[Lowfer] Beacon Schematic

[J D]

 >>> What is the purpose of the resistor between the crystal and pin 10?

That one has three main functions, each of which affects stability in 
one way or another. It is often omitted in microprocessor applications 
if precision is not too critical, but generating stable RF for slow 
transmission modes is pretty demanding by comparison. (Also, I am given 
to understand some uP chips have those resistors built in.)

The series resistor limits current, which is important for crystals in 
certain frequency ranges, especially when operated at higher voltages. 
In extreme cases, excess current can damage the crystal, but that is 
rare in a circuit like this. It's still better for a crystal not to be 
subject to any more internal heating than necessary, though.

Second, it provides a little isolation from the complex and often poorly 
defined output impedance of the gate.

Third, and probably most important for our purposes, setting the value 
of the series resistor roughly equal to Xc of that first capacitor (at 
the crystal fundamental frequency) gives it the appropriate voltage lag 
to meet the phase criteria for oscillation.  Without it, the external 
feedback path will "adjust" the operating frequency as necessary to 
achieve the required 180 degree total shift between pins 10 and 11, but 
it will no longer occur precisely at the crystal's natural resonance.

Regards,
John Davis