[ARC5] Crystal work-around question. OT

[J Mcvey]

 I should have tried it by inverting the division...it didn't look right... 
Thanks for straightening me out.

I tried the series cap for heck of it , and it works.A series 130pf brought the frequency from 10.2396 to 10.24087.Yes, it overshot, but proof of concept!220 pf got it 10.24040 
So my guess is a 330 pf would be in the sweet zone.
It was an interesting experiment for use in a pinch.
This particular xtal is used as a reference , yet no adjustment was provided. Either the xtal is on the money or replace it are the choices.

    On Wednesday, September 23, 2020, 4:09:38 PM EDT, Tom Lee <tomlee at ee.stanford.edu> wrote:  
 
  Or, to burn the corpse after beating it to death, just divide the shift in hertz by the base frequency in MHz. The result is automatically in ppm. No powers of ten to keep track of.
 
 --Tom
 -- 
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu On 9/23/2020 12:38, Mike Morrow wrote:
  
 
#yiv6144880103 #yiv6144880103 -- DIV {margin:0px;}#yiv6144880103  #yiv6144880103 #yiv6144880103 -- DIV {margin:0px;}#yiv6144880103   To beat the subject to death: 
  ppm = 1,000,000 (crystal freq - desired freq) / desired freq ppm = 1,000,000 (10,239.6 kHz - 10,240 kHz) / 10,240 kHz ppm = -39.0625 
  Mike / KK5F
 
 
-----Original Message----- 
 From: J Mcvey via ARC5 
 Sent: Sep 23, 2020 2:07 PM 
 To: "arc5 at mailman.qth.net" , Tom Lee 
 Subject: Re: [ARC5] Crystal work-around question. OT 
 
  I think it's a bridge too far if i did the calculation correctly: 
  The Xtal is 10.240 MHZ 
  so (10.24* 10^6/400) * 10^6 = 25600 PPM.
  WOW. Guess I'll just have to wait for the part to com in...
      On Wednesday, September 23, 2020, 2:46:13 PM EDT, Tom Lee <tomlee at ee.stanford.edu> wrote:  
  
     What matters is not the absolute frequency shift, but its value normalized to the nominal frequency. Without knowing the latter, it's impossible to say whether 400Hz is a little or a lot.
 
 Shifting frequency up or down by tens of ppm is easy. Shifting by more than 100ppm puts you into the "difficult" range, while attempting to shift by 1000ppm puts you into the "you might get there, but the crystal isn't really controlling things any longer."
 
 Adding a series capacitance is easy enough, so give it a shot and see. A practical maximum is a value somewhat below the holder capacitance, but the true limit depends on the topology of the oscillator circuit, among other factors.
 
 -- Cheers,
 Tom
 -- 
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu  On 9/23/2020 11:26, J Mcvey via ARC5 wrote:
  
 
     The crystal in question is a sealed HC25U type. The frequency is low by about 400 Hz, so it can't be padded. Could it be "brought up" with an adjustable series cap? I have a new one on order, but was wondering if anyone had a "fix" without replacing the crystal? 
  I suspect too much 'pulling" would cause instability if there was a way. What say you guys?
   
        
    
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