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

Tue Feb 16 00:56:11 EST 2016

Hah,  Well, I seem to have started something,

But to comment on come of the comments, first, the cathode is NOT in the signal path.  While it has a function and can effect certainly the gain of the signal path, it is not in the signal path. The signal path goes from the grid to the plate and hence onward. The cathode is often at ground.  That would not do a lot for the signal.

Secondly, a single crystal is commonly used without any tricky dick resonating inductance or more commonly capacitance, to provide effective selectivity and one only need look at the copious literature to see that this is true.

I rest my case, that just placing the crystal in the signal path at the grid before biasing would provide an interesting experiment.  Not sure what else would happen for sure, but I think it is worth a look in the case of that 2830 kHz IF and crystal.  The inherent loss resistance – even at resonance – is small compared to the grid input resistance and certainly compensated for by the gain of the stage.

B

From: Milsurplus [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of hwhall at compuserve.com
Sent: Monday, February 15, 2016 10:48 PM
To: kargo_cult at msn.com; milsurplus at mailman.qth.net
Subject: Re: [Milsurplus] Quick n dirty crystal selectivity - it no workee

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 < <mailto:kargo_cult at msn.com> kargo_cult at msn.com>
To: milsurplus < <mailto:milsurplus at mailman.qth.net> 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?> mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of  <mailto:hwhall at compuserve.com> hwhall at compuserve.com
Sent: Monday, February 15, 2016 7:58 PM
To:  <mailto:kargo_cult at msn.com> kargo_cult at msn.com;  <mailto:milsurplus at mailman.qth.net> 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 < <mailto:kargo_cult at msn.com> kargo_cult at msn.com>
To: tetrode < <mailto:tetrode at googlegroups.com> tetrode at googlegroups.com>; milsurplus < <mailto:milsurplus at mailman.qth.net> 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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