[Lowfer] Beacon Schematic

[John Hamer]

Thanks Dave.

 I guess the devices I have used already had the resistors in them.

Regards,
John Hamer
Lowfer JH

On Tue, Nov 24, 2020, 10:54 PM Dave ZL3FJ <2c39a at silverbears.nz> wrote:

> For reference, I put a copy of a Rakon design note about this oscillator
> configuration in my Dropbox here-
>
>
> https://www.dropbox.com/s/x65jfhbg46ab85i/IC_CRYSTAL_OSCILLATOR_CIRCUITS.pdf
> ?dl=0
>
> 73
>  Dave, ZL3FJ
>
> -----Original Message-----
> From: lowfer-bounces at mailman.qth.net [mailto:
> lowfer-bounces at mailman.qth.net]
> On Behalf Of J D
> Sent: Wednesday, November 25, 2020 12:10
> To: lowfer at mailman.qth.net
> Subject: Re: [Lowfer] Beacon Schematic
>
>  >>> 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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