Hi
What result does selecting the wrong capacitance have? Does it skew the frequency, alter the shape of the waveform produced, or something entirely different?
Best Regards.
Thank you very much for your helpful answers.
This depends on the application of the crystal. If you are using it for a clock then the fo will decrease with too much capacitance in increase with too little. Typical errors are any where from 20 to 100ppm/pF so if you need good clock accuracy, you should be careful with the capacitance (this includes stray capacitance from your PCB as well). Also, if you go too far away from the resonant frequency the crystal may fail to oscillate at all or go into spurious mode.
For a quartz microbalance it is not so critical since they are often "driven" at frequencies other than the resonant frequency. Overtone driving is common. Typically you are trying to match the impedance of the RF driver output.
For an actuator (like a speaker) it is even less important since you will most likely be driving the crystal with a range of frequencies. You may want to select a capacitor that prevents oscillations for this application.
Asymmetric oscillations are not likely in crystal oscillators so a capacitor mismatch isn't going to change the wave "shape", just the amplitude.
My Dear Colleague'
The answer, it could be:
May not oscillate.
May not oscillate reliably.
May have low to very low output.
May oscillate on wrong frequency (may be an undertone or an overtone).
For extra points may jump between modes (not usual)
May start up very slowly. And/or not always.
May produce less sinusoidal waveform than expected.
May shatter crystal (not usual in modern applications).
May work perfectly.
Factor of 2 in capacitance usually OK, but not always.
Note that some ceramic cap compositions are far worse in tempco, initial accuracy and general stability than others.
Depends on IC used, layout, how Murphy is feeling and phase of Moon.
Layout can be extremely important. Loop area enclosed by capacitor and crystal leads forma part of the oscillator circuit. Manufacturers often give quite specific advice on crystal and capacitor layout. Note it.
Crystal oscillators approach more closely to analog and to black magic than sane people want to get on a good day. In many cases they "just work" and the arcanery is well hidden. You MAY find that capacitance that is off by a factor or 2 or even 5x still works OK. Or, you may not. When they don't work as hoped they can be most obdurate obstinate obnoxious odious and generally overwhelmingly annoying.
Attached is a good article" Title: The effect of load capacitor on the crystal", you might find it useful. you might see the rests too.
Well Malek , capacitors if used as load are primarily filters . Effectively they smooth the waveform, so no skew but removes them .
Specifically in terms of controls they are attributed as a pole in the system design(s- domain-Frequency). These poles (depending how many capacitors you use) help you in looking at stability and coupling to next stage .
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