[time-nuts] Testing the TAC, and a question about ADEV

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Dec 28 18:04:58 UTC 2012

Fabio Eboli wrote:
> Hello
> How could I test the time to analog converter
> we talked few posts ago?
> Something that can be done with things I have
> or can easily find.
One method is to use a statistical fill the buckets technique to measure 
the linearity.
To do this one needs to use an incoherent source to trigger the 
interpolator and plot a histogram of the results.
A noisy RC oscillator would be useful for this but care should be taken 
to avoid injection locking.
To achieve useful measurement in a reasonable time interval a trigger 
rate somewhat greater than 1Hz is required.
If the interpolator has 1024 time delay bins then ~ 100,000 trigger 
events are required to achieve a bin width measurement error of 10%.

Otherwise a series of measurements of a set of accurately known delays 
is required.
Useful results can be produced by measuring the delay between various 
outputs of a shift register clocked at a sequence of different measured 
> I was thinking that would be nice to try to
> feed it with signals similar to the real ones,
> but that can be controlled: PPS + 10MHz reference,
> without the PPS tipical jitter.
One should characterise the interpolator linearity etc first.
> To do so I was thinking to use the PPS from
> one of the 2 FE5680 and the 10MHz from the
> second ad use these to simulate the real signals.
> I can tune the frequency of the Rb with the
> serial interface (tested and working).
> I'd like to verify both the resolution and
> repeteability.
> I was trying to figure the approx jitter
> I will have using the Rb like I said above,
> so I'm giving an eye to this diagram from
> John Miles page: http://www.ke5fx.com/rb.htm
> If I'm understanding correctly, that 5680
> at 1s should have most of the jitter (95% +-2sigma)
> into an interval +- 3.08x10^-11 wide, i.e. about 62pS
> on the PPS signal (non considering the PPS buffer
> inside the 5680). Is this how it work, or I'm mistaking?
> This is from only one unit, but both unit will have
> the jitter, how to take into account the jitter
> from both?
> Is there a better method to make this test?
> I could try to test the TAC alone feeding it
> with a 100 to 200nS pulse, but I dont know
> from where to start to generate a clean stable
> and repeteable pulse.
You dont need one (see above) as long as you have the means to 
accumulate the results of 100,000 measurements or preferably more.
> I have the 2 5680, the counter with it's 10MHz
> inputs and outpus, and a Vectron OCXO that came
> with one of the 5680, with these markings:
> OCXO500-18 63.897600MHz
> 34537 A0715
> and...
> soldering iron, solder and scrap electronics :)
> Thanks,
> Fabio.

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