simply looked at the marked frequency, it would not tell you if it's for transmit or receive. Though the ones I've seen did mark them with "R" or "T". The point is that if it's a receive crystal, it wouldn't be 1/3 of 27.125 but 27.125-IF and then divided by three. IFs were frequently 455KHz But not always! Many cb sets used an if frequency of 1600khz plus or minus a few 100khz. I have some old Lafayette if cans at 1650khz that came out of an old cb set. Some also used double conversion with a first if anywhere from 1.5mhz to 13mhz and a down conversion to 455khz. So that receive rock could be anywhere in frequency. |
Ken Scharf ) writes:
Steve Nosko wrote: "Michael Black" wrote in message ... PaoloC ) writes: Hi. I have spent part of the weekend trying to resonate al old CB XTAL at its fundamental frequency. The XTAL is labelled 27.125 MHz, with a fundamental of about 9.041 MHz, which falls into 18m HAM band when multiplied by two. I assume 27MHz XTALs are 3rd overtone.... Could the crystal be a receive crystal? CB crystals tended to show the channel number or the frequency of the channel, and so if you simply looked at the marked frequency, it would not tell you if it's for transmit or receive. Though the ones I've seen did mark them with "R" or "T". The point is that if it's a receive crystal, it wouldn't be 1/3 of 27.125 but 27.125-IF and then divided by three. IFs were frequently 455KHz But not always! Many cb sets used an if frequency of 1600khz plus or minus a few 100khz. I have some old Lafayette if cans at 1650khz that came out of an old cb set. Some also used double conversion with a first if anywhere from 1.5mhz to 13mhz and a down conversion to 455khz. So that receive rock could be anywhere in frequency. Which is why I didn't specify the IF. I thought the original poster might be listening on 1/3 the marked frequency, but if it's a receive crystal it wouldn't be there. And while assuming an offset of 455KHz might be a good 1st guess, one could listen there on a receiver and still not find something. It still doesn't answer the question of whether the crystal is oscillating if you don't hear anything at the expected frequency, because it might be on some other frequency. Years ago, when I would fiddle with oscillators, I'd put the SP-600 that I had at the time on it's highest band, 30 to 54MHz. Then with the oscillator on, I'd spin that knob. A few good spins got it across the dial. I'd find a spot where there was a carrier, and then do some fine tuning. Then I'd shift down in frequency, looking for a submultiple of that frequency, until I found the actual operating frequency. A digitally tuned receiver let's you find the expected frequency much faster, but you still need to tune around if the crystal isn't at the expected frequency. Michael VE2BVW |
Ken Scharf ) writes:
Steve Nosko wrote: "Michael Black" wrote in message ... PaoloC ) writes: Hi. I have spent part of the weekend trying to resonate al old CB XTAL at its fundamental frequency. The XTAL is labelled 27.125 MHz, with a fundamental of about 9.041 MHz, which falls into 18m HAM band when multiplied by two. I assume 27MHz XTALs are 3rd overtone.... Could the crystal be a receive crystal? CB crystals tended to show the channel number or the frequency of the channel, and so if you simply looked at the marked frequency, it would not tell you if it's for transmit or receive. Though the ones I've seen did mark them with "R" or "T". The point is that if it's a receive crystal, it wouldn't be 1/3 of 27.125 but 27.125-IF and then divided by three. IFs were frequently 455KHz But not always! Many cb sets used an if frequency of 1600khz plus or minus a few 100khz. I have some old Lafayette if cans at 1650khz that came out of an old cb set. Some also used double conversion with a first if anywhere from 1.5mhz to 13mhz and a down conversion to 455khz. So that receive rock could be anywhere in frequency. Which is why I didn't specify the IF. I thought the original poster might be listening on 1/3 the marked frequency, but if it's a receive crystal it wouldn't be there. And while assuming an offset of 455KHz might be a good 1st guess, one could listen there on a receiver and still not find something. It still doesn't answer the question of whether the crystal is oscillating if you don't hear anything at the expected frequency, because it might be on some other frequency. Years ago, when I would fiddle with oscillators, I'd put the SP-600 that I had at the time on it's highest band, 30 to 54MHz. Then with the oscillator on, I'd spin that knob. A few good spins got it across the dial. I'd find a spot where there was a carrier, and then do some fine tuning. Then I'd shift down in frequency, looking for a submultiple of that frequency, until I found the actual operating frequency. A digitally tuned receiver let's you find the expected frequency much faster, but you still need to tune around if the crystal isn't at the expected frequency. Michael VE2BVW |
In article , PaoloC
writes Hi. I have spent part of the weekend trying to resonate al old CB XTAL at its fundamental frequency. The XTAL is labelled 27.125 MHz, with a fundamental of about 9.041 MHz, which falls into 18m HAM band when multiplied by two. I assume 27MHz XTALs are 3rd overtone. Since the circuitry is/will be digital, the oscillator is one gate of a 74HC14. 470ohm resistor from gate output to the parallel of 1Mohm//XTAL. 10pF and 33pF (from the junkbox) capacitors to ground on each side of the XTAL. No oscillation (I have no oscilloscope, I use my HF receiver to troubleshoot oscillators at known frequencies). If I replace the XTAL with a 10.000 MHz rock the oscillation is loud and clear. I have never built something with an overtone XTAL. I know that I need an output resonating circuit if I want to extract the 3rd harmonic. Do I need the same if I want the fundamental? Are overtone XTALs "harder" to resonate? Are those old CB XTALs 3rd overtone? I assume my 27MHz XTAL works. :-) Thanks in advance for all suggestions, Paolo IK1ZYW Could be a 27 MHz fundamental or 10.7 above or below 27MHz. -- ddwyer |
In article , PaoloC
writes Hi. I have spent part of the weekend trying to resonate al old CB XTAL at its fundamental frequency. The XTAL is labelled 27.125 MHz, with a fundamental of about 9.041 MHz, which falls into 18m HAM band when multiplied by two. I assume 27MHz XTALs are 3rd overtone. Since the circuitry is/will be digital, the oscillator is one gate of a 74HC14. 470ohm resistor from gate output to the parallel of 1Mohm//XTAL. 10pF and 33pF (from the junkbox) capacitors to ground on each side of the XTAL. No oscillation (I have no oscilloscope, I use my HF receiver to troubleshoot oscillators at known frequencies). If I replace the XTAL with a 10.000 MHz rock the oscillation is loud and clear. I have never built something with an overtone XTAL. I know that I need an output resonating circuit if I want to extract the 3rd harmonic. Do I need the same if I want the fundamental? Are overtone XTALs "harder" to resonate? Are those old CB XTALs 3rd overtone? I assume my 27MHz XTAL works. :-) Thanks in advance for all suggestions, Paolo IK1ZYW Could be a 27 MHz fundamental or 10.7 above or below 27MHz. -- ddwyer |
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