[time-nuts] Question about frequency counter testing

Glenn Little WB4UIV glennmaillist at bellsouth.net
Sun May 27 13:03:31 EDT 2018


It appears that I replied to the wrong message, please ignore.

Glenn

On 5/27/2018 11:52 AM, Oleg Skydan wrote:
> Hi!
>
> From: "Magnus Danielson" <magnus at rubidium.dyndns.org>
>>>> You build two sums C and D, one is the phase-samples and the other is
>>>> phase-samples scaled with their index n in the block. From this you 
>>>> can
>>>> then using the formulas I provided calculate the least-square phase 
>>>> and
>>>> frequency, and using the least square frequency measures you can do
>>>> PDEV. The up-front processing is thus cheap, and there is meathods to
>>>> combine measurement blocks into longer measurement blocks, thus
>>>> decimation, using relatively simple linear processing on the block 
>>>> sums
>>>> C and D, with their respective lengths. The end result is that you can
>>>> very cheaply decimate data in HW/FW and then extend the properties to
>>>> arbitrary long observation intervals using cheap software 
>>>> processing and
>>>> create unbiased least square measurements this way. Once the linear
>>>> algebra of least square processing has vanished in a puff of logic, it
>>>> is fairly simple processing with very little memory requirements at
>>>> hand. For multi-tau, you can reach O(N log N) type of processing 
>>>> rather
>>>> than O(N^2), which is pretty cool.
>>>
>>> I had some free time today to study the document you suggested and do
>>> some experiments in matlab - it was very useful reading and 
>>> experiments,
>>> thanks!
>>
>> Thanks for the kind words!
>>
>>> It looks like the proposed method of decimation can be
>>> efficiently realized on the current HW.
>
> I had some free time yesterday and today, so I decided to test the new 
> algorithms on the real hardware (the HW is still an old "ugly 
> construction" one, but I hope I will have some time to make normal HW 
> - I have already got almost all components I need).
>
> I had to modify the original decimation scheme you propose in the 
> paper, so it better fits my HW, also the calculation precision and 
> speed should be higher now. The nice side effect - I do not need to 
> care about phase unwrapping anymore. I can prepare a short description 
> of the modifications and post it here, if it is interesting.
>
> It works like a charm!
>
> The new algorithm (base on C and D sums calculation and decimation) 
> uses much less memory (less than 256KB for any gaiting time/sampling 
> speed, the old one (direct LR calculation) was very memory hungry - it 
> used 4xSampling_Rate bytes/s - 20MB per second of the gate time for 
> 5MSPS). Now I can fit all data into the internal memory and have a 
> single chip digital part of the frequency counter, well, almost single 
> chip ;) The timestamping speed has increased and is limited now by the 
> bus/bus matrix switch/DMA unit at a bit more then 24MSPS with 
> continuous real time data processing. It looks like it is the limit 
> for the used chip (I expected a bit higher numbers). The calculation 
> speed is also much higher now (approx 23ns per one timestamp, so up to 
> 43MSPS can be processed in realtime). I plan to stay at 20MSPS rate or 
> 10MSPS with the double time resolution (1.25ns). It will leave a 
> plenty of CPU time for the UI/communication/GPS/statistics stuff.
>
> I will probably throw out the power hungry and expensive SDRAM chip or 
> use much smaller one :).
>
> I have some plans to experiment with doubling the one shoot resolution 
> down to 1.25ns. I see no much benefits from it, but it can be made 
> with just a piece of coax and a couple of resistors, so it is 
> interesting to try :).
>
> All the best!
> Oleg UR3IQO
>
>
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Glenn Little                ARRL Technical Specialist   QCWA  LM 28417
Amateur Callsign:  WB4UIV            wb4uiv at arrl.net    AMSAT LM 2178
QTH:  Goose Creek, SC USA (EM92xx)  USSVI LM   NRA LM   SBE ARRL TAPR
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