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

[Brian]

Hello All,

I think we should listen to Bill.
One mistake I suspect the rest of you are making is assuming that if you put a capacitor in series with a crystal, that is a series resonant circuit.
The first thing that most of you are missing is that the series and parallel resonances refer to within the crystal, not the circuit outside the crystal. Go look in any book on crystal oscillators. In the book you may be lucky enough to find a chart of frequency versus impedance. If you look at resonant frequencies, you will see that the parallel and series resonant frequencies are very close. For instance, see Figure 5.3 on p22 of Marvin Frerking (1978) Crystal oscillator design and temperature compensation, Van Nostrand, NY.

Please note email etiquette: I have only left attached those emails dealing directly with series and parallel crystal resonances.

73 de Brian, VK2GCE.
BE, PhD, CPEng, FIEAust

On Thursday, February 18, 2016 8:27 AM, Ian said in answer to Bill:

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
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