[ARC5] Receiver Selectivity-add cap to REDUCE bandwidth?

[Bruce Long]

I think there is another way to add selectivity to an ARC5 receiver using a gimmick capacitor that does not rely upon regeneration.  I have 
hesitated posting until I could find the original reference but since I 
have been unable to find the reference to date here goes.

I remember seeing somewhere in a 1930's vintage radio handbook the fact 
that inductive coupling of two parallel resonant circuits has the 
opposite polarity of capacitive coupling of two resonant circuits.  If 
correct, this means if you have two inductances that are mutually coupled as part of a two 
pole resonant circuit adding a small coupling capacitance from the hot 
side of the primary side resonant circuit to the hot side of the 
secondary side resonant circuit will in fact reduce the total mutual 
coupling.  IIRC this also causes an out of band notch at the frequency 
where the mutual inductive and the top coupled capacitive coupling have 
equal and opposite signs and complete cancellation results.

I am really busy at work at the moment so i cannot do much more with this at this time but maybe someone here will be interested enough to take 
the ball and run with it.  I see two ways to proceed, an experimental approach 
and a modelling approach.

If you have a sweep generator and can generate a graphical image of the IF filter passband shape it ought not be too hard to add a small gimmick 
or other small value capacitance from the hot side of the primary to the hot side of the secondary and see what happens   I expect minor 
re-tuning of both sides will be necessary to judge the true effect.

Alternative you could model a two pole resonant band pass filter with mutual 
inductance coupling in a circuit modelling software and then add the 
small coupling cap and observe the effect.

If anyone is interested in this second approach i might be able to find 
time to come up with a circuit model for a mutual inductance two pole 
bandpass filter that you can use as a starting point for your 
considerations.

Good luck    bruce    KJ3Z

Sorry for the double email Kenneth   forgot to click "reply all" the first time




________________________________
 From: Kenneth G. Gordon <kgordon2006 at frontier.com>
To: Arc5 at mailman.qth.net 
Sent: Monday, May 20, 2013 2:22 PM
Subject: Re: [ARC5] Receiver Selectivity
 

On 20 May 2013 at 16:42, Fuqua, Bill L wrote:

> It appeared to work great with my 40 meter receiver. But you had to
> remove AVC from the that IF stage so that feedback was independent of
> RF gain and avc levels.

Ah! That makes sense: I never did that at the time some 50 years 
ago...obviously I should have, if I had known about it.

>  I could set it right on the edge of oscillation and get great CW
>  selectivity. But not so much as compared to modern radios.

No, but the improvement you did get was substantial and made the receiver 
very much more usable on 40 meters.

Ken W7EKB
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