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

Magnus Danielson magnus at rubidium.dyndns.org
Fri Dec 28 19:24:10 UTC 2012

Hi Fabio,

On 28/12/12 17:26, 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.
> 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.
> 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.

Not a bad approach.

Recall that you do not need to run it as seldom as 1s, you can run it 
more often. You can allow for a wide set of frequency offsets that way.

> 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.

It should be a good way to sweep through the various delays in a 
sequence, which with fairly simple post-processing would reveal both the 
jitter and non-linearity.

> 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?

Noise jitter adds like power, so same noise would give you a 1.41 
multiplication up.

> 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.

Having two 5680s isn't a bad solution, as you can tweak one of them to 
be a suitable rate off from the other to slowly sweep the range. 
Processing of that should allow you to characterize your interpolator 
with not to much of coding magic. By measuring over longer period would 
allow you to see just how temperature-sensitive you are.

So, you just want to produce a 1 PPS, so a PIC divider such as TADD-2 
would be suitable if you don't have PPSes on your 5680s.

> 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 :)

You could do a bit of damage with that :)


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