[time-nuts] Re: ADEV interpretation

[Bob kb8tq]

Hi

Run through the math ….

500 ps is the resolution number on the counter. Is it the one sigma number as well? 
Probably best to test it. Some HP counters do *much* better. Others … errr .. not so much.
Simple test is to feed your LO PPS signal into both channels. You need a bit of cable in on
one side to keep the two inputs from hitting at the same time. 

500 ps *if* it is the ADEV floor of the counter at 1 second, is 5x10^-10. That will be well below
your 5x10^-10 by the time you get to 30 seconds. 

The F9T has a “sawtooth” of about 4 ns. At one second that would move things around
by 4x10^-9. That is *not* the same thing as an ADEV of 4x10^-9. How one translates
to the other is a bit of a “that depends” sort of thing. (ADEV will be lower than P-P sawtooth).

4 ns at 40 seconds should be around 1x10^-10. Since that’s the upper bound on sawtooth
to ADEV, it (by it’s self) is not the issue. 

Uncorrected GPS is getting into things as well. It may not add much to the sawtooth. A lot
depends on how you have things set up.

The internal oscillator on the 53131 *should* be doing quite a bit better than 5x10^-10 at
30 seconds. That assumes it has been warmed up and is working correctly ( = it’s not broke). 
A simple check would be to see how it looks compared to other OCXO’s you have lying 
around. Most will be better than 5x10^-11 over the 30 to 100 second range. 

So my input to the question would be “d) none of the above”. :) :) . Time to check this and 
that to see where the issue lies. 

Bob

> On Apr 4, 2021, at 7:52 AM, Marek Dorsic <marek.dorsic at gmail.com> wrote:
> 
> Hello,
> 
> I would like to ask your knowledge to help me interpret this basic ADEV result.
> Assume following measurement setup:
> PPS output of Ublox RCB-F9T connected to channel 1 of a HP53131A counter.
> 10MHz output of the counter's internal oscillator (opt. 010, medium stability oven) divided with PICDIV to 1 PPS connected to channel 2 of the counter.
> Counter set to measure time interval from channel 1 to channel 2.
> After processing the readings, the floor of ADEV is 5x10^-10 at about 30-100 seconds interval.
> Which of the following is true?
> a) ADEV shows the real performance of the internal oscillator,
> b) ADEV floor is 5x10^-10 because of the restricted counter time interval resolution (500ps),
> c) ADEV floor is 5x10^-10 because of the ZED-F9T PPS jitter (after saw correction) of the gate start input channel.
> 
> 
> 
> Thanks in advance
>     .marek
> 
> 
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