[ARC5] Crystal work-around question. OT

[Tom Lee]

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" , 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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