[time-nuts] Re: Build a 3 hat timestanp counter

Wed May 25 14:37:27 UTC 2022

Thanks for your answer and the many suggestions what can be improved. 

The first picture shows my concept for a prototype. 

The input shaper consists of a 4:1 transformer with differential output
and a TLV3501 comparator. The digital part with divider and start/stop
logic fit for all 3 channels into a XCR3064XL CPLD. Maybe it is better
to separate the channels later. The MC is a STM32H743 and runs with 450
MHz (pll), clocked from the reference frequency. TDC7200 used as TDC. 
The measurement with spi readout takes about 5Âµs, so I decided for 10Âµs
(100 kHz) sample time. 

The second picture shows the measuring timing inside CPLD. 

The TDC7200 runs in mode 1 and supplies only the fine time. The MC runs
a 10 MHz (reference) counter with 3 capture channels as coarse time. So
I only have to read the fine timer and the calibration register from the
TDC. 
The TDC cannot measure from 0, so a reference cycle is added. (t = x
+100ns). 

For the averaging I had thought of a linear regression. 

Hans-Georg 

Am 2022-05-25 01:18, schrieb Magnus Danielson via time-nuts:

> Hi,
> 
> The first limit you run into is the 1/tau slope of the measurement setup. This is often claimed to be white phase modulation noise, but it is also the effect of the single-shot resolution of the counter, and the actual slope level depends on the interaction of these two.
> 
> So, you might want to try a simple approach first, just to get started. Nothing wrong with that. You will end up want to get better, so I will try to provide a few guiding comments for things to think of and improve.
> 
> So, in general, try to use as high frequency as you can so that as you average down, your sqrt(f/f0) gets as high as possible as the benefit will be 1/sqrt(f/f0) where f is the oscillator frequency and f0 is the rate after average.
> 
> As you do ADEV, the f0 frequency will control your bandwidth.
> 
> The filter effect of the averaging as you reduce and sub-sample will help to some degree with anti-aliasing, but rather than doing averaging, consider doing proper anti-aliasing filtering as the effect of aliasing into these measures is established and improvements into the upcoming IEEE Std 1139 reflect this. In short, aliasing folds the white noise and straight averaging tends to be a poor suppressor of aliasing noise.
> 
> For white phase modulation (WPM) the expected ADEV response depends linearly with the bandwidth of the measurement filter. It's often modelled as a brick-wall filter, which it never is. For classical counters, the input bandwidth is high, then the sampling rate forms a Nyquist sampling frequency, but wide band noise just aliase around that. Anti-aliasing filter helps to reduce or even remove the effect, and then the bandwidth of the anti-aliasing filter replace the physical channel bandwidth. If the anti-aliasing is done digitally after the counter front-end, you already got some aliasing wrapping, but keeping that rate as high as possible keep the number of overlays low and then filtering-wise reduce it will get you better result.
> 
> For aliassing effects, see Claudio Calosso of INRIM. Great guy.
> 
> This is where the sub-sampling filter approach is nice, since a filter followed by sub-sampling removes the need to produce all the outputs of the original sample rate, so filter processing can operate on the sub-sampled rate.
> 
> As your measures goes for higher taus in ADEV, the significant amount of the ADEV power will be well within the pass-band of the filter, so just making sure you have a flat top avoids surprises. For shorter taus, the anti-aliasing filter will be dominant, so assume first decade of tau to be waste.
> 
> I say this to guide you to get the best result with the proposed setup.
> 
> The classical three-cornered hat calculation has a limitation in that it becomes limited by noise and can sometimes result in non-stable results. The Grosslambert analysis is more robust, since it is essentially the same as doing the cross-correlation measurement. The key is that you average down before squaring where as in the three-cornered hat to square early and is unable to surpress noise of the other sources with as good quality. For Grosslambert analysis, see FranÃ§ois Vernotte series of papers and presentation. FranÃ§ois is another great guy. I spent some time discussing the Grosslambert analysis with Demetrios the other week. I think I need to also say that Demetrios is a great guy too, not to single him out, but he really is.
> 
> There is another trick up the sleeve thought. If you do the modified Allan deviation (MDEV) processing, it actually integrate the sqrt() trick with measurement, achieving a 1/tau^1.5 slope for the WPM. This will push it down quicker if you let it use enough high rate of samples, so that you hit the flicker phase-modulation slope (1/tau), the white frequency modulation slope (1/tau^0.5) and finally flicker frequency modulation (flat) quicker. The reference levels will be different from ADEV for the various noise-types, but that you can look up in tables and correct for.
> 
> Cheers,
> Magnus
> 
> On 2022-05-24 18:37, Hans-Georg Lehnard via time-nuts wrote: 
> 
>> Hi,
>> 
>> my Name is Hans-Georg Lehnard from Germany and I'm new here, worked as a
>> developer for hardware then for software and last as a system developer.
>> Now I'm retired and I can play with hardware again ;-).
>> 
>> I have:
>> 
>> 4 x 20MHz Rubium (TEMEX MCFRS-1),
>> 2 x 10MHz HP10811-60111
>> 1 x Samsung UCCM GPSDO
>> 1 x FA2 counter.
>> lots of OCXO
>> 
>> and try to build a house standard that I can trust and qualify my
>> oscillators.
>> Reproducible measurements with the FA2 in 10s precision mode I trust to
>> 10E-11.
>> The short-term stability of the HP oscillators cannot be measured with
>> it, or both are defective.
>> The FA2 is not suitable for short-term measurements of 0.01 ... 1s.
>> 
>> For measurements against a reference frequency, the stability of the
>> reference must be 5 to 10 times better than the measured frequency, and
>> I don't have that. Now there are 2 options DMTD mixer or 3-hat
>> measurements.
>> Because I'm a digital person I chose the 3-hat method.
>> 
>> The idea is now to divide the 3 measuring frequencies (20 or 10 MHz)
>> down to 100Khz and to measure the phases with a TDC against the next
>> reference edge. Average the measurement results until I am down to 0.001
>> ... 1 s. That should improve the 100ps resolution of a TDC7200 far
>> enough and can also be output via RS232.
>> 
>> Are my thoughts correct and could it work ?
>> 
>> Hans-Georg
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