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

[hans-georg at lehnard.de]

Hi,  

I know Michael and his work, we have private email contact and I also
have a meter with TDC7200 from him.  

I would like to solve 2 problems. 

1.) I have 4 working Temex MCFRS-01 Rb oscillators (stability up to
4*10E-11 per month) and no better reference against I can measure in the
range of seconds. Therefore the idea with the 3-hat principle. 

2) I have 2 HP 10811 OCXO and want to measure the short time stability
(< 1s) in the range of 10E-12. 

With the BG7TBL FA2 counter I can make long time measurements over hours
against my Samsung UCCM GPS and with the HPE1740A short time
measurements up to 1.6s with 50ps resolution. 

Both problems can not be solved with the FA2 and also not with the Mino
counters. 

I will now try to optimize only one channel as recommended by Magnus and
bring the raw timestamps to the Pc and evaluate them there. 
Then we can see if the faster sampling and averaging can solve my
problems. 

Hans-Georg 

Am 2022-05-31 22:53, schrieb zfe via time-nuts:

> 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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