[time-nuts] Re: Build a 3 hat timestanp counter (hans-georg at lehnard.de)

[zfe]

Acam, now ScioSense, has also the 4 channel GPX2 that achieves up to
35Msps over LVDS. It has the same time resolution as the AS6501.

Michael Nowak (http://www.mino-elektronik.de/) build nice simple little
counters with the TDC7200 and the AS5601 (the later is not on his
website, I think) that do gapless measurements and have the option to
use internally linear regression (the AS6501 version with more than
100.000 sps). He kindly provided me with several revisions of prototypes
of both counters for testing.

In loopback measurements the improvements of linear regression are as
expected. The problem is that the performance of linear regression can
break down when DUT and reference have the same (stable) frequency  with
a small unfavorable offset. Unfortunately this is easily the standard
situation if you test two 10MHz oscillators against each other. Things
get worse the shorter the measurement time is. I think the three
cornered hat approach will not necessarily eliminate that problem, it is
more for overcomming the limits of the reference oscillator.

Achieving a good part of the higher performance is possible if you
adjust the frequencies of DUT and reference carefully and if they are as
stable as supposed. But that is a messy process where I am always in
doubt if the displayed result is the performance of the DUT/reference or
a problem with the counter, unless I do several cross checks.
Best would be the possibility to use  reference with a frequency skew to
the DUT.

I attached a quick example with Michael Nowaks AS6501 counter:

There are three loopbacks one without linear regression, and two with
linear regression (with 1s and 10s measurement time).

Then I compare a 10MHz OXCO, with good short time stability, to a well
performing FE-5680
Without linear regression the Allan deviation below 100s is due to the
AS6501 resolution and not due to the oscillators.
Next measurements with linear regression.
If I match the frequencies to about 1mHz the limits come mostely from
the FE-5680.
If I adjust to 0.5Hz offset the linear regression breaks down to the raw
AS6501 performance and worse.

To demonstrate that that OCXO performs well with 0.5Hz offset, I use it
as reference to measure a fine 16.384MHz OCXO. What you see in the plot
is, more or less, the performance of the 10MHz-OCXO.
There is also a plot of the 16.384MHz OCXO against the FE-5680, with and
without linear regression.
Combining both you see that linear regression gives improvements - you
get at least about 10^-12 with 1s measurement time. I have no better
10MHz oscillator to explore the limits with 1s measurement time.
But if you decrease the measurement time you fast get in messy
measurements that give you headache with attributing the culprit.
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