[time-nuts] GPS 1PPS ultimate accuracy

Tom Van Baak tvb at LeapSecond.com
Thu Jan 15 13:38:02 EST 2015

Hi Andrea,

Making measurements of quartz oscillator aging is much easier than you think and requires minimal equipment. In particular all you need is a GPS 1PPS and a simple counter. No need to worry about sawtooth. Any $20 GPS/1PPS receiver will work.

Consider this rough example of measuring for one week a OCXO with 1e-10/day frequency drift rate.

Say on day 1 it is 1e-10 low in frequency. It will lose 0.1 ns per second. That's a very small amount and expensive to measure. But who cares. You're not trying to measure phase noise, or short-term stability, or frequency; all you're trying to measure is frequency drift. So let it run all day. By the end of the day those 0.1 ns have added up to 8.6 us. One data point.

8 microseconds is a lot. You can measure this with a $1 PIC or a $10 Arduinio. Use either time interval or timestamping methods. See PIC examples at www.leapsecond.com/pic/ although you can turn just about any microcontroller into a microsecond, or sub-microsecond counter and results over serial or USB to a PC for logging.

On day 2 it has drifted to be 2e-10 low in frequency, so it will lose an additional 17.2 us of time, etc. Another data point.

By the end of the week, the slowly aging oscillator is 7e-10 low in frequency and will lose 60 us that day.

This simple experiment would give you 7 data points which would nicely show your oscillator drift rate. You could collect data more than once a day if you wanted, like every hour or every minute. Differentiate the time error to get frequency error. Differentiate frequency to get frequency drift rate. Or just do a quadratic fit of the raw time error data.

It requires so little hardware that you could easily let it run for a month, or year and collect wonderful data. The more the oscillator drifts the larger the time measurements are so the easier they are to measure with accuracy.

This setup might even work for Rubidium. On the one hand Rubidium drift rates are 10x to 100x less than OCXO so your times will not grow nearly as rapidly, making precise measurements more difficult. On the other hand, Rubidium drift rates are so low that you would want to measure for months instead of weeks. In the end the two factors may balance themselves.

So I don't think you need nanosecond counters or fancy sawtooth corrected GPS timing receivers or GPSDO or measurements every second. A slowly aging oscillator is very easy to measure, mostly because, in order to measure aging you need many days or weeks or months of data. The longer the measurement time, the less it matters what the resolution of the counter (or the GPS 1PPS) is or how quickly you collect data.


----- Original Message ----- 
From: "Andrea Baldoni" <erm1eaae7 at ermione.com>
To: <time-nuts at febo.com>
Sent: Monday, January 12, 2015 2:59 AM
Subject: [time-nuts] GPS 1PPS ultimate accuracy

> Hello all.
> I am planning to do some experiments to evaluate the aging of oscillators
> (this one of the reasons I'm willing to buy the Milleren without EFC).
> What I would like to do exactly is to sample the total of a counter (of
> suitable number of bits, taking in account the fact that it will overflow)
> whose clock is the DUT.
> The sampling interval could come from a (long time based on a) sawtooth
> uncorrected PPS from a cheap GPS, a sawtooth corrected from a good one (perhaps
> the Lucent GPSDO), or a computer using NTP.
> Each of these sources should reach a goal stability (say, 1 part in 10^13)
> after averaging them on a different (and very high I suppose) number of
> seconds (averaging them for an infinity number of seconds should give the
> stability of the underlying reference clock, but I'm willing to stop sooner...).
> I know there's no reason to go 1E-13 when the Milliren couldn't go that far,
> but the DUT may be also something else like a FE-5680A).
> The sawtooth uncorrected GPS receiver may never yeld a good stability in the
> short term, but in the long one it should as well because the internal clock
> jitter would average results.
> If I'm using the correct teminology, after what tau the ADEV graph of the
> different references intersect the 1E-13?
> By the way, the stability of the TAI is known or, because it's
> the reference one, it has zero deviation for definition (so you can reach
> its ultimate stability through GPS really only at the infinity...)?
> Best regards,
> Andrea Baldoni
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