[time-nuts] Phase measurement of my GPSDO

Fri Apr 3 15:20:57 UTC 2020

Hi Bob

knowing that my counter's noise floor is terrible (even though I still
don't understand why) I tried to measure the ADEV and MDEV of my GPSDO
against another GPSDO.
>From the graphs, everything below tau=10s is, I would say, rubbish. But I
tend to mistrust these complete results, as I have no means of finding out
whether my reference is so bad or my own GPSDO. The reference is an eBay
GPSDO, and as we all know, these are sometimes of doubtful pedigree.
But still, below the 10s tau, the ADEV and MDEV are so close to the noise
floor that I would say this measurement is useless.

But it still does not explain why my 5335A is so bad.

Tobias
HB9FSX

On Thu, Apr 2, 2020 at 10:17 PM Bob kb8tq <kb8tq at n1k.org> wrote:

> Hi
>
> What you have measured *is* the noise floor of a 5335 when trying
> to use it to measure ADEV. Anything past the numbers on your plot
> will be “past” what the 5335 can “see”. Indeed, even when you get
> close to those numbers, things may get a bit weird due to the fact
> that you are measuring counter “noise” plus device noise.
>
> Bob
>
> > On Apr 2, 2020, at 3:13 PM, Tobias Pluess <tpluess at ieee.org> wrote:
> >
> > 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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> >>
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