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

Wed Feb 17 16:13:42 EST 2016

BillI stand by my statement that a series reactance put in series with a crystal operating at series resonance shifts the minimum impedance(resistance) frequency with little if any change in the real component of of the series impedance.
I think there is a terminology issue here.   Please note my use of the phrase" combined series resonance frequency".

      From: "Fuqua, Bill L" <wlfuqu00 at uky.edu>
 To: Bruce Long <coolbrucelong at yahoo.com>; 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: Wednesday, February 17, 2016 3:46 PM
 Subject: RE: [ARC5] Receiver Filter Adaptor Simple ARC 5 xtal receiver?

  I am sorry, the only way to get the minimum impedance across a crystal, even with a series resistance, is at its series resonance mode which is at its
natural mechanical vibration frequency. You can shift its parallel resonance, Z is highest at this resonance by a capacitance, and some times it is not obvious that
it is operating in parallel resonance mode. 
  I work with crystals and design filters all the time. 
73
Bill wa4lav

________________________________________
From: Bruce Long [coolbrucelong at yahoo.com]
Sent: Wednesday, February 17, 2016 9:30 AM
To: Fuqua, Bill L; 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?

Well Bill at one level you are correct.  A crystal is a slab of quartz.  No external reactance is going to change the mechanical resonant frequency.

However an external reactance will shift the combined series resonance frequency of the series reactance-xtal combination.  To further confuse the issue the majority of the quartz crystals made are not resonant at the marked frequency.

A 10 MHz, 20 pf load crystal is "series" resonant at 10 MHz only if you connect it in series with a 20 pF capacitance.

If you  replace the 20 pF load capacitance with a short, the crystal series resonant frequency will be less than 10 MHz.

If you replace the 20 pF series capacitor with a 39 pF series capacitor the combined series resonant frequency will still be less then 10 MHz

If you replace the 20pF series capacitor with a 10 pF series capacitor the combined series resonance frequency will be above 10 MHz

How much shift are we talking about/

Two weeks ago at work I shifted a 13 MHz, cheap fundamental mode AT cut strip resonator over 400 ppm with an 8:1 variation of the series capacitor.

Bruce  KJ3Z

________________________________
From: "Fuqua, Bill L" <wlfuqu00 at uky.edu>
To: Bruce Long <coolbrucelong at yahoo.com>; 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 9:32 PM
Subject: RE: [ARC5] Receiver Filter Adaptor Simple ARC 5 xtal receiver?

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<mailto:arc5-bounces at mailman.qth.net>] on behalf of Bruce Long via ARC5 [arc5 at mailman.qth.net<mailto:arc5 at mailman.qth.net>]
Sent: Tuesday, February 16, 2016 12:47 PM
To: David Stinson; kgordon2006 at frontier.com<mailto:kgordon2006 at frontier.com>; ARC5 at mailman.qth.net<mailto:ARC5 at mailman.qth.net>; Milsurplus at mailman.qth.net<mailto: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<mailto:arc5 at ix.netcom.com>>
To: kgordon2006 at frontier.com<mailto:kgordon2006 at frontier.com>; ARC5 at mailman.qth.net<mailto:ARC5 at mailman.qth.net>; Milsurplus at mailman.qth.net<mailto: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><mailto: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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