[Lowfer] Beacon Schematic

[jrusgrove at comcast.net]

Good info Dave ... thanks for posting.

Jay W1VD




----- Original Message -----
From: Dave ZL3FJ <2c39a at silverbears.nz>
Reply-To: Discussion of the Lowfer (US, European, & UK) and MedFer bands <lowfer at mailman.qth.net>
To: 'Discussion of the Lowfer (US, European, & UK) and MedFer bands' <lowfer at mailman.qth.net>
Sent: 11/24/2020 10:54:15 PM
Subject: Re: [Lowfer] Beacon Schematic
________________________________________________________________________________

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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