[Lowfer] Beacon Schematic

[John Hamer]

John,

Great answer and thank you!

Regards,
John Hamer
Lowfer JH

On Tue, Nov 24, 2020, 6:11 PM J D <listread at lwca.org> wrote:

>  >>> 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
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