VCXO frequency isn't high enough

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

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.

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.

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

Yeah, I probably expect too much from it. On a side note, IT WORKS. I can hear stations, but the noise (man made) is something awful. It really needs a narrow audio filter.

I found some stuff on those, the look pretty neat. I may have to head to the dollar store and see what I can find. ;-)

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.

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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. ;-)

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

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.

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

In message , W3JDR writes

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.]

This is more-or-less what I said. The influence of the relatively large series trimmer capacitor will be pretty small.

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!

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).]

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.

I have to agree with Joe. Basically there is no such thing as an crystal oscillator in "parallel resonance". However there are oscillators that use the crystal in the feedback path to add substantial phase shift. Such as in the Pierce oscillator where it behaves inductive. The phase shift changes so rapidly that it can still make a low-drift oscillator.

In the book by Matthys where he compares various oscillators there is one in chapter 10.6 where the deviation from the (series) resonant point is the highest. It is a circuit where the crystal sees a very high impedance as opposed to regular circuits where highest Q is obtained with very low impedance. This in effect makes the crystal load to be around C0 of the crystal with some output and input capacitance. And therefore it is probably the smallest effective physical series capacitance obtainable and thus the highest frequency.

Looking up the Franklin oscillator you mentioned, I notice this also is providing a high impedance to the resonant elements. So yes, it seems a valid way of implementing an alternative to Matthys' example.

Now also cancelling the effect of C0 of the crystal by adding parallel inductance might push it a bit further. Right now I would not be able to predict the effect on loaded Q of the crystal. Lowering Q is normally not done, but in this case we are primarily in quest for wide pulling range right?

In a low impedance Butler (overtone) oscillator I have seen C0 cancellation by using parallel inductance as well. There sometimes is an L plus series R used to lower the Q of the L/C0 combination. This seems not appropriate for a high impedance oscillator circuit. I would expect best effect if the Q of the inductor is high (low Rs).

Sorry this is still theory. I have no examples of VCXO in this context.

Cheers,

Joop

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.

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.

Not sure about the resistor not seeing the circuit.

the statement all crystal circuits operate in series mode stems from the fact that the internal eq circuit of a crystal is a series LC, any capacitance in parallel with the crystal can only be in series with the internal motional capacitance.

If your lucky you may be able to find a spurious node, but it would be hard to make it oscillate at this point.

Colin =^.^=

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

In message , Joop writes

I've eventually found some good plots of the frequency responses of crystals (see below).

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.

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

Get the right freq crystal.

Jimmie