[time-nuts] Phase measurement of my GPSDO

[Tobias Pluess]

Hello all

in the meantime I figured out most of my problems and my GPSDO is working
now with some very ugly prototype code. Today, I wanted to do some ADEV
measurements.
My plan was to compare the 1PPS generated from my GPSDO to the 1PPS of my
Oscilloquartz STAR4; unfortunately I have nothing else (like Rb or so)
which is perhaps more stable. So I try with the STAR4 and see where I get.
However, before I did any meaningful measurements, I wanted to see what the
noise floor of my test equipment is.
Again, unfortunately I have nothing better than a HP 5335A with 1ns
resolution in TIC mode. I measured the noise floor of the TIC as follows:
the 1PPS output of my GPSDO was connected to a resistive power splitter,
and then, one output of the splitter went to channel A of the TIC (START
signal) while the other output from the splitter went first to a long cable
and then to channel B. With this, I achieved about 16ns of delay.
I then used the TIC together with Timelab and measured the ADEV of this
setup.
As far as I understand, if the delay of the cable stays constant (which it
does as long as it is not moved and the temperature stays the same), all I
see in the ADEV plot is the ADEV of my counter itself. Right?

So I let this test run for one hour (collected 3600 samples), and the
result looks terrible. See the attached file. I did the test twice; once I
used the STAR4 GPSDO as external reference for the counter, and once I used
its internal reference, which is a HP 10544A oven. As one can see, the ADEV
at 1sec is between 7e-10 and 8e-10. I don't know yet what numbers I can
expect from my GPSDO, but from datasheets of commercial GPSDOs I saw that
the ADEV shortly after powerup should be in the 1e-11 region. So how does
one measure such low ADEVs?

To me, it appears that the ADEV at 1sec is roughly the counter's
resolution; a bit less due to averaging. If I take averaging over 3600
samples into account, I think I could expect maybe ~1ns/sqrt(3600) =
16.7e-12 as ADEV at 1 second, but we can clearly see that this is not the
case. So there are two interesting questions arising:

a) I think the ADEV is so high because of the quantization error of the
counter. Assume the time interval measured is right at the transition from,
say, 15ns to 16ns, even the smallest amount of noise will produce some
alternating readings of 15ns and 16ns, which, in turn, results in an ADEV
around 1e-9, right? Further, why is this effect not averaged out with
sqrt(# of samples)?

b) if I want to measure 1e-11 or even 1e-12 at 1sec - what resolution does
my counter need? If the above was true, I would expect that a 1ps
resolution (and an even better stability!) was required to measure ADEV of
1e-12, The fact that the (as far as I know) world's most recent,
rocket-science grade counter (some Keysight stuff) has "only" 20ps of
resolution, but people are still able to measure even 1e-14 shows that my
assumption is wrong. So how are the measurement resolution and the ADEV
related to each other? I plan to build my own TIC based on a TDC7200, which
would offer some 55ps of resolution, but how low could I go with that?


Best regards
Tobias
HB9FSX



On Fri, Mar 20, 2020 at 5:34 PM Bob Q <bobqhome at live.com> wrote:

> I have seen differences between both UCT and Oscilloquartz 8663 ocxo’s.
> The attached plot shows an example. Both boxes use Ublox LEA-6T receiver,
> surveyed in, AD5680 DAC 18 bit DAC, same level shift circuit and same
> control circuit. The reference is an LPRO-101. The Oscilloquartz ocxo was
> purchased used. Both UCT ocxo’s (only the better one is shown) were
> purchased new and have 100’s of operating hours. I have also seen
> differences with constant EFC control voltage. The differences limit what
> performance you can achieve._______________________________________________
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