Quartz Crystal Specification Confusion.

A crystal looks like a series LC in parallel with a capacitor. The series LC characteristics come from the chuck of quartz, while the parallel capacitance comes from the metalization and case.

When it's operated in series resonant mode the only real determinant is the series LC part -- so in theory things are nice and stable. When it's operated in the parallel resonant mode the series LC part must supply some inductive reactance to counter the capacitive reactance of the external crystals. As this external capacitance (and the case capacitance) changes, the resonant frequency changes also.

So at resonance the crystal just looks like an LC circuit, with the crystal acting as the inductor and the external capacitance acting as the capacitor.

This was all a very long way to lead up to the following: Consider two circuits. Circuit A is a series-resonant LC circuit, working into a very low impedance -- what is it's resonant frequency? Circuit B is a parallel-resonant LC circuit, working into a very high impedance -- what is _it's_ resonant frequency?

.-------o-----o | | --- | C --- | | .-. R = | | | really low .-------o--------o------o | | | | | | | '-' | | .-. C| | C| --- | | R = L C| | L C| C --- | | really high C| | C| | '-' | | | | | | | | | === === === === === GND GND GND GND GND

Circuit A Circuit B (created by AACircuit v1.28.6 beta 04/19/05

Next question: Does it matter whether the author thinks the circuit is "really" in series or parallel?

I am probably wrong but, just to confuse the issue, one thing you might want to think about is wether you are dealing with a series or parallel 'cut' crystal. That refers to the way the crystal is dealt with mechanically with hugely massive whirling blades bathed in blood for cooling that splashes all over the place as they grind grindingly through the very body of the pure crystal of life.

All horribly horror show.

Anyway, I'm not sure about this, but one way after you metalllise the ends, after treatment with the acidic, in boiling vats of viciously hot metal that will scar ten thouasand fanatics of the ancient days such that they will bow down to the commands as given by those who are truly worthy...

Cough

One way, the houses are lined up so you excite the streets and the other way they are lined up so you jump up and down on the roofs. Yea verily such is the way that the pure can be selected or we can produce overtones. Such it was moted, let it moted be.

Let us pray.

DNA

The crystal looks like a series LC resonator, therefore the external capacitance is always in series with the internal motional capacitance (wich is extremly small), even though it maybe in parallel with the crystal itself.

Colin =^.^=

Picture any two terminal device. How do you connect another two terminals device to it, without the result being seen as either a pair of devices in parallel, or a pair of devices in series? The terms series and parallel, in this case, refer to the resonance mode. Series resonance hits minimum and real impedance at resonance, while parallel resonance hits real and maximum impedance, when in resonance. A crystal is a series resonator, all by itself. You must connect capacitance, effectively across it to produce parallel resonance across the crystal.

Yes, and with a specific value of capacitance, to achieve the specified resonant frequency.

Well, none small enough to affect the resonant frequency, which is based on a mechanical capacitance it the femto farad range.

Yes. It is intended to be operated so that it achieves minimum real impedance.

Yes. Intended to operate as a parallel resonance, that achieves maximum and real impedance at resonance.

The varactor acts as a variable parallel capacitor, as far as the resonance goes. It may be in series with some larger capacitance to close the loop around the crystal.

If you want the filter to have a tunable resonance near the crystal's specified resonant frequency, then you need one specified for parallel resonance, so that you can trim the exact resonance on either side of that frequency, with a variable capacitance that varies through the specified load capacitance.

Yes... of course!!

'If it had been a snake, it would have bitten me..'

Thanks. Sometimes the obvious is hard to see for some of us. Jim WB5KYE

There's a heck of a lot of confusion about this.

A crystal is a crystal is a crystal. It has no intrinsic "series" or "parallel" flavor.

If you hook a crystal up between a signal generator and a voltmeter, and slowly and carefully tune the generator across the band, you'll notice a peak, that's where the crystal is acting a a series-tuned circuit, presenting a low impedance at a very narrow frequency band (often just cycles wide), then a little ways off, the voltmeter goes to darn near zero over another narrow band-- there the crystal is acting as a very high impedance, a parallel resonance.

To get the crystal to osciallte in its series mode, you have to put it between the output and the input of an amplifier-- feeding back CURRENT, which will preferentially happen at it's low impedance point, its series resonance. If you see a crystal with both ends above ground, that's usually a series-resonance oscillator circuit.

To get the crystal to oscillate in its parallel mode, you have to put it between the output and the input of an amplifier-- feeding back VOLTAGE , which will preferentially happen at it's HIGH impedance point, its parallel resonance. If you see a circuit where the crystal has one side at ground, that's usually a parallel oscillation circuit.