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From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: Voltage Controlled Xtal Oscillator
Summary: Multivibrator is spectrally impure (not good for crystal)
Keywords: Oscillators VCO VCXO
Message-ID: <8071@milton.u.washington.edu>
Date: 25 Sep 90 02:50:56 GMT
References: <935@hls0.hls.oz>
Organization: University of Washington, Seattle
Lines: 31


In article <935@hls0.hls.oz> geoff@hls0.hls.oz (Geoff Bull) writes:
>
>I want to build a 60 MHz VCXO.
>Can I simply replace the capacitor on a MC1658 VCO with a capacitor
>and crystal in series?
>
        Not really.  The 1658 is a multivibrator, and few (if any)
crystals will operate reliably with all the frequencies that it'll
be generating (it MIGHT work if you put a series LC in, to limit the
out-of-band power).  Usually, one uses a damped crystal (put a resistor
across it if you have to), with a simple oscillator like

                |
  +-------------+
  |             |
  |            /
  +--|O|--+---|    NPN transistor
  |       |   |
  =       =    \
  |       |     |
 GND     GND   GND

	Resistors omitted to confuse the neophytes.

(I hope the ASCII graphics isn't too terrible).  Tuning would be by using
a varactor diode/capacitor combination for one of the two capacitors.
As nearly as I can remember, some crystals come specified for VCO
operation (usually very narrow band).

                        John Whitmore