[time-nuts] Homebrew frequency counter, need help
Bob Camp
kb8tq at n1k.org
Fri Dec 12 12:41:24 EST 2014
Hi
> On Dec 12, 2014, at 11:44 AM, Li Ang <lllaaa at gmail.com> wrote:
>
> Yes, you are right. 5650_5650 is sig=ref case. prs10_5650 is sig=prs10 and
> ref=5650 case.
>
> Since I really want to reduce the noise, what is the best test set you
> suggest? All the frequency source I have: FE5650 Rb , PRS10 Rb , MV89a*2
> OCXO, Thunderbolt GPSDO, 8663-XS*2 OCXO, Wenzel 100M OCXO(not sure).
>
> How to analyze the ADEV plot to get information about noise?
Since you are running TimeLab, there are multiple plots you can do to look at what is going on. Each one will show you slightly different data. It’s probably best to look at all of them in order to work things out. So far, there is nothing obvious in your ADEV plots that points to a single issue. That’s partially because of the short length of the runs. Most of the data seems to be going as 1/ sqrt( tau). That’s a lot like the sqrt (N) thing when averaging noise. It’s the expected result …
Bob
>
> Thanks
>
>
> 2014-12-13 0:31 GMT+08:00 Bob Camp <kb8tq at n1k.org>:
>>
>> Hi
>>
>> In your original data:
>>
>> Is the blue “5650 to 5650” trace looking at the same source for input and
>> reference?
>>
>> Is the green “PRS10 to 5650” trace looking at two independent sources (one
>> reference and the other as input)?
>>
>> I’m guessing that the answer is yes in both cases.
>>
>> Bob
>>
>>
>>> On Dec 12, 2014, at 10:40 AM, Li Ang <lllaaa at gmail.com> wrote:
>>>
>>> Hi Bob,
>>> The job done by linear regression is to reduce the uncertainty. This
>>> counter is designed to use continous timestamp method. My current design
>>> can measure 9000 times/second. If I only use the 1st and last one to
>>> calculate, it's the traditional recipocal + interploator method. This is
>>> what you can see on the chart named "without linear regression". The
>>> uncertainty of slope(the frequency ratio of ref and signal ) is
>> contributed
>>> by these 2 measurements. With linear regression of all 9000 data within
>> one
>>> second, the uncertainty will reduced to smaller one. (I really can't
>>> remember the ratio. Something like sqrt(9000)).
>>>
>>>
>>> while(1) {
>>> double t3; //fraction part of refcnt, measured by tdc_gp22
>>> double ref_curr;
>>> uint32_t sig_curr;
>>> static double ref_start;
>>> static uint32_t sig_start;
>>> uint32_t refcnt, sigcnt;
>>> const uint32_t gate_time = 1000; // 1000ms gate time
>>>
>>> if (i == 0) {
>>> init_regression(&rv);
>>> cpld_rst();
>>> timestamp(&refcnt, &sigcnt, &t3);
>>> ref_start = refcnt - t3;
>>> sig_start = sigcnt;
>>> i++;
>>> continue;
>>> }
>>>
>>> timestamp(&refcnt, &sigcnt, &t3);
>>>
>>> regression_enter_data(&rv, refcnt - t3 - ref_start, sigcnt -
>> sig_start);
>>> if (msecond < gate_time) {
>>> i++;
>>> continue;
>>> } else {
>>> t = regression_slope(&rv) ; // with linear regression
>>> printf("\r\nFreq=%.*f", 14, t);
>>> ref_curr = refcnt - t3;
>>> sig_curr = sigcnt;
>>>
>>> t = CalcFreq(ref_curr - ref_start, sig_curr -
>>> sig_start); // without linear regression
>>> printf(" Interpolated=%.*f", 12, t);
>>>
>>> msecond = 0;
>>> i = 0;
>>> }
>>> }
>>>
>>> 2014-12-12 21:18 GMT+08:00 Bob Camp <kb8tq at n1k.org>:
>>>>
>>>> HI
>>>>
>>>>> On Dec 12, 2014, at 4:04 AM, Tom Van Baak <tvb at LeapSecond.com> wrote:
>>>>>
>>>>> This large posting is from Li Ang.
>>>>> /tvb
>>>>>
>>>>> ----- Original Message -----
>>>>> From: Li Ang
>>>>> To: Discussion of precise time and frequency measurement
>>>>> Sent: Thursday, December 11, 2014 7:37 AM
>>>>> Subject: Re: [time-nuts] Homebrew frequency counter, need help
>>>>>
>>>>>
>>>>> Hi Bob,
>>>>> I've sent the PCB to the factory and I am waiting for the new board.
>>>> This time, it's a 4-layer borad and changed from CPLD to FPGA. This is
>> the
>>>> first time of FPGA & 4-layer project. Hope everthing be OK.
>>>>
>>>> Very nice looking. I hope it works !!!
>>>>
>>>>> TPS79333 as the LDO for TDC. Better PSRR and noise spec than before
>>>> (XC6206). Analog and digital parts have their dedicated LDO.
>>>>>
>>>>>
>>>>> While I'm waiting the the new board. I did a test with PRS10 & FE5650
>>>> with current board. It's strange that the 20s adev of "without linear
>>>> regression" is better than "with linear regression”
>>>>
>>>> Be careful pre-processing ADEV data. There are a variety of statistical
>>>> “traps” you can fall into. An overly simple explanation is that ADEV
>> looks
>>>> at noise and that most pre-processing is a filter. Filters take out
>> noise.
>>>> Finding one that only takes out the “bad noise” and keeps the “good
>> noise”
>>>> can be quite difficult.
>>>>
>>>> What exactly are you doing in your linear regression computation?
>>>>
>>>> Bob
>>>>
>>>>>
>>>>
>> .<bottom_layer.GIF><power_plane.GIF><top_layer.GIF><adev.gif>_______________________________________________
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