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
frequencies.
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.
Bruce
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