Re: Variable Capacitance and Inductance

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

Tesla list wrote:
> 
> Original poster: "David Thomson by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <dave-at-volantis-dot-org>
> 
> Hi Gary,
> 
> Radios are tuned to crystals precisely because inductor/capacitor only
> circuits do drift.
> 
> Dave

	Many (but by no means all) "modern" radios and TV's use frequency
synthesizers whose frequency is controlled by a crystal.  The reason is
one of cost, not stability.  It's cheaper to build a synthesizer (chip
less than a buck) with a crystal (perhaps half a buck) than to build a
good L-C variable frequency oscillator.  Up until the last few years
that wasn't the case, and probably literally billions of very
satisfactory radios and similar devices were built without crystal
control.  With care it's not hard to get a stability of one part in
10,000 (100 parts per million) with a good "VFO", which is at least as
good as a mediocre crystal like a typical computer clock crystal.

	Your typical "cheapie" transistor radio uses an ordinary L-C tuned
circuit.  Take one apart and look at it.

Ed

Ed