[time-nuts] Question about frequency counter testing
Azelio Boriani
azelio.boriani at gmail.com
Thu Apr 26 03:06:00 EDT 2018
Very fast time-stamping like a stable 5GHz counter? The resolution of
a 200ps (one shot) interpolator can be replaced by a 5GHz
time-stamping counter.
On Thu, Apr 26, 2018 at 12:28 AM, Bob kb8tq <kb8tq at n1k.org> wrote:
> Hi
>
> Unfortunately there is no “quick and dirty” way to come up with an accurate “number of digits” for a
> math intensive counter. There are a *lot* of examples of various counter architectures that have specific
> weak points in what they do. One sort of signal works one way, another signal works very differently.
>
> All that said, the data you show suggests you are in the 10 digits per second range.
>
> Bob
>
>> On Apr 25, 2018, at 3:01 PM, Oleg Skydan <olegskydan at gmail.com> wrote:
>>
>> Dear Ladies and Gentlemen,
>>
>> Let me tell a little story so you will be able to better understand what my question and what I am doing.
>>
>> I needed to check frequency in several GHz range from time to time. I do not need high absolute precision (anyway this is a reference oscillator problem, not a counter), but I need fast high resolution instrument (at least 10 digits in one second). I have only a very old slow unit so, I constructed a frequency counter (yes, yet another frequency counter project :-). I is a bit unusual - I decided not to use interpolators and maximally simplify hardware and provide the necessary resolution by very fast timestamping and heavy math processing. In the current configuration I should get 11+ digits in one second, for input frequencies more then 5MHz.
>>
>> But this is theoretical number and it does not count for some factors. Now I have an ugly build prototype with insanely simple hardware running the counter core. And I need to check how well it performs.
>>
>> I have already done some checks and even found and fixed some FW bugs :). Now it works pretty well and I enjoyed looking how one OCXO drifts against the other one in the mHz range. I would like to check how many significant digits I am getting in reality.
>>
>> The test setup now comprises of two 5MHz OCXO (those are very old units and far from the perfect oscillators - the 1sec and 10sec stability is claimed to be 1e-10, but they are the best I have now). I measure the frequency of the first OCXO using the second one as counter reference. The frequency counter processes data in real time and sends the continuous one second frequency stamps to the PC. Here are experiment results - plots from the Timelab. The frequency difference (the oscillators are being on for more than 36hours now, but still drift against each other) and ADEV plots. There are three measurements and six traces - two for each measurement. One for the simple reciprocal frequency counting (with R letter in the title) and one with the math processing (LR in the title). As far as I understand I am getting 10+ significant digits of frequency in one second and it is questionable if I see counter noise or oscillators one.
>>
>> I also calculated the usual standard deviation for the measurements results (and tried to remove the drift before the calculations), I got STD in the 3e-4..4e-4Hz (or 6e-11..8e-11) range in many experiments.
>>
>> Now the questions:
>> 1. Are there any testing methods that will allow to determine if I see oscillators noise or counter does not perform in accordance with the theory (11+ digits)? I know this can be done with better OCXO, but currently I cannot get better ones.
>> 2. Is my interpretation of the ADEV value at tau=1sec (that I have 10+ significant digits) right?
>>
>> As far as I understand the situation I need better OCXO's to check if HW/SW really can do 11+ significant digits frequency measurement in one second.
>>
>> Your comments are greatly appreciated!
>>
>> P.S. If I feed the counter reference to its input I got 13 absolutely stable and correct digits and can get more, but this test method is not very useful for the used counter architecture.
>>
>> Thanks!
>> Oleg
>> 73 de UR3IQO
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