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Oscillator range issue. Coil VS crystal

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neazoi

neazoi

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Hi, the oscillator section of my transmitter **broken link removed** covers 1.8MHz to 24MHz using crystals.
If I replace the crystal with an inductor is can cover frequencies from <1.8MHz to 40MHz.
What is the reason for this difference?
Why the use of the coil make the oscillator oscillage at higher frequencies, whereas the crystals cannot?
Maybe it is the problem of the crystals?
 

very interesting question.
my view is that ,it may not be due to problem of crystals.

when you substitute a crystal in your oscillator ,
you are introducing a series resonant circuit.

when you substitute a coil , its inductance is
part of the resonant circuit that you have designed.

so taking all the circuit components into consideration , the condition is not supportive
for the series resonace into oscillation beyond 24MHz.
 
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    neazoi

    neazoi

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Because Q is higher for inductor.
Higher Q of oscilator, higher frequency.
 

tony_lth said:
Because Q is higher for inductor.
Higher Q of oscilator, higher frequency.
Click to expand...
The crystal has higher Q, not the inductor.

- - - Updated - - -

srizbf said:
very interesting question.
my view is that ,it may not be due to problem of crystals.

when you substitute a crystal in your oscillator ,
you are introducing a series resonant circuit.

when you substitute a coil , its inductance is
part of the resonant circuit that you have designed.

so taking all the circuit components into consideration , the condition is not supportive
for the series resonace into oscillation beyond 24MHz.
Click to expand...

So what if I add a series inductor with the crystal? This might "fool" the oscillator so that it can oscillate beyond 24MHz?

Initially I thought that there is a problem with the crystals, because all crystals above 20MHz or so, are harmonic. Also, I am not sure, but I suspect that the less the crystal material, the higher the loss, so the less signal out of it? I am not sure about it.
 

So what if I add a series inductor with the crystal? This might "fool" the oscillator so that it can oscillate beyond 24MHz?
Click to expand...

i think you can, but not sure.
now the added inductor is in series with the crystal series circuit. knowing the internal parameters of the crystal , the additional inductance value is to be decided.
whether combined one will be as stable as original , you may have to do experiment.

Initially I thought that there is a problem with the crystals, because all crystals above 20MHz or so, are harmonic. Also, I am not sure, but I suspect that the less the crystal material, the higher the loss, so the less signal out of it? I am not sure about it.
Click to expand...

this reasoning sounds logical for me.
since harmonics have less amplitude , your circuit may not be able to pickup.
 

Did you verify that the circuit is not working with fundamental crystals > 24 MHz or is it just a guess?
 

FvM said:
Did you verify that the circuit is not working with fundamental crystals > 24 MHz or is it just a guess?
Click to expand...

As it is the circuit does not work with crystals above 24MHz. Only if I alter the gain loop capacitors, oscillator starts again. However, without altering any capacitors and when using an inductor in place of the crystal, oscillation is reliable to 40MHz. That is the weird thing to me, but I believe it proves that the oscillator is capable of oscillating at higher frequencies without altering any components. Either way, the original schematic has been coppied from an article that claimed the oscillator to operate on 1-30MHz crystals.
 

I believe you know that the impedance of a crystal in resonance is different from an inductor.

Why not accept that the circuit parameters must to be adjusted?
 

FvM said:
I believe you know that the impedance of a crystal in resonance is different from an inductor.

Why not accept that the circuit parameters must to be adjusted?
Click to expand...

So it is an impedance problem you guess?
 

I guess that downscaling the capacitor values of the colpitts oscillator can increase the workable frequency range.
 

FvM said:
I guess that downscaling the capacitor values of the colpitts oscillator can increase the workable frequency range.
Click to expand...

Yes, I have tried so and it worked. The question is why when using an inductor no downscaling of the capacitors is needed for the oscillator to iscillate?
Is it the different crystal impedance maybe as you said?
 

Years ago I spent some time testing crystals, the good ones had an ESR* less then 100 ohms, bad ones higher. Perhaps this figure rises for higher frequency crystals and not for low value inductors.
* Equivalent Series Resistance
Frank
 
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chuckey said:
Years ago I spent some time testing crystals, the good ones had an ESR* less then 100 ohms, bad ones higher. Perhaps this figure rises for higher frequency crystals and not for low value inductors.
* Equivalent Series Resistance
Frank
Click to expand...

It shounds logical. The crystals above 20MHz or so, showed less output signal and this might be due to the possible higher ESR. I had to tune the source bias potentiometer to alter the gain of the oscillator, to compensate for this loss. However, the gain loop caps may play a role on this, so it is hard to tell.

However, I also have some (very few) crystals at 17MHz that refuse to oscillate, so the gain loop caps do not play a big role on this frequency. These crystals are mostly the shorter packages not the longer HC-49, so they are probably tuning fork crystals (guess).
 

Please notice that even if the crystal ESR is constant over frequency, it results in lower circuit Q if the capacitor values aren't scaled down with increasing frequency.
 
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FvM said:
Please notice that even if the crystal ESR is constant over frequency, it results in lower circuit Q if the capacitor values aren't scaled down with increasing frequency.
Click to expand...

It shounds like a combination of both factors. That is why when I decrease the gain loop capacitors, I can make the oscillator oscillate in a bit higher frequencies using crystals.

However the capacitor thing alone, does not explain why when inserting an inductor, everything works fine without scaling. My only guess it the ESR factor mentioned.
 

Most of the crystals (> 90% of them) with frequencies higher than 24 MHz are not fundamental crystals, and they are overtone.
Usually overtone oscillators requires a different oscillator topology, other than Colpitts that you use. Butler or Pierce are preferred for overtone.
Try to probe the oscillation output from the source of the oscillator transistor (BF246C) and might be a chance to get oscillations when replace the inductor with crystals higher than 24 MHz.
 

vfone said:
Most of the crystals (> 90% of them) with frequencies higher than 24 MHz are not fundamental crystals, and they are overtone.
Usually overtone oscillators requires a different oscillator topology, other than Colpitts that you use. Butler or Pierce are preferred for overtone.
Try to probe the oscillation output from the source of the oscillator transistor (BF246C) and might be a chance to get oscillations when replace the inductor with crystals higher than 24 MHz.
Click to expand...

Interesting, I will try that!
My initial guess was the thing about overtone crystals you mentioned. I will be happy if this is the case.

Another thing, is to try to find a crystal above 24MHz that is fundamental. If this oscillates, then we will be sure that this is the case.

The author of the original oscillator circuit, claimed that it was able to oscillate with crystals from 1-30MHz. However in the same article, he has a picture of crystals inside the same noce enclosure, like they have been ordered from the same company. See here https://www.ab4oj.com/test/imdtest/main.html at about the half of the page.
This make me believe more that the crystals he has used were special fundamental.
What do you think?

Any cheap source of fundamental crystals above 24MHz to try?
 

Today you can find fundamental crystals up to 150 MHz, but they are expensive. As I mentioned there is a small percent of usual crystals that oscillate on fundamental up to 30 MHz (less than 10%). For example all of the old 27 MHz CB band crystals, actually were the third overtone of 9.xxx MHz. I've seen people making 9MHz filters using CB crystals.
The easiest way to distinguish between an overtone and a fundamental crystal is that a fundamental crystal will never oscillate on a sub-harmonic frequency.
 
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    neazoi

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If you look for small SMD crystals, 50 or even 80 MHz fundamental mode is pretty standard. Their crystal disc is much smaller than with classical can packages.
 
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    neazoi

    neazoi

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