[time-nuts] I've designed a GPSDO, but how "good" is it?

Bob Camp kb8tq at n1k.org
Sun Aug 16 19:32:14 EDT 2015


Let’s say you want to measure a GPSDO over it’s entire range. The only practical approaches
are comparison based. You either need a fancy cesium standard / hydrogen maser or you compare
to another GPSDO. If you are making GPSDO’s having more than one around is a practical answer.
To take common mode issues out of the setup, you don’t really want to compare two identical GPSDO’s.
They are a lot cheaper than an atomic clock, you buy several.

Next you need something to measure with. Ideally you want a floor around 1x10^-13 for a 1 second 
single shot measurement. No fair taking a million 1 second measurements at 1x10^-10 and averaging
to get 1x10^-13. You can buy fine pieces of test gear that will do a single shot below 1x10^-13. The 
TimePod is one example. There are many others. I know of no “low cost” off the shelf gear that will get you there.

The 1x10^-13 is a bit arbitrary. Since you will likely improve resolution as tau increases, you could
simply ignore short tau testing. A 1x10^-9 (1 ns) single shot counter will get you to 1x10^-13 at 1,000
seconds. That’s a bit long of a wait. You want > 10 readings and prefer > 100 for characterizing this kind 
of thing. A minimum run time of a day is pretty slow. It also makes it quite hard to figure out what’s going
on with things like filter tuning.

The most practical answer to the measurement question is either a single mixer or a dual (or triple or
quad..) down conversion system. You can build one that will get you at least to the 1-2x10^-12 at one second 
level without a lot of crazy work. You can get to 10X better than that with a bit of care. With a single mixer
you will need an offset reference source for comparison. With a double or higher, you can directly compare two
devices on the same frequency.

If this sounds a bit crazy - welcome to the modern world. Hardware development takes a back seat to
firmware and testing. True both on cost and time.

So what does the setup look like:

Two or more eBay GPSDO models that have fairly well documented performance.

Likely at least two of one of the models so you can compare against published data.

Some pretty good buffering and splitting to pass the signals around.

A couple of older counters and GPIB to talk to them. (LOTS of alternatives here)

An OCXO as the heterodyne “offset” for your mixers.

Distribution, splitting, amplification for the OCXO (takes care of nasty cross talk issues)

A mixer for each device you will test plus a limiter / amplifier. (each goes to it’s own counter)

Yes, there is some stuff there. It’s not a lot of stuff. Commercial gear will be in the > $5K 
(and maybe >> $5K) range. Four 5334’s or 5335’s plus three or four GPSDO’s should be under 
$1K if you shop a bit.


So how about hiring the job out? Well a run is likely to take a week or two of equipment time. 
That’s about 300 hours. The tradeoff cost on the first run is $3 per hour. That’s a lot if gear tied 
up for $3 ….Let’s say you find a lab and indeed it’s only $1,000. Back comes the report. Here
is what your device looks like over that period.

Trust me on this, the result will not make you happy. The first real test if anything like this 
never is great, Next question - what’s wrong? You make some guesses. You change some
things. You get it all boxed back up. Back to the out side test. Crunch, out goes another $1K.

How many passes to expect? A year or so of testing is not uncommon on this sort of thing.
Even if you get one pass of testing free, you will have lots of passes after that …

You can’t afford to pay for commercial testing. It’s far more likely to be $30 a machine hour than $3.If 
you are serious about designing one of these beasts, a lab setup is essential. 


> On Aug 16, 2015, at 4:00 PM, Charles Steinmetz <csteinmetz at yandex.com> wrote:
> Nick wrote:
>> The GPS reference is a 1 pps signal   * * *
>> The oscillator itself (Connor Winfield DOT050V 10 MHz) has a short-term (though they don't say how short that term is) stability of 1 ppb.   * * *
>> The feedback loop takes samples over a 100 second period. That gives me an error sample with a granularity of 1 ppb. I keep a rolling sample window of 10 samples to get an error count over 1000 seconds.
> The limitations are fundamental.  The GPS signal has stability in the e-13 range, BUT only when averaged for 10,000 seconds or more (tau greater than or equal to 10,000 seconds).  It is actually quite noisy over short periods -- it can be as bad as e-7 over one second.  So, a GPSDO relies on its local oscillator for stability for periods shorter than 1000 seconds or so.  A good OCXO can provide stability in the e-10 range at tau in the 1 second to 100 second range, but even a very good TCXO cannot do that well.
> The point of a GPSDO is to rely on the stability of the local oscillator at short tau, where it is better than the GPS stability, and to cross over to the greater GPS stability at longer tau by disciplining the LO to GPS.  You have chosen a local oscillator with not very good stability at short tau, so your stability at short tau will not be very good (compared to well-designed GPSDOs using OCXOs) -- likely 3 to 4 orders of magnitude worse.  Additionally, your disciplining algorithm does not sound as if it gets the most out of the parts you are using.
> In order to measure the stability of your GPSDO, you will need a time interval counter (preferably), or a frequency counter, with a resolution at least 1nS, preferably 20pS or so.  You can use another one of your GPSDOs as a "reference" and attribute half the rms error to each one -- but preferably you would use a reference at least 10x better than the DUT at all tau of interest.  You will collect data for long enough to get stability numbers for the longest tau you care about (at least 30 minutes), then use a program like TimeLab to calculate the stability from the raw data.
> Sorry to say, you can do much better buying one of the Lucent or Trimble boxes for $150 on ebay.
> Best regards,
> Charles
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.

More information about the time-nuts mailing list