[ARC5] ARC-2: IF Regeneration is a "No"

[Fuqua, Bill L]

  The problem with having a variable IF is that the Q changes with frequency. 
R remains relatively constant but Xc and Xl changes with frequency. This is a problem 
when trying to improve a receiver's IF bandpass with out having to constantly be changing
the regeneration control My impression is that the person making the modifications wants
to improve the IF filtering and not fool with it afterwards. If the IF frequency was constant
as in the ARC 5 this would be no problem. 
  By the way, the Q multipliers have tuned circuits in them which would have to track as well.
  It is possible that the ARC-2 could be made the IF bandpass narrower, however, they may not 
have due to the difficulty of making the higher Q IF stages track. This may also be an issue. 
I don't see a neat clean solution to this problem. Other than making only one transformer narrow and
keeping the others broad, just as they are.  
73
Bill wa4lav

________________________________________
From: arc5-bounces at mailman.qth.net [arc5-bounces at mailman.qth.net] on behalf of Bruce Long [coolbrucelong at yahoo.com]
Sent: Wednesday, April 02, 2014 9:54 PM
To: David Stinson; arc5 at mailman.qth.net
Subject: Re: [ARC5] ARC-2: IF Regeneration is a "No"

how to implement a negative impedance generator.

Basically you need a voltage amplifier with hi input impedance, low output impedance and zero degrees transmission phase shift at the operating frequency.  A high speed op-amp would work nicely.  Set the gain to equal - say- +2.  Connect a feedback resistor of value R from the amplifier output to the amplifier input.

Now connect the amplifier input - through dc blocking capacitor- to one of the IF filter nodes.   The impedance looking into the amplifier input ( now connected to the IF filter pole)  = (G-1)R where G is the amplifier gain, and R is the feedback resistance.

Notice of the the impedance will be real if G has no phase shift and if the feedback impedance is really a resistor with insignificant strays.

You can think of the feedback resistor as providing positive regeneration.

This circuit is the basis of ham Q multipliers although the ham literature tends not to explain these circuits in this way.

So basically the suggestion is to build a small Q-multipler for each node of the bandpass filter.  I still think the filter coupling capacitor the 5pf guy must be made smaller.    Low value coupling capacitors are hard to buy off the shelf.   You can use the standard gimmick capacitor or you can put three capacitors in a Tee network with the tee network having the effect of a single low value capacitor but made from- taking numbers from the air- three 4.7pf caps.

Again adding the discrete negative impedance converters might be more trouble than it is worth.  If I had the time I would give it a shot--- but I have an unfair advantage- I have a network analyzer for trouble shooting.


On , Bruce Long <coolbrucelong at yahoo.com> wrote:

Light feedback ie regeneration around an active  ie amplifying stage does create a negative impedance but this negative impedance has a real (resistive) and an imaginary ( reactive) component.  It is the reactive component that shifts the filter center frequency.    This is why the suggestion of adding an entirely seperate negative impedance generator using an independent amplifying section not the existing IF amplifier.  I am sure this can be mad to work but might not be worth the trouble.


On Wednesday, April 2, 2014 7:52 PM, David Stinson <arc5 at ix.netcom.com> wrote:


----- Original Message -----
From: <millerke6f at aol.com>


>I noted the NO to regeneration, but why not.....

As noted in a earlier post, even "light" feedback
radically changed the bandpass and resonance
of the IF amplifiers and this effect varies with tuning,
ruining the fine tracking of the variable IF and RF stages.
This would be OK if the ARC-2 was to be parked on
3885 KC and never move again, but that's not my goal.
"Adjusting" the regeneration while tuning requires
boogering the front panel for a new control
and in my world, that is also an automatic "no."
So my list of possible options is getting thin ;-).

TNX DE Dave AB5S


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