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

[hwhall at compuserve.com]

Yes, there is that. Oh, well. My memory is trying to recall another stunt I though I'd seen for fiddling with coupling... seems like it involved a 1-turn ring between coils. Or maybe I'm out to lunch on that!
 

 Wayne
WB4OGM

 

-----Original Message-----
From: Hubert Miller <kargo_cult at msn.com>
To: milsurplus <milsurplus at mailman.qth.net>
Sent: Mon, Feb 15, 2016 9:04 pm
Subject: Re: [Milsurplus] Quick n dirty crystal selectivity - it no workee



You can’t cancel out the resistance tho, of the original winding. The Q is about ½ of all the other IF coils as is.
-Hue 
 
From: Milsurplus [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of hwhall at compuserve.com
Sent: Monday, February 15, 2016 7:58 PM
To: kargo_cult at msn.com; milsurplus at mailman.qth.net
Subject: Re: [Milsurplus] Quick n dirty crystal selectivity - it no workee
 
>
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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