[time-nuts] Averaging effects

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Dec 27 19:13:40 UTC 2010


Tom Van Baak wrote:
>> I've used various arming rates to the DTS-2070C from 1 Hz to 10 kHz 
>> and also varied the averaging block size accordingly such that I will 
>> get one reading every second.
>
> That's a very nice plot; textbook perfect. Thanks for posting it.
>
> What GPIB data rate did you actually get for the 10 kHz trace?
> When JohnM ran the same test on my 2070C it was closer to
> 0.74 s instead of 1.0 s. I'm wondering if you found a way to
> get your DTS pacing more accurately.
>
>> There is a downside to this approach which should be understood, it 
>> will also averaging out the white noise of the DUTs.
>
> Correct. A similar white noise effect can happen if you average
> the raw data itself. See the plot at the bottom of:
> http://www.leapsecond.com/pages/adev-avg/
>
Its a little more complicated than that.
The measured ADEV depends on the associated filter bandwidth (typically 
for 1Hz sampling one uses a low pass (for the phase fluctuations) filter 
bandwidth of 0.5Hz or less).
When one uses a time interval counter the counter input system noise 
bandwidth may be as high as 100MHz (5370A/B) or 500MHz or more (DTS2070) 
whereas the crystal oscillator buffer amp (principal source of OCXO 
white phase noise floor) may have a somewhat lower bandwidth. The time 
interval counter method severely undersamples the phase noise spectrum 
leading to aliasing effects.
Averaging of this type creates a low pass filter that will reduce the 
system noise to a large extent whilst not greatly affecting the 
measurement as the equivalent filter bandwidth will still be much larger 
than 0.5Hz and the equivalent filter response is far from ideal.


>> The V-shape of the curves comes from the white-noise limit slope (low 
>> taus) and the drift-rate limit (high taus). I have not performed any 
>> drift rate compensation and the OSA 8600 has only been heated a few 
>> days, so the drift is still a bit high.
>
> Did they flatten out with HDEV? A couple of days warm-up will
> be enough so that the frequency drift over just 10 minutes can
> be safely removed in software. JohnM -- is there a linear trend
> removal option to TimeLab that works on the freq series just like
> works on the time (phase) series?
>
>> A peculiar effect is that to make good readings for low-tau values I 
>> need to trim the oscillators to be very near each other. Otherwise 
>> there will be a polution of the lower taus compared to my selected 
>> good plots. This polution and the slope is insensitive to any drift 
>> rate compensation, so Hadamard analysis does not help.
>
> Have you tried better isolation between the two BVA and
> the DTS to make sure it isn't that? How does the DTS input
> isolation spec compare to say a TSC 51xxA?
>
>> I have not been able to pin-point how this frequency offset effect 
>> really works with ADEV, but currently I suspect it has something to 
>> do with quantization and averaging... but I haven't had time to 
>> verify that.
>
> Yes, this sounds plausible. I suspect the DTS2070 (or HP5370
> or SR620) will give better looking (though not necessarily more
> truthful) results if you hit the very same interpolator bin on every
> sample. I can send you examples of this.
>
> Do you think this might be happening in your case? If so
> you might be able to make it even more visible if you use
> one of the BVA as the ext ref to the DTS. Or use the same
> BVA for both inputs (or all three inputs!).
>
> Some cheaper counters avoid this effect with clock dithering.
> Or you essentially get dithering for free with any lesser grade
> DUT. But in this experiment you're using two BVA and a DTS
> so there is almost no noise to begin with. I would think this is
> very test setup you would use for exposing an interpolator
> in-a-rut effect if that is what you were trying to do.
>
> /tvb
>
Bruce




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