04-08-2022, 08:20 PM
Put 10K in series with crystal to gnd. Scope across crystal. Drive the 10K with a signal gen at 10 MHz to 45 MHz. If the crystal is O/C you'll see no effect. If there is something between about 10MHz and 15 MHz, then it's a 3rd overtone crystal. A 5th Overtone is unlikely, but you can try 5MHz to 10MHz too. It's impossible to predict what the fundamental frequency is for an overtone mode as it's the physical shape of the crystal.
See https://en.wikipedia.org/wiki/Tuning_fork to understand why an overtone is not a integer number of fundamental as harmonic multipliers always are.
Sub-miniature crystals actually are tuning forks, which allows a low frequency in a small package. Higher frequencies often use a disk or rectangular piece of quartz.
Overtone circuits
https://www.robkalmeijer.nl/techniek/ele...index.html
Overtone crystals are not well suited to a combined modulator-oscillator, but the oscillator is best driving a separate mixer.
If you want a combo single device modulator/mixer and oscillator you need a fundamental crystal, but even then there is risk of oscillation ceasing at lower carrier levels (greater modulation depth) and FM effects at higher levels.
See https://en.wikipedia.org/wiki/Tuning_fork to understand why an overtone is not a integer number of fundamental as harmonic multipliers always are.
Sub-miniature crystals actually are tuning forks, which allows a low frequency in a small package. Higher frequencies often use a disk or rectangular piece of quartz.
Overtone circuits
https://www.robkalmeijer.nl/techniek/ele...index.html
Quote:Most overtone crystals, being of the plated variety, are incapable of dissipating much heat. This means that the crystal oscillator must be operated at low power level, and with no more feed-back than is necessary to maintain good starting characteristics under load. The crystal oscillator should never be thought of as a power-generating device, and this is particularly true of overtone oscillators. The oscillator should generate a stable signal; stepping up the power should be left to succeeding stages.Many Crystals marked at over 30Mhz are actually overtone, so without a crystal osc circuit for the required parallel or series overtone mode it may run at the fundamental or not at all.
When fundamental crystals are used on overtones, the frequency of oscillation may not be an exact multiple of the marked frequency. And the frequency will be different for series or parallel resonance. Moral: When working anywhere near band edges, have some accurate means of checking frequency; a crystal marking is no guarantee that you will be inside the band.
The common test for self-oscillation, pulling out the crystal to see if oscillation stops, is not applicable to most overtone circuits. The capacitance of the crystal and its holder is a part of the feed-back circuit. If there is self-oscillation present, it will almost invariably stop when the crystal is removed.
Overtone crystals are not well suited to a combined modulator-oscillator, but the oscillator is best driving a separate mixer.
If you want a combo single device modulator/mixer and oscillator you need a fundamental crystal, but even then there is risk of oscillation ceasing at lower carrier levels (greater modulation depth) and FM effects at higher levels.
