[ARC5] Crystal work-around question. OT

Tom Lee tomlee at ee.stanford.edu
Wed Sep 23 19:52:27 EDT 2020 

Poor contacts to the crystal faces usually makes the frequency go up 
(they act much like an extra series capacitor), but poor contacts can 
have lots of effects, so perhaps it can go the other way.

If the crystal package isn't hermetic, or contaminants were sealed into 
the package at manufacture, the oscillation frequency can diminish over 
time, as contaminants find their way to the surface and add mass. A good 
high-amplitude vibration can shake them off for a time. Perhaps that 
happened here. Hard to say. But I'm happy that the frequency is spot on 
now. Hope it stays that way for as long as you need it to!

--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 16:44, J Mcvey wrote:
> I have accidentally found a "fix" for the Low end xtal -maybe...
> As part of the testing process , I put the xtal on my aa600 to see 
> what it looked like.
> It read about 1 khz lower on the analyzer than when it was in circuit.
> I suspect this was due to the shunt capacitance of the analyzer and 
> the binding post adapter.
>
> Anyway, when i stuck it back in the radio , the xtal was at 10.240074! 
> Now it's a keeper!
> Perhaps the aa600 drove it so hard, it polished the contacts?
>
>
>
>
>
>
> On Wednesday, September 23, 2020, 7:28:52 PM EDT, Tom Lee 
> <tomlee at ee.stanford.edu> wrote:
>
>
> Congratulations on a successful experiment! A combo of a cap and a 
> trimmer would allow you to dial it in, if you really needed to nail 
> the target until the new crystal arrives.
>
> Btw, your result underscores that it is possible for elements in the 
> circuit other than the crystal to shift frequency. While waiting for 
> your new crystal, it might be worthwhile to check other parts of that 
> oscillator, just in case there's a non-crystal reason for the 400Hz 
> error. I once went a couple rounds with a Colpitts that was 
> persistently off frequency because of a dodgy ground connection to one 
> of the caps. It looked ok to the casual eye, but there was a hairline 
> crack. Refreshed the joint, and it popped to within a few ppm of the 
> right frequency. I will not tell you how long it took me to get to 
> that point.
>
> --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 14:24, J Mcvey wrote:
> 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> <mailto: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:
> 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" <mailto: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> <mailto: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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