In the ARRL (2011) Section 9.5.4 (talking about a series mode quartz crystal oscillator) it says:

To build a practical oscillator from this circuit, choose an inductor with a reactance of about 300 Ω at the wanted frequency and calculate C1 in series with C2 to resonate with it. Choose C1 to be 3 to 4 times larger than C2.

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This is the schematic they provide.

1) Is there a reason for choosing 300 ohms instead of say 1k?

2) Does the choice of reactance depend upon whether you want to drive? the crystal at an overtone


It's all about XL relative to the 1 kohm resistor.

If XL is too high in value relative to the 1 kohm, then the tank circuit will be too low in Q and the circuit may not oscillate. If XL is too low, the Q will be higher and there may be too much distortion in the sine wave output. Also, with a highly resonant tank it may be more troublesome to align its resonant frequency with that of the crystal i.e. it may also be more fiddly to set up as well as producing a more distorted output when aligned.

Also, there is no great benefit taking XL much lower in value than that needed for reasonable sine wave oscillation.

I recommend that you use a free sim tool and play around with values. You can mimic the crystal if you search on-line for an equivalent circuit.

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  • \$\begingroup\$ I'd only add that this circuit's "output" should be attached to a high-impedance load, otherwise oscillations may cease (because loop gain falls too low). A low-impedance load might attach to Q1's emitter, or to the junction of C1,C2 \$\endgroup\$ – glen_geek Aug 6 '18 at 15:12

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