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

[Jim Candela]

Hi George, et, all,

What a wonderful write-up on a poorly understood yet important circuit.
My story about a crystal filter is when I obtained an RCA AR-88 receiver. 
These are a late 1930's design used in WW2, mainly by the US allies via the 
Lend-Lease.
My AR-88 came without the wiring harness, and missing two front panel rotary 
switches. Someone literally butchered the wiring out of there with little 
regard for what was left. To make matters worse, the IF can used in the 
crystal filter was also missing. This became a labor of love and hate. 
Cosmetically this thing was an '8' and electrically it was a '2' at best.
The switches and wiring harness were not too hard to recreate. Changing 
leaking capacitors and out of tolerance resistors was not that hard either 
except all that work took up gobs of time.
The area that kicked my kester was that crystal filter. I actually took a 
similar IF transformer from my junk box, and rewound one coil so that it had 
a center tap. This took a little bit of cut and try to get the various 
parameters in line such as the coupling coefficient, and resonant 
frequencies between the two coils to be similar. When I finally got it all 
working I had the center frequency shifting as I moved the selectivity knob 
from 1-5. Nothing I tried made it any better. I could make it worse 
though..that was easy.
Sometime later someone got me the correct IF can from a RCA CR-88. I don't 
recall the IF frequency on the CR-88 but it was lower. I discovered quite by 
accident that if I changed the fixed coil shunt capacitors to the AR-88 
value, that the resonant frequency came up to be tunable at 455 Khz. So it 
appeared that I had the correct IF can now, so I performed surgery once 
again to install the new IF Can.
After a bit of fiddling, tuning, and poking I quickly realized that the 
thing behaved exactly the same way as it did with my home made IF can!
In the end I learned to tune in an AM signal with the selectivity 'tight' 
and then open it up as signal and noise level permitted. Tuning this way 
seems to hide the frequency shift pretty well.

Regards,
Jim
WD5JKO

----- Original Message ----- 
From: "George Maier" <George at MaierGroup.com>
To: "'Ron.K3PID'" <ron.k3pid at sbcglobal.net>; 
<central-electronics at mailman.qth.net>
Cc: "kim.peck" <kim.peck2 at verizon.net>
Sent: Wednesday, September 03, 2008 10:33 PM
Subject: RE: [Central-Electronics] Crystal filter question(s).


> 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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