[Central-Electronics] Crystal filter question(s).

[George Maier]

Hi Ron:

I passed your question along to Kim - WA1PBU, a friend of mine who has
forgotten more about crystal technology than I will ever know. 
Here's his answer:

"This is a classic single pole crystal filter.  The main obstacle to a sharp
crystal filter, is the electrode capacitance (of the crystal), which tends
to short out the series LC model of the crystal.  There is usually a
transformer involved somewhere in the filter.  The transformer winding must
have a center tap, and that is grounded (at RF at least), so that the
transformer ends are 180 degrees out of phase relative to each other.  The
"phasing" capacitor, is adjusted to be exactly the same as the electrode
capacitance.  The voltage from the winding end away from the crystal,
exactly cancels the voltage coupled through the electrode capacitance (we
used to use the word "neutralization") leaving only the voltage which will
pass the series LC of the crystal.  

This LC, called the motional reactances (comes from mechanical movement of
the quartz), is possessed of very high Q, and it is loaded Q that sets the
filter slope.  Adding a resistor across the crystal reduces the loaded Q,
and widens the filter response (also changes the loss quite a bit).  In a
good design, some of the resistance was in series, and some in parallel, to
attempt to keep the loss, so overall receiver gain, constant (sort of).  I
believe another name for this type, is "half lattice" (don't hold me to
that, as it's late as I type this)."

Hope this answers your question....

73, George - W1LSB

-----Original Message-----
From: central-electronics-bounces at mailman.qth.net
[mailto:central-electronics-bounces at mailman.qth.net] On Behalf Of Ron.K3PID
Sent: Wednesday, September 03, 2008 8:49 PM
To: milsurplus at mailman.qth.net; johnson at mailman.qth.net;
Heathkit at mailman.qth.net; central-electronics at mailman.qth.net;
boatanchors at mailman.qth.net
Subject: [Central-Electronics] Crystal filter question(s).

In a "classic" IF crystal filter such as the one used in many of the
Hammarlund receivers and as described in many handbooks where a crystal is
placed in an IF stage in a bridge configuration with a "phasing" capacitor
on the other leg, resistance is sometimes added in steps to decrease Q and
widen the passband. Is it normal to see a shift in the center frequency as
well as a widening of the passband with changing R? If so, can someone
explain the function?

Regards

K3PID
Ron H
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