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

[Bruce Long]

I suppose this is not relevant to the topic ( ARC 5 receiver bandwidth reduction) at hand but there is a trick in multipole resonant bandpass filter design of introducting coupling between non-adjacent resonant poles.  Consider a three pole resonant bandpass filter with three parallel resonant circuits each coupled with a top coupled capacitor ( or alternatively with inductive or mutual inductive coupling.  If i remember correctly when everything is correctly tuned up there is a 90 degree shift in the voltage across each resonant circuit.  So if you couple the third pole to the first pole you have a 180 degree phase shift and have created a notch at the center of the passband.  Now slightly detune something and you can move the notch off the center of the bandpass response off to the shoulder where it is very useful for improving the filter shape factor and rejecting nearby signals---such as you would like to be able to do if you want to operate in
 the cw portion of the band with a phone transmitter also operating in the immediate area.  As the interstage transformers in the ARC5 have only two resonant poles this trick is not immediately useful although it might be possible to add a third tuned circuit inside the IF transformer can using a toroidal inductor to essentially eliminate mutual coupling and to get a high Q resonator small enough to fit inside the can.

Lots of interesting possibilities here but alot of work is required,.




________________________________
 From: Bill Cromwell <wrcromwell at gmail.com>
To: arc5 at mailman.qth.net 
Sent: Tuesday, May 21, 2013 6:43 PM
Subject: Re: [ARC5] Receiver Selectivity-add cap to REDUCE bandwidth?
 

Well ya know,

I recall some printed discussion about rf filters - probably in one of 
the ARRL handbooks - that icluded something called a Kohn filter. The 
filters under discussion were for use between the antenna and receiver 
and would protect a CW receiver from an SSB receiver on the same band an 
at the other end of the table at Field Day. Or vice verse. I have always 
intended to build one (or more) and I wondered if they would be useful 
in an IF strip. The signals are coupled in and out of the ones I saw 
differently but that top or bottom coupling cap is as described here. 
Since I intend to build a filter like that I will also investigate  
using it with those IF cans in the IF strips of my 40 and 80 meter 
command receivers. I already had in mind to build a separate 
Q-multiplier on a spare IF can and not make the IF amplifier(s) 
regenerate - a la a Drake or Heath outboard Q-multiplier.

I think my sweeper is functional.

73,

Bill  KU8H

On 05/21/2013 04:38 PM, Bruce Long wrote:
>
> 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
>
>

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