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

Ian Wilson ianmwilson73 at gmail.com
Wed Feb 17 16:27:42 EST 2016


If you ignore the 'holder' capacitance, a crystal is essentially a large L
in series with a small C. You can change the resonant frequency of this
combination a small amount by adding series C (increase) or series L
(decrease). This is how 'rubbering' a crystal works.

The holder capacitance, etc, place a physical limit on how far you can
pull a particular crystal + stray capacitances.

Note that the "superVXO"s work by adding extra, identical, crystals in
parallel. This makes the equivalent more complicated but in some cases
decreases the effective Q, allowing much more pullability.

73, ian K3IMW


On Wed, Feb 17, 2016 at 1:13 PM, Bruce Long via ARC5 <arc5 at mailman.qth.net>
wrote:

> Bill
> I 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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