Frequency crystal motional parameters

 

The motional inductance (Lm) and motional capacitance (Cm) of radio frequency crystals are of importance when designing crystal filters for narrowing bandwidth of a device (usually a radio). 

I soldered a circuit together and used a test setup invented by a HAM (Hobby Amateur Radio) guy. The math looked scary but you just plunk in your numbers. 

You test 50 or so crystals and then group the 10 most similar ones and use them.

Cs = size of the capacitor in the circuit.

Co = measure the capacitance of the crystal with a meter.

^f = the delta (change) between measuring the crystal with and without the capacitor switched on in the circuit. Subtract one frequency from the other.

F = the first capacitor off in the circuit measurement.

            Capacitance of the crystal by itself:

Little circuit board kit I soldered up (not the capacitor on/off switch to the left):

Frequencies with/without the capacitor turned on:

The motional parameters of that radio frequency crystal (I was off by some decimal places, lol):

Crystal Testing Circuit

Crystal Testing Circuit 

Quick oscillator circuit, powering a 16MHz crystal. 

Powered by 2.4vDC:

Hex inverter pinout:

Breadboard circuit:

16MHz on the frequency meter:

It worked dead on with a 16MHz crystal, ok with slightly higher frequency crystals but was unstable at lower frequencies. Just need to adjust capacitors and resistors.

This uses a 74hc04 hex inverter IC chip.