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VCXO frequency isn't high enough
colin wrote:
"Anthony Fremont" wrote in message ... colin wrote: "Anthony Fremont" wrote in message ... colin wrote: Its only a pull of ~100ppm, this should be easily pullable for most fundamental xtals. I'd like to get close to 500ppm if possible. do you need that much ? I "need" an increase of about .0546%. Isn't that about 546ppm? what freq you need ? 3581.5kHz to zero beat with the desired signal. whats the colourburst crystal freq ? 3579.545kHz. aha ok, thats a fair bit, but maybe within range, as as been said at a certain frequency the crystal becomes a complete open circuit. I didn't design it, but allot of folks seem to have had success with getting it working. Like one poster said, I'm already within range to copy the signal, but it would be about 1.5kHz. That's a tad high in pitch for my taste. I was hoping to be able to get to the other side of zero beat, but perhaps that was wishful thinking on my part. I've managed to get it 500Hz higher than the marked frequency so I should probably be happy with that. After all it's a crystal and it was designed to be operated well within 50ppm of its marked freq. ofc if the tolerenace all add up against you you might find it hard. I would also try reduce the 100pf caps on the sa602 too. 100pf is higher than most crystals I use would like, 50pf or 20pf or if youve got some spare trimmers ... you can adjust the ratio too, say just reduce the one accross pin 6-7 I figured that they were voltage dividers to set the amount of feedback, but I can certainly see how they could have an effect. Since I'm close to where I need to be, I will try a couple of 33pF caps to see what happens. no they are involved in setting the frequency too, in order for you circuit to work it needs to resonate, with the 2 100pf the input where the crystal is looks like a capacitor with some negative impedance, the circuit with your crystal, inductor, and trimmer must be inductive, it then forms a resonant ciruit with the capacitance of the input. usualy the crystal would just be operated so that it looks inductive. Ok, I had to read that a few times to get it. A period between ".....100pf" and "the input..." there would have been quite helpful. ;-) Yeah I kinda got lost in my own explanation myself. didnt have much time to explain. Don't get me wrong, I appreciate the information. the negative part of the input impedance must be stronger than the loss in the tuned circuit. this is affected by the ratio of the 2 100pf capacitors. So they do act like a voltage divider of sorts and shunt some of the oscillator output to ground and some back to the input. this circuit is an emiter folower wich has no voltage gain, so actually they operate in the opposite way wich is kinda confusing but let me explain ... consider a typical tuned circuit with LC and a tap in the L, driving the circuit at the tap acts as a step up, but the crystal is the inductor wich make it difficult to put a tap here, but at resonance the 2 capacitors can act in the same way and provide a voltage step up. but you can see there is a curent loop involving all these components in series, this is what sets the frequency. If you decrease the capacitor accross the 2 pins of the ic this will increase the voltage accros it and so give more drive, as wel as increase the frequency. another way to look at it is if you consider that ground is at the base then the capacitors are in fact a voltage divider wich feed into the emiter of a comon base amplifier. a crystal can apear to be a very high inductance at resonance, at the point where you want to operate it probably has very high inductance indeed. you can determnine the eqv inductance by using the equivalent internal inductance and capacitance. you need to find the mutual capacitance of the crystal wich is hard to find man specs wich tel you this but it is often something like 14ff for example. (0.014pf) you can then work out the eqv series inductance for it to resonate with the std load wich may be 20pf. you can then work out what inductance the crystal will apear to have at the frequency you want. and hence the series capacitance you need. you might find the inductance is so high that you need less than 1pf or it has become capacitive. Cool, a way to figure out just how high you can pull it and how to attain a certain frequency. I'll probably stick to tinkering though. ;-) Thanks allot for the detailed explanation. :-) The pulling range is usually equal to the motional capcitance over the crystal self capacitance, so for 14ff and 20pf this gives 700ppm, but actually at this extreme its unusable in this circuit as its required to be inductive. as said by some1 else the farther you pull it the worse the performance. Thanks again for the info. I think I'll go tinker with it a bit. My new scope shippped yesterday and it will be here tomorrow, yaaaayyyyyy. ;-) |
VCXO frequency isn't high enough
In message .com,
writes On Mar 13, 12:02 pm, "Anthony Fremont" wrote: Hello all, I was playing around and saw that my junk box had all the parts so I started tossing this together:http://newenglandqrp.org/files/w1aw-receiver.jpg The problem is (well I think it's a problem) is that I'm all the way down to a 10pF cap for the crystal trimmer and the highest frequency I can get out of it is still less than 3580kHz. Pleae correct me if I'm wrong, but I'm thinking that the 20uH inductor is supposed to pull the colorburst crystal high in frequency The inductor will pull the crystal down in frequency, as others have suggested. In fact it is extremely difficult to pull a crystal's series resonance up in frequency more than a few Hz. This is because the crystal's parallel resonance is just above its series resonance. If you put a capacitor in series with the crystal the series resonant frequency goes up...BUT...if you approach the parallel resonant frequency you can no longer get a low impedance resonance condition since the crystal's parallel resonance makes the crystal look like an open circuit, regardless of what you put in series with it. If you want to understand this better, try to find a reference with a good discussion of the equivalent circuit of the quartz crystal resonator. the only one I know of at present is Kenneth K. Clarke and Donald T. Hess, Communication Circuits: Analysis and Design, Addison- Wesley Publishing Co., 1971. It may be a bit hard to find outside a good university library. As Ian Jackson suggested a parallel inductor might work....this modifies the parallel resonance. Ian..is there a schematic available for that ? I'd be interested in what actually worked. Steve I'll try and draw what I have in mind, and post it in alt.binaries.schematics.electronic. However, in the meantime, let me try and explain. The explanation may not be absolutely 100% complete, or even 100% correct, but it may help in moving a crystal more HF than it wants to go. Sorry that it's a bit rambling! In Anthony's circuit (http://newenglandqrp.org/files/w1aw-receiver.jpg), the crystal will probably be functioning as a series-tuned circuit. As Steve has stated, a crystal suddenly goes into parallel resonance just HF of its series resonance. This limits how far the series resonance can be pulled HF by the addition of a series trimmer capacitor. However, if this parallel resonance can be removed (or moved further HF), it should be possible to move the crystal further HF. The technique described certainly does work with VHF overtone crystals (between 50 and 200MHz), but should also work with HF crystals working on their fundamental frequencies. A crystal is a mechanical device, but can be represented as being a series-tuned L-C circuit. (Call these L1 and C1.) Also, across the two is a parallel C (C2). Forget about losses (represented by a resistor). [Note: L1 and C1 are not actual electrical components, and only appear to have these values at or near to the L1-C1 resonant frequency. However, C2 essentially is a physical electrical capacitor consisting of the plating on each face of the crystal, with the crystal as the dielectric between.] L1 is very large (possibly 1H or more, depending on the frequency). C1 is very small (say only a few pF or even a fraction of a pF - again depending on the crystal frequency). [So adding a relatively large series trimmer capacitor has very little effect on the frequency.] C2 is typically around 5pF, regardless of frequency. Imagine doing a test where you look at the resonant frequency of a crystal, using a signal generator. This feeds an RF signal through a crystal, into a 50 ohm load. You measure throughput of the crystal by measuring the voltage across the load. Swing the sig gen frequency slowly from LF to HF, through the resonant frequency of L1-C1. [Let's forget about C2 for the moment.] Below the resonant frequency of L1-C1, the L1-C1 circuit acts like a small capacitor, so there is very little throughput. Above the series resonant frequency of L1-C1, the L1-C1 circuit acts like a large inductor, so again there is very little throughput. However, when you hit the series resonance of L1 and C1 (F1), reactance of L1 and C1 cancel. The crystal acts like a short-circuit (or nearly so) and there is a large throughput. Because the L-C ratio is very high, the resonance peak is very sharp. The effect of C2 across the L1-C1 circuit is to produce a second (parallel) resonant circuit. VERY slightly HF of the L1-C1 resonance, C2 resonates with effective inductance of the L1-C1 circuit. This produces a parallel resonant circuit (F2). Another way of looking at it is that L1 resonates with the series combination of C1 and C2 (so F2 must be higher than F1). The parallel resonance is, of course, a high impedance, where there is almost no throughput through the crystal. As a result of this double resonance, the crystal acts as a series-tuned circuit at F1 (one you want), and a parallel-tuned circuit at F2. The transition between the two is very sudden. The frequency response peak of the throughput is very lopsided, and gets chopped off suddenly on the HF side. The difference between F1 and F2 is very small (a few Hz to a few kHz, depending on the frequency and type of the crystal). If F1 is lower than you want, and you add an external series trimmer capacitor to try and pull the crystal L1-C1 series resonance HF, you effectively hit a brick wall with the parallel resonance at F2. The parallel resonance will block any throughput at (or near) this frequency. A possible solution is to neutralize C2. [Note: Neutralization is a technique sometimes required when using VHF crystals, as C2 may be large enough to allow the oscillator to free-run, instead of being locked to the frequency of L1-C1. However, it may also be used with advantage, as described below.] You can neutralize C2 by adding an inductor across the crystal (ie in parallel with C2). The value required is that which parallel-resonates with C2 at the crystal frequency. In effect, C2 no longer exists. With C2 neutralized, there is no longer a sudden transition from the wanted series resonance F1 to the unwanted parallel resonance F2. The peak the response curve of the throughput of the crystal (at F1) is now nice and symmetrical, without the sudden cutoff at F2. In practice, the actual F1 peak will probably be somewhat more HF than before, and the crystal should be more pullable with a series capacitor. Finally, if you reduce the value of the inductor so that its resonance with C2 is somewhat higher than the crystal frequency, this tends to pull the F1 resonance peak even higher in frequency. However, if you overdo this, the oscillation will probably unlock from the crystal, and start to free-run. As I said, sorry for the ramble. Ian. -- |
VCXO frequency isn't high enough
In message , Anthony Fremont
writes Anthony Fremont wrote: Doodling with reactance formulas, it appears that 20uH (coincidence?) would offset 100pF of capacitance fairly well by having a an opposing reactance (well resistance at this point) of about 450 Ohms at s/.well resistance at this point.// It's just inductive reactance, I need more coffee. ;-) 3581kHz, the same as 100pF. I'll try putting the coil I made in parallel and see what happens. Hopefully it won't short the oscillator and kill my 15 year old NE602, I only have two spares. Should I be afraid to do this? Does it need something to block DC current? If you do try an inductor across the crystal, make sure that you still do have a DC blocking capacitor somewhere in the path to ground (as provided by the existing C2 trimmer). Ian. -- |
VCXO frequency isn't high enough
Ian Jackson wrote:
In message .com, writes On Mar 13, 12:02 pm, "Anthony Fremont" wrote: Hello all, I was playing around and saw that my junk box had all the parts so I started tossing this together:http://newenglandqrp.org/files/w1aw-receiver.jpg The problem is (well I think it's a problem) is that I'm all the way down to a 10pF cap for the crystal trimmer and the highest frequency I can get out of it is still less than 3580kHz. Pleae correct me if I'm wrong, but I'm thinking that the 20uH inductor is supposed to pull the colorburst crystal high in frequency The inductor will pull the crystal down in frequency, as others have suggested. In fact it is extremely difficult to pull a crystal's series resonance up in frequency more than a few Hz. This is because the crystal's parallel resonance is just above its series resonance. If you put a capacitor in series with the crystal the series resonant frequency goes up...BUT...if you approach the parallel resonant frequency you can no longer get a low impedance resonance condition since the crystal's parallel resonance makes the crystal look like an open circuit, regardless of what you put in series with it. If you want to understand this better, try to find a reference with a good discussion of the equivalent circuit of the quartz crystal resonator. the only one I know of at present is Kenneth K. Clarke and Donald T. Hess, Communication Circuits: Analysis and Design, Addison- Wesley Publishing Co., 1971. It may be a bit hard to find outside a good university library. As Ian Jackson suggested a parallel inductor might work....this modifies the parallel resonance. Ian..is there a schematic available for that ? I'd be interested in what actually worked. Steve I'll try and draw what I have in mind, and post it in alt.binaries.schematics.electronic. However, in the meantime, let me try and explain. The explanation may not be absolutely 100% complete, or even 100% correct, but it may help in moving a crystal more HF than it wants to go. Sorry that it's a bit rambling! In Anthony's circuit (http://newenglandqrp.org/files/w1aw-receiver.jpg), the crystal will probably be functioning as a series-tuned circuit. As Steve has stated, a crystal suddenly goes into parallel resonance just HF of its series resonance. This limits how far the series resonance can be pulled HF by the addition of a series trimmer capacitor. However, if this parallel resonance can be removed (or moved further HF), it should be possible to move the crystal further HF. The technique described certainly does work with VHF overtone crystals (between 50 and 200MHz), but should also work with HF crystals working on their fundamental frequencies. A crystal is a mechanical device, but can be represented as being a series-tuned L-C circuit. (Call these L1 and C1.) Also, across the two is a parallel C (C2). Forget about losses (represented by a resistor). [Note: L1 and C1 are not actual electrical components, and only appear to have these values at or near to the L1-C1 resonant frequency. However, C2 essentially is a physical electrical capacitor consisting of the plating on each face of the crystal, with the crystal as the dielectric between.] L1 is very large (possibly 1H or more, depending on the frequency). C1 is very small (say only a few pF or even a fraction of a pF - again depending on the crystal frequency). [So adding a relatively large series trimmer capacitor has very little effect on the frequency.] C2 is typically around 5pF, regardless of frequency. Imagine doing a test where you look at the resonant frequency of a crystal, using a signal generator. This feeds an RF signal through a crystal, into a 50 ohm load. You measure throughput of the crystal by measuring the voltage across the load. Swing the sig gen frequency slowly from LF to HF, through the resonant frequency of L1-C1. [Let's forget about C2 for the moment.] Below the resonant frequency of L1-C1, the L1-C1 circuit acts like a small capacitor, so there is very little throughput. Above the series resonant frequency of L1-C1, the L1-C1 circuit acts like a large inductor, so again there is very little throughput. However, when you hit the series resonance of L1 and C1 (F1), reactance of L1 and C1 cancel. The crystal acts like a short-circuit (or nearly so) and there is a large throughput. Because the L-C ratio is very high, the resonance peak is very sharp. The effect of C2 across the L1-C1 circuit is to produce a second (parallel) resonant circuit. VERY slightly HF of the L1-C1 resonance, C2 resonates with effective inductance of the L1-C1 circuit. This produces a parallel resonant circuit (F2). Another way of looking at it is that L1 resonates with the series combination of C1 and C2 (so F2 must be higher than F1). The parallel resonance is, of course, a high impedance, where there is almost no throughput through the crystal. As a result of this double resonance, the crystal acts as a series-tuned circuit at F1 (one you want), and a parallel-tuned circuit at F2. The transition between the two is very sudden. The frequency response peak of the throughput is very lopsided, and gets chopped off suddenly on the HF side. The difference between F1 and F2 is very small (a few Hz to a few kHz, depending on the frequency and type of the crystal). If F1 is lower than you want, and you add an external series trimmer capacitor to try and pull the crystal L1-C1 series resonance HF, you effectively hit a brick wall with the parallel resonance at F2. The parallel resonance will block any throughput at (or near) this frequency. A possible solution is to neutralize C2. [Note: Neutralization is a technique sometimes required when using VHF crystals, as C2 may be large enough to allow the oscillator to free-run, instead of being locked to the frequency of L1-C1. However, it may also be used with advantage, as described below.] You can neutralize C2 by adding an inductor across the crystal (ie in parallel with C2). The value required is that which parallel-resonates with C2 at the crystal frequency. In effect, C2 no longer exists. With C2 neutralized, there is no longer a sudden transition from the wanted series resonance F1 to the unwanted parallel resonance F2. The peak the response curve of the throughput of the crystal (at F1) is now nice and symmetrical, without the sudden cutoff at F2. In practice, the actual F1 peak will probably be somewhat more HF than before, and the crystal should be more pullable with a series capacitor. Finally, if you reduce the value of the inductor so that its resonance with C2 is somewhat higher than the crystal frequency, this tends to pull the F1 resonance peak even higher in frequency. However, if you overdo this, the oscillation will probably unlock from the crystal, and start to free-run. As I said, sorry for the ramble. Ian. OMG are you kidding, don't be sorry. Thank you way so much!!! :-))) You should set your clock to way in the future and repost that message so it sticks around for a while. ;-) So to make a long story short I need to put an inductor of roughly 400uH across the crystal to cancel the 5pF of C2. Wow that's a ton of inductance, but about 27 turns on a FT50-43 ferrite torroid ought to do it. I'll let you know how that works out. I found another crystal, unfortunately it's identical and possibly from the same batch. I haven't tried it yet, but I'm not expecting any miracles. I'm tired of burning my fingers unsoldering parts, so I'm goint to tinker on the breadboard with another 602 set up just for the oscillator testing with capacitor changes. I will apply the new coil to the soldered up version though. The receiver hears, as we just had a storm earlier and I could hear lightning crashes in the distance. In my narrow tuning range, I can hear what is likely the carrier of a broadcaster too, or maybe my TV. Later tonight when the band opens up some more, I should hear something from W1AW hopefully. Thanks again |
VCXO frequency isn't high enough
|
VCXO frequency isn't high enough
In message , Anthony Fremont
writes Ian Jackson wrote: As I said, sorry for the ramble. Ian. OMG are you kidding, don't be sorry. Thank you way so much!!! :-))) You should set your clock to way in the future and repost that message so it sticks around for a while. ;-) So to make a long story short I need to put an inductor of roughly 400uH across the crystal to cancel the 5pF of C2. Wow that's a ton of inductance, but about 27 turns on a FT50-43 ferrite torroid ought to do it. I'll let you know how that works out. I found another crystal, unfortunately it's identical and possibly from the same batch. I haven't tried it yet, but I'm not expecting any miracles. I'm tired of burning my fingers unsoldering parts, so I'm goint to tinker on the breadboard with another 602 set up just for the oscillator testing with capacitor changes. I will apply the new coil to the soldered up version though. The receiver hears, as we just had a storm earlier and I could hear lightning crashes in the distance. In my narrow tuning range, I can hear what is likely the carrier of a broadcaster too, or maybe my TV. Later tonight when the band opens up some more, I should hear something from W1AW hopefully. Thanks again As you say, at around 3.5MHz, you will need a fairly large inductor to resonate with 5pF. An alternative might be to make a bridge circuit, where you actually use another (5pF) capacitor to balance out the unwanted 5pF. I used to use an extremely simple balancing circuit to make accurate measurements of the resonant frequencies and ESRs (equivalent series resistance) of VHF crystals, and it should be possible to use something similar in an oscillator. However, maybe someone out there can advise on a tried-and-tested circuit which will definitely work. [There's no point in re-inventing the wheel!] Ian. -- |
VCXO frequency isn't high enough
On Wed, 14 Mar 2007 06:26:39 -0500, "Anthony Fremont"
wrote: Arv wrote: My W1AW receiver uses the crystal with just a 5-47 pf variable capacitor...no inductor, and it nets right on frequency. Try shorting Story of my life. ;-) the inductor and see if this gets you closer to the required frequency. Not all color burst crystals were created equal. If you have another crystal you might want to try it. That seems to be the common consensus. I suspect my crystal is just too good. ;-) Your ferrite will not be saturating at the small amount of signal you are sending through it as part of an SA-602 oscillator. Thanks, I know very little about these things. --- Here's a good tutorial: http://www.foxonline.com/techdata.htm -- JF |
VCXO frequency isn't high enough
John Fields wrote:
On Wed, 14 Mar 2007 06:26:39 -0500, "Anthony Fremont" wrote: Arv wrote: My W1AW receiver uses the crystal with just a 5-47 pf variable capacitor...no inductor, and it nets right on frequency. Try shorting Story of my life. ;-) the inductor and see if this gets you closer to the required frequency. Not all color burst crystals were created equal. If you have another crystal you might want to try it. That seems to be the common consensus. I suspect my crystal is just too good. ;-) Your ferrite will not be saturating at the small amount of signal you are sending through it as part of an SA-602 oscillator. Thanks, I know very little about these things. --- Here's a good tutorial: http://www.foxonline.com/techdata.htm Thanks John. :-) I was referring to core saturation and when to suspect it/materials/etc, but I can sure stand to learn a few things more about crystals too. That's pretty good information in the link you posted. If anyone knows about crystals it should be Fox. ;-) I had never tried pulling one high before, only tweaking them down a little to get them on frequency. I can pull this one low several kcs without much of a problem other than stability, but it sure doesn't want to go any higher than about 500Hz above spec. I'm going try the parallel inductance trick to see if I can get the frequency higher, that should prove interesting. I like doing reality vs. theory experiments. ;-) |
VCXO frequency isn't high enough
Just to set the record straight....
Someone suggested that you need to 'neutralize' the parallel resonance so the series resonance can be tuned toward it. This is completely wrong! The series resonance is, for practical purposes, invariant. The motional parameters (L and C) of the series resonance are such high reactances (small capacitance; high inductance) that external components have only a tiny influence on the series resonance. The series resonant frequency is the lower of the two crystal 'resonances'. The parallel resonance is above it. When you make a VCXO with any substantial tuneability, you're probably operating the crystal at its parallel resonance. This leads to the common observation that you can 'pull' a crystal up in frequency more than you you can pull it down. You can only pull the parallel resonance to approach the series resonant frequency, but you can't pass it because the crystal is effectively a short-circuit at that frequency. Also for the record, the crystal's quartz only has one fundamental and significant natural resonance - the series resonance. The so-called 'parallel resonance' is actually a controlled spurious resonance caused by the holder capacitance. At frequencies above series resonanve, the crystal's RLC equivalent looks inductive, and at some frequency the holder capacitance will resonate that net inductance. Joe W3JDR "Ian Jackson" wrote in message ... In message , Anthony Fremont writes Ian Jackson wrote: As I said, sorry for the ramble. Ian. OMG are you kidding, don't be sorry. Thank you way so much!!! :-))) You should set your clock to way in the future and repost that message so it sticks around for a while. ;-) So to make a long story short I need to put an inductor of roughly 400uH across the crystal to cancel the 5pF of C2. Wow that's a ton of inductance, but about 27 turns on a FT50-43 ferrite torroid ought to do it. I'll let you know how that works out. I found another crystal, unfortunately it's identical and possibly from the same batch. I haven't tried it yet, but I'm not expecting any miracles. I'm tired of burning my fingers unsoldering parts, so I'm goint to tinker on the breadboard with another 602 set up just for the oscillator testing with capacitor changes. I will apply the new coil to the soldered up version though. The receiver hears, as we just had a storm earlier and I could hear lightning crashes in the distance. In my narrow tuning range, I can hear what is likely the carrier of a broadcaster too, or maybe my TV. Later tonight when the band opens up some more, I should hear something from W1AW hopefully. Thanks again As you say, at around 3.5MHz, you will need a fairly large inductor to resonate with 5pF. An alternative might be to make a bridge circuit, where you actually use another (5pF) capacitor to balance out the unwanted 5pF. I used to use an extremely simple balancing circuit to make accurate measurements of the resonant frequencies and ESRs (equivalent series resistance) of VHF crystals, and it should be possible to use something similar in an oscillator. However, maybe someone out there can advise on a tried-and-tested circuit which will definitely work. [There's no point in re-inventing the wheel!] Ian. -- |
VCXO frequency isn't high enough
On Mar 16, 5:49 am, "W3JDR" wrote:
The series resonance is, for practical purposes, invariant. The motional parameters (L and C) of the series resonance are such high reactances (small capacitance; high inductance) that external components have only a tiny influence on the series resonance. Yes....this is the point of a VCXO...to allow an almost infinitessimally small, but still useful, variation about the crystal frequency while maintaining most of the crystal's stability. The series resonant frequency is the lower of the two crystal 'resonances'. The parallel resonance is above it. When you make a VCXO with any substantial tuneability, you're probably operating the crystal at its parallel resonance. This leads to the common observation that you can 'pull' a crystal up in frequency more than you you can pull it down. Nearly all VCXO's I've run across work the other way. You can pull the frequency down substantially while maintaining good stability (typically on the order of 0.1%), but not up. This certainly applies to the circuit for which the original poster provided a link. Do you have any examples of practical circuit schematics which use parallel resonance and which can be pulled substantially up in frequency ? I assume it should be possible to do with a parallel inductor, for example in a Franklin oscillator circuit, but as was pointed out the inductor values can be inconveniently large. Steve |
VCXO frequency isn't high enough
In message lDuKh.7998$vV3.3900@trndny09, W3JDR writes
Just to set the record straight.... Someone suggested that you need to 'neutralize' the parallel resonance so the series resonance can be tuned toward it. This is completely wrong! It may be 'completely wrong', but my experience with getting out-of-spec (too LF) VHF overtone crystals up to the required frequency indicates that it does enable the oscillator to work at a slightly higher frequency than it 'wants to'. This is because the throughput peak of the series resonance moves HF when the sudden parallel resonance is removed. [The assumption is that oscillation occurs at the peak of the series resonance, which may not be entirely true.] The series resonance is, for practical purposes, invariant. The motional parameters (L and C) of the series resonance are such high reactances (small capacitance; high inductance) that external components have only a tiny influence on the series resonance. This is more-or-less what I said. The influence of the relatively large series trimmer capacitor will be pretty small. The series resonant frequency is the lower of the two crystal 'resonances'. The parallel resonance is above it. When you make a VCXO with any substantial tuneability, you're probably operating the crystal at its parallel resonance. Lots of technical information calls the actual parallel resonance 'anti-resonance', and indicates that there is an 'area of parallel resonance' between the true series resonance and the spurious parallel resonance. In this area, the impedance of the crystal rapidly changes from being zero (at the series resonant frequency) to infinitely inductive (at the anti-resonant frequency). In many oscillator circuits, the oscillation occurs neither at the series resonant nor the parallel (anti-) resonant frequencies. Instead, the actual frequency of oscillation will be determined by some value of this inductance and the external capacitors, and also on the phaseshift and amplitude of signal throughput through the crystal. All very complicated! This leads to the common observation that you can 'pull' a crystal up in frequency more than you you can pull it down. You can only pull the parallel resonance to approach the series resonant frequency, but you can't pass it because the crystal is effectively a short-circuit at that frequency. And neither can you use external elements to pull the series resonance very far HF, because it runs into the parallel resonance. From my experience, a swept frequency response through a crystal shows that the throughput peak of the series resonant frequency never really reaches its full amplitude before it starts to get pulled down in parallel resonance hole. Neutralizing the shunt capacitance prevents the parallel resonance from occurring so close to the series resonance. As a result, the frequency response throughput curve becomes symmetrical, and the actual peak is somewhat further HF. Certainly, my oscillators (which were supposed to operate at the true series resonance of the crystal) DID move HF when I neutralized the crystal. [Note that the full frequency response of a crystal with a parallel neutralizing inductor, from DC to well above the crystal frequency, consists of a broad notch centred on the crystal frequency (the parallel resonance of the parallel capacitance of the crystal and the neutralizing inductor). In the centre of the notch is a very narrow bandpass (the series resonance of the crystal).] Also for the record, the crystal's quartz only has one fundamental and significant natural resonance - the series resonance. The so-called 'parallel resonance' is actually a controlled spurious resonance caused by the holder capacitance. At frequencies above series resonanve, the crystal's RLC equivalent looks inductive, and at some frequency the holder capacitance will resonate that net inductance. Exactly so. At the parallel (anti-) resonance, the reactance of the crystal suddenly jumps from being infinitely inductive to being infinitely capacitive (0p). As you move further HF, it stays capacitive, progressively decreasing in reactance. The parallel resonance therefore presents a brick wall, beyond which external capacitors cannot resonate with the inductive reactance of the crystal. However, if you neutralize the crystal, you kill the sudden transition from series to parallel resonance, and the frequency range over which the crystal is inductive is considerably extended. This should enable the resonance with external capacitors to extend further HF than when the crystal is not neutralized. As I originally said, neutralization of the crystal was a suggestion, rather than a panacea. I still reckon that should work. It's worth a try. Unfortunately, the size required for the inductor (which resonates with the crystal parallel capacitance of appx only 5pF) is rather large. If neutralization DOES help, a brute force method of allowing a somewhat smaller inductor to be used would be to deliberately add MORE parallel capacity, and lower the value of the inductor to suit. A more elegant method would be to build the crystal into a simple bridge circuit, so that a neutralizing capacitor could be used instead of an inductor. However, I appreciate that the object of the exercise is to make a simple receiver, and it would be somewhat incongruous to need a very complicated circuit just for the crystal. Ian. -- |
VCXO frequency isn't high enough
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VCXO frequency isn't high enough
On Mar 13, 10:02 am, "Anthony Fremont" wrote:
Hello all, ... supposed to pull the colorburst crystal high in frequency then the adjustable cap should be able to tweak it back down. It's only possible to pull a crystal a little way from its center frequency by external components. If you put a resistor in parallel with the crystal (to lower its Q) you can increase the pull range. |
VCXO frequency isn't high enough
On Fri, 16 Mar 2007 22:51:34 GMT, "colin"
wrote: The reason for the parallel inductance in the overtone mode is the low impedance of the crystal self capacitance at high frequencies, this on its own can be quite low, and the series resistance at overtone can be quite high, so this can allow the tank circuit to dominate rather than the crystal unless it is cancelled. I know why the inductor is usually necessary. The thing is that for the purpose of the Butler the effect of C0 can be neutralized. The question is whether it can be made to have a similar effect in the "pulling arena" as well. Of course it should not have to much side effects in normal operation of the oscillator. In the overtone butler there is also another LC resonance circuit present that determines the possible operating frequency (read desired overtone). Such a thing might be necessary in most circuits where an inductor is placed in parallel with a crystal. Joop |
VCXO frequency isn't high enough
In message , Joop
writes On Fri, 16 Mar 2007 22:51:34 GMT, "colin" wrote: The reason for the parallel inductance in the overtone mode is the low impedance of the crystal self capacitance at high frequencies, this on its own can be quite low, and the series resistance at overtone can be quite high, so this can allow the tank circuit to dominate rather than the crystal unless it is cancelled. I know why the inductor is usually necessary. The thing is that for the purpose of the Butler the effect of C0 can be neutralized. The question is whether it can be made to have a similar effect in the "pulling arena" as well. Of course it should not have to much side effects in normal operation of the oscillator. In the overtone butler there is also another LC resonance circuit present that determines the possible operating frequency (read desired overtone). Such a thing might be necessary in most circuits where an inductor is placed in parallel with a crystal. Joop I've eventually found some good plots of the frequency responses of crystals (see below). http://g4oep.atspace.com/crystal_fil...rystal_filters ..htm#4)%20Single-Crystal%20Ladder. This info from G4OEP is about filters, rather than VXOs, but it graphically illustrates how a crystal can be neutralised in a bridge circuit, using a small capacitor rather than large inductor. The capacitor will be the same value as the parallel capacity of the crystal. The plots in Figs 9, 10 and 12 also show how the throughput peak moves around to some extent as the neutralization is adjusted. The circuit is essentially the same as I used when I was testing a load of crystals to see if they met spec wrt frequency accuracy and ESR. You should be able to use the circuit of the single crystal filter (in-to-out, without the resistive padding etc) as the resonant part in a VXO, and adjust the neutralization to pull the frequency of oscillation. It's worth a try. Ian. -- |
VCXO frequency isn't high enough
"W3JDR" wrote in message news:lDuKh.7998$vV3.3900@trndny09... Just to set the record straight.... Someone suggested that you need to 'neutralize' the parallel resonance so the series resonance can be tuned toward it. This is completely wrong! The series resonance is, for practical purposes, invariant. The motional parameters (L and C) of the series resonance are such high reactances (small capacitance; high inductance) that external components have only a tiny influence on the series resonance. The series resonant frequency is the lower of the two crystal 'resonances'. The parallel resonance is above it. When you make a VCXO with any substantial tuneability, you're probably operating the crystal at its parallel resonance. This leads to the common observation that you can 'pull' a crystal up in frequency more than you you can pull it down. You can only pull the parallel resonance to approach the series resonant frequency, but you can't pass it because the crystal is effectively a short-circuit at that frequency. Also for the record, the crystal's quartz only has one fundamental and significant natural resonance - the series resonance. The so-called 'parallel resonance' is actually a controlled spurious resonance caused by the holder capacitance. At frequencies above series resonanve, the crystal's RLC equivalent looks inductive, and at some frequency the holder capacitance will resonate that net inductance. Joe W3JDR "Ian Jackson" wrote in message ... In message , Anthony Fremont writes Ian Jackson wrote: As I said, sorry for the ramble. Ian. OMG are you kidding, don't be sorry. Thank you way so much!!! :-))) You should set your clock to way in the future and repost that message so it sticks around for a while. ;-) So to make a long story short I need to put an inductor of roughly 400uH across the crystal to cancel the 5pF of C2. Wow that's a ton of inductance, but about 27 turns on a FT50-43 ferrite torroid ought to do it. I'll let you know how that works out. I found another crystal, unfortunately it's identical and possibly from the same batch. I haven't tried it yet, but I'm not expecting any miracles. I'm tired of burning my fingers unsoldering parts, so I'm goint to tinker on the breadboard with another 602 set up just for the oscillator testing with capacitor changes. I will apply the new coil to the soldered up version though. The receiver hears, as we just had a storm earlier and I could hear lightning crashes in the distance. In my narrow tuning range, I can hear what is likely the carrier of a broadcaster too, or maybe my TV. Later tonight when the band opens up some more, I should hear something from W1AW hopefully. Thanks again As you say, at around 3.5MHz, you will need a fairly large inductor to resonate with 5pF. An alternative might be to make a bridge circuit, where you actually use another (5pF) capacitor to balance out the unwanted 5pF. I used to use an extremely simple balancing circuit to make accurate measurements of the resonant frequencies and ESRs (equivalent series resistance) of VHF crystals, and it should be possible to use something similar in an oscillator. However, maybe someone out there can advise on a tried-and-tested circuit which will definitely work. [There's no point in re-inventing the wheel!] Ian. -- |
VCXO frequency isn't high enough
On Tue, 20 Mar 2007 12:08:57 +0000, Ian Jackson
wrote: You should be able to use the circuit of the single crystal filter (in-to-out, without the resistive padding etc) as the resonant part in a VXO, and adjust the neutralization to pull the frequency of oscillation. It's worth a try. Ian. The images primarily showed how the stopband notch moved around. So I decided to put it into spice. Compared to L compensation it does seem to have a benefit as higher frequency peak. It also is not troubled by the side-effect of L-compensation as passing lower and higher frequencies than those around the crystal frequency. Trying to make the most of the balancing compensation I placed a small capacitor in series with the crystal. This moves the pass band frequency up a bit more. But the smaller the series cap, the less pronounced the peak seems to be. Also the circuits starts to attenuate more and more. This might cause difficulty in an oscillator setup where the loop gain should stay more than one. Also with the balanced compensation circuit, the phase is changed around the peak frequency. Without series cap around -45 degrees, climbing to -83 with 12pF. This should be accounted for in the feedback loop of an oscillator. (A properly dimensioned L-compensated crystal does not change phase.) But all in all it might be a (complex) method of shifting the working frequency up. Joop |
VCXO frequency isn't high enough
"Anthony Fremont" wrote in message ... Jamie wrote: You need to reduce the inductor.. that is causing a down swing in your freq. Thanks, I took it out and the frequency increased by only about 70Hz, but it did increase. :-) Do you know of anything else I can do to increase the frequency by about another couple of kcs? Get the right freq crystal. Jimmie |
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