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

[hwhall at compuserve.com]

>
My thinking was to simply unwind turns from this winding and add capacitance to resonate. The winding is 2-pi and i still don't know if i'd peel off the near winding or the more distant winding - to lower the coupling.
>

 

 How about adding turns in the reverse direction, thus cancelling some of the inductance? (a) it might be easier and (b) it would be reversible.

Wayne
WB4OGM


-----Original Message-----
From: Hubert Miller <kargo_cult at msn.com>
To: tetrode <tetrode at googlegroups.com>; milsurplus <milsurplus at mailman.qth.net>
Sent: Mon, Feb 15, 2016 6:41 pm
Subject: [Milsurplus] 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 idea was to be something like the low-Z 'transfilters'
used in some transistor circuits. I thought later that maybe the cathode resistor needed an RFC in 
series, otherwise the low resistance would shunt the crystal. But i never did that experiment, would
have taken more rewiring for my limited time. But - is this a workable idea, or is there a 'gotcha' - 
something i'm not accounting for?
 
I did try the grid-to-plate gimmick capacitor to make the stage regenerative, but i didn't like that at
all. Too much like a poor implementation of  regenerative receiver. You don't want the selectivity 
stage gain-controlled separately. In my experience a q-multiplier is much better. I did see some
'conversion' article probably in CQ Magazine that did build a Q-multiplier for the 2830 kHz IF. 
I am curious how that really worked out. f/Q is the 3db selectivity and you can see 2830/Q is going
to be a LOT more than 455/Q. The crystal does have a very high Q and i thought in this low impedance
point, the cathode circuit, the parallel capacitance of the crystal holder would be insignificant. 
Obviously i missed something. I am almost up for a rematch with this challenge.
 
While i'm at it: another question to the wisdom of the members. The USAAF radio receiver BC-348,
the last IF is overcoupled. I see all the other IF windings are 1.5 ohms DC with 250 pF resonating.
The secondary of the last IF can is 5 ohms DC resonated with 125 pF. My thinking was to simply
unwind turns from this winding and add capacitance to resonate. The winding is 2-pi and i still 
don't know if i'd peel off the near winding or the more distant winding - to lower the coupling. BUT -
i had another idea: what if i just get another first or second IF can from a junker, and use that to 
replace this entire original overcoupled IF can? I know in selecting IF transformers, there always
was one specified for 'diode load', but i have no idea what kind of impedance such a transformer
has.
Ideas - advice?
tnx
-Hue Miller 

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