[time-nuts] Beginners GPS locked frequency counter question

Jim Lux jimlux at earthlink.net
Sun Nov 1 09:03:38 EST 2015

On 10/31/15 7:32 PM, Bob Camp wrote:
> Hi
> So … how good is the “calibrate and go” (not the tone on second channel) approach likely to be?
> If it’s a bare crystal or normal XO (not a TCXO) that is supplying the clock, the crystal will follow some
> fairly well known curves. Which one of the curves it follows will depend entirely on the individual sample
> you have. What I see on my sound card will not be what you see on a different sample of exactly the same
> sound card installed in a similar machine.

having just done a bunch of measurements over 4 days on some crystals of 
this type (16 MHz, nothing special), I can shed some light on it.

over half a day sitting on my desk, they fluctuated about 0.1 ppm  (peak 

AVAR is about 1E-9 at tau=10sec, rising to 2e-7 at 10,000 seconds (after 
taking out the temperature dependence)

There is a strong temperature dependence.. with temperature changes of 6 
degrees (C) the frequency change is on the order of 6ppm (90 Hz out of 
16 MHz)

> It’s a pretty good bet that your device will change by 0.1 to 1 ppm per degree C. Yes it can be a bit better. It
> can also be a whole lot worse. Your room environment is likely to move 1 to 3 C per hour if you have the heat
> or air-conditioning turned on. If you are in an unheated garage … who knows. If you take the most likely 0.5 ppm/C tempco
> and the most likely 2C change you get a “wobble” of about 1 ppm. The period of this wobble probably will be
> in the 30 to 90 minute range.

yes, that's about right..
Although my office doesn't have that kind of fluctuation..
And I ran the 4 day test with the oscillators in a cardboard box over 
the weekend, so that damps short term variations.

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