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

[Fuqua, Bill L]

Using the crystal to bypass the cathode resistor requires minimum resistance at resonance. This is the series mode of the crystal. You can't change the
series resonance frequency of the crystal is a capacitor.
73
Bill wa4lav

________________________________________
From: ARC5 [arc5-bounces at mailman.qth.net] on behalf of Bruce Long via ARC5 [arc5 at mailman.qth.net]
Sent: Tuesday, February 16, 2016 12:47 PM
To: David Stinson; kgordon2006 at frontier.com; ARC5 at mailman.qth.net; Milsurplus at mailman.qth.net
Subject: Re: [ARC5] Receiver Filter Adaptor Simple ARC 5 xtal receiver?

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<mailto: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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