[time-nuts] A first MDEV plot...

[Bruce Griffiths]

Bert, VE2ZAZ wrote:
> Hi Everyone,
>
>
> This is my first Allan Deviation plot. It consists of the PPS output of an Oncore GT+ (Navigation-grade) GPS vs. an Ovenaire 10MHz OCXO divided down to 1Hz via an HP 5328A counter. The HP 5370A is free running on its 10811-60101 OCXO. The control/plotting is done by KE5FX's T.I. utility software.
>
> The curve clearly shows the limitations of the GPS at Tau=1s... But what else can be said of the GPS, or of the test setup when looking at the plots?
>
> One thing I see is that the bottom valley is much more pronounced when plotting MDEV, whereas with ADEV the curve barely breaks. How much more revealing is the MDEV curve in this case, and what effect is shown here? What dominant factor causes the valley. Is it hitting a noise floor, a true GPS performance deterioration or is is simply a calculation artefact?
>
> I use the HP 5328A as a divider by 10 million. This is a very useful feature if you own one of these counters. I presume that a straight line plot gives a warm feeling about the divider performance, but it would be great if someone on this list who owns a HP 5328A could characterise the Timebase Output when dividing by 10 million. I will try some measurements using the 5370A and one of my OCXOs, but I am not sure I own what is necessary to make valuable measurements.
>
> So much to learn! So much to try!
>
> Thanks,
>
> Bert.
>
>    
Bert

If you only accumulated data for a bit over 10,000 seconds you cant 
place much reliance in the plot much beyond about Tau=3000 secs as the 
error bars become a litle large.
Accumulating data for longer (eg 1 day or more) will improve the 
confidence in the plto at around 10,000 seconds.

Eventually for sufficiently large tau the noise contribution from the 
gps receiver will be equal to that of the OCXO.
For larger tau the noise from the OCXO will dominate and the curve will 
turn up as the OCXO instability eventually increases for increasing Tau.

The plots probably indicate that the divider noise isn't worse than the 
GT specification with SA on.
For your particular antenna and SA off the noise should be a little lower.

To get a better idea of the divider noise would require using a time 
interval counter like your 5370A.

I'd connect the PPS from the divider to the START input of the 5370A and 
the divider input signal to the STOP input.
This minimises the effect of the 5370A timebase error.

If possible save the measurements so you can calculate ADEV for the setup.

If the divider has low jitter you'll end up measuring the ADEV of the 
5370A itself.

Bruce