[time-nuts] Homebrew frequency counter, need help

Magnus Danielson magnus at rubidium.dyndns.org
Sun Dec 14 11:36:17 EST 2014

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On 12/12/2014 06:15 PM, Bob Camp wrote:
> 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.
>
> In the “both same (5650 / 5650) case” your linear regression filtering is faking you out a bit. The SR620 counter has exactly this same issue. That’s probably for the same reason. It’s a fine test to see if you have various problems under control. It’s not a perfect way to estimate the number of digits you will get on a real measurement. Using two independent sources is a better way to do that.
>
> When you have two identical signals, the TDC noise is the main issue. All the edges are arriving in the same relation to each other (same timing). The linear regression is (obviously) good at suppressing the sort of noise the TDC has. With two independent signals the noise is more complex. The edges arrive at various times relative to each other. More things contribute to the total noise. The linear regression is having a harder time suppressing that sort of noise. In some cases (as you observed) the linear regression is actually making things worse.
>
> If Magnus was here, he would be tossing empty beer bottles at me and saying — see Bob, sqrt(N) doesn’t always work ….

Indeed, except I would not be tossing empty beer bottles at you, I might
jokingly attempt do, but never actually throw it. One has to realize
that the quantization noise of the TIC may seem to process as if it
where white phase noise, but it isn't random noise, it is a systematic
noise and if you fool around with the systematics is may work for you or
against you.

I do consider to pass another bottle of good beer to Bob for good
behavior. :)

> The filtering process used does need to be adapted to the noise of the total system.

It's one of the forgotten parameters, and I've even seen good Sam Stein
stand up and say "we used to do this wrong" on the same point, you need
to publish and consider the bandwidth of your processing, as it *will*
affect the ADEV plot (but only MDEV and TDEV somewhat).

>>
>> 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,
>
> If the MV89’s are in good working condition, they are the best thing to compare.The have the best ADEV of the group you have available I would check them for output level and stability before I trusted them. There are a lot of defective parts on the market. People get some, sort them and sell the bad ones. The bad ones just keep getting re-sold again and again … My guess is that they were good parts at one time and they got damaged when pulled off boards. If you use them, keep them on power at all times. Any OCXO will do better if you run it that way.

In order to test if systematics is messing badly with you, measure the
ADEV of the oscillator as it is steered (and stabilized) to a number of
different frequencies. For larger offsets to the counter reference,
multiple beatings occurs within the regression interval. You want that
number to be an even number of beats, or the beat count to be so large
that the phase of the last beat does not care. Linear regression helps
out, as it weighs out the outermost measures compared to the central
one, making the beating at the beginning and end not care as much.

These are *systematic* noise effects, and as you play around with
systematics and processing, you might have the systematics works for or
against you, but at the same time, the random noise you try to measure
will suffer the processing filtering, and you need to recall that. If
you balance these properly, you can make good and correct measurements,
it's just that few do.

Oh, and only use ADEV, MDEV and TDEV to estimate random noises, system
noises as they show up there should be estimated separately and removed
from the random noise estimates. They have *way* different behaviors.

Cheers,
Magnus - considering what beer will be best to start the evening with
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