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

[Charles Steinmetz]

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