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

[Hubert Miller]

It's near the end of my day, and i'm disinclined to do a lot of work; i'm
feeling lazy. But i took a quick look at 
http://www.electronics-tutorials.ws/oscillator/crystal.html

They work the example of a 10 MHz crystal with an equivalent inductance of
2.55 mH and q ~ 25,000. Shure Brothers tell me the Xl for this inductance at
10 MHz is ~150k ohm.
You can see where this is going.
Now, are you quite sure the Rs of my crystal is 10K ?

If you don't like this, look at this intuitively: think of a series crystal
filter in a receiver, with a load in the 10s of kohms - such as the BC-312
receiver, or a transistor circuit, where the crystal is working
right into the transistor base. IF the crystal actually had an Rs of 10k,
the effective selectivity of the crystal filter would be very low, with a q
at most in the low double digits.

Eventually, i may be able to give you an actual real number for 2837 kHz - i
have a couple of crystal impedance test sets - need to set them up and learn
to use them.
-Hue 


-----Original Message-----
From: Milsurplus [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of
AKLDGUY .
Sent: Monday, February 15, 2016 6:47 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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