The crystal has higher Q, not the inductor.Because Q is higher for inductor.
Higher Q of oscilator, higher frequency.
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.
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.
Did you verify that the circuit is not working with fundamental crystals > 24 MHz or is it just a guess?
Higher Q of oscilator, higher frequency.
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?
I guess that downscaling the capacitor values of the colpitts oscillator can increase the workable frequency range.
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
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.
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.
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