[Milsurplus] [Glowbugs] Quick n dirty crystal selectivity - it no workee

[Bill Carns]

Also this seems like the long way around Robin's Barn.

Seems like the right thing to do with that crystal is to insert it in the
signal path.  I think they call that a "crystal filter".

:-)

-----Original Message-----
From: Milsurplus [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of
AKLDGUY .
Sent: Monday, February 15, 2016 8:47 PM
To: milsurplus <milsurplus at mailman.qth.net>
Subject: Re: [Milsurplus] [Glowbugs] Quick n dirty crystal selectivity - it
no workee

> Many years back, i had an idea for improving the selectivity of the 6 
> - 9 MHz 'Command Sets' receiver.
> The IF is 2830, IIRC.  I took a crystal from an old 2-MHz boat radio,
> 2837 kHz, and put it across the first IF cathode resistor, with the 
> bypass cap formerly there, disconnected. It didn't do a darn thing, no 
> change in selectivity or sensitivity at all.

The equivalent circuit of a crystal in a holder consists of four components.
There is a capacitance across the pins due to the spacing between them and
the spacing across the quartz plates. Also across the pins are a
capacitance, an inductance, and a resistance, all three in series.

At series resonance (as in your cathode circuit) the reactance of the latter
capacitance negates the reactance of the inductance, so their combined
impedance is zero. Typical values for the resistance range from 10K up.
There was no effect, since 10K in parallel with the much lower value of
cathode resistor has no effect.

73 de Neil ZL1ANM
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