[ARC5] Receiver Filter Adaptor Simple ARC 5 xtal receiver?

[Bruce Long]

You are right Dennis  The modern strip AT and tuning fork type crystals have too much series resistance.  An older and biger conventional type crystal might have sufficiently low RS to work well.   A solid-state negative resistance generator would be easy to make at 80 kHz.  It might also be possible to use a small toriodal transformer connection to the crystal to lower the effective series resistance sufficiently to  make the filter work.
Another idea would be to put the xtal in the feed back path of a fairly fast op-amp and use the Miller effort to reduce the effective series resistance
 

      From: Dennis Monticelli <dennis.monticelli at gmail.com>
 To: Bruce Long <coolbrucelong at yahoo.com> 
Cc: David Stinson <arc5 at ix.netcom.com>; "kgordon2006 at frontier.com" <kgordon2006 at frontier.com>; "ARC5 at mailman.qth.net" <ARC5 at mailman.qth.net>; "Milsurplus at mailman.qth.net" <Milsurplus at mailman.qth.net>
 Sent: Tuesday, February 16, 2016 2:54 PM
 Subject: Re: [ARC5] Receiver Filter Adaptor Simple ARC 5 xtal receiver?
   
Bruce,
I looked into a XTAL filter for the ARC-5.  It will work for the high IF versions but not the low freq IFs due to high crystal resistance.  For example an 85KHz crystal Rs would be 50 to 100K.  By contrast a 2MHz rock would exhibit around 200 ohms.
Various forms of negative resistance generators would work well and could be added with less invasiveness than inserting solid state active IF filters in a low IF receiver.  They also work with the higher IFs, albeit with less shape factor than a crystal filter.
Dennis AE6C


On Tue, Feb 16, 2016 at 9:47 AM, Bruce Long via ARC5 <arc5 at mailman.qth.net> wrote:

There may be a minimum mod way to add a xtal filter to a ARC-5 receiver.  I thought of this several years ago but have not had the time to give it a try.  The ARC-5 receiver has two tubes serving as IF amplifiers.  Both tubes have a cathode resistor to ground bypassed by a by pass capacitor.  I thought about disconnecting the cathode bypass capacitor and replacing it with a crystal at ( or near) the IF frequency.   The xtal will serve as a cathode bypass but only at the xtal amplifier series resonant frequency.  A small variable capacitor could be added in series with the xtal to adjust its in -circuit resonant frequency.  The first and second IF stages could be adjusted to slightly different frequencies to give a basic bandpass characteristic at the cost of a little gain.
Receiver gain will be affected by the xtal series resistance so attention to this parameter is warranted.  If xtal series resistance is a problem it ought ot be possible to add a transistor or op-amp negative resistance generator to cancel most of the xtal Rs thereby wiggling past that possible limitation.
As we are talking about disconnecting only lead and adding a two port network to the cathode node and ground, restoration to original condition should be quite easy.

      From: David Stinson <arc5 at ix.netcom.com>
 To: kgordon2006 at frontier.com; ARC5 at mailman.qth.net; Milsurplus at mailman.qth.net 
 Sent: Tuesday, February 16, 2016 11:53 AM
 Subject: Re: [ARC5] Receiver Filter Adaptor
  
Thank you for this, Ken.  Good info.
This level of circuit changes are forbidden in "my world," but
the ideas and concepts are still fertile fields to plow.
73 Dave S.

----- Original Message ----- 
From: <kgordon2006 at frontier.com>
Subject: Re: [ARC5] Receiver Filter Adaptor


> Well, David, et al. Your musings are well taken here.
>
> Now, considering a simple method of increasing selectivity in 
> our BAs, we
> might take a look at Mike Murphy's WU2D youtube video "ARC-5
> Command Receivers Part-4" on selectivity.
>
> Near the end of this video, Mike shows how he improved the 
> selectivity of a
> BC-455 by modifying the first IF transformer into a crystal 
> filter.

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