[time-nuts] Averaging effects

[WarrenS]

Thanks Bruce, your feedback is always helpful and welcomed.

The TPLL is not a cure all for everything. It has it's own set of problems 
and trade-offs,
but I have found it to be simple, viable alternative that can achieve good 
results with a minimum of parts and cost.

As far as injection locking between the reference osc and the DUT,
I tried it with and without buffers, and have not seen any problems or 
difference at below 1e-12 using a 10811 as the reference.
But buffer(s) are always a good idea for many applications, then again most 
high end things
I've tried it on like Cs, Rb and active Masers have good buffers already on 
them.

What one need to watch out for that can cause injection locking problems,
(or more accurately slow Phase/freq modulation) is if there is another Osc 
around that is not on the exact same frequency,
That can certainly cause problems like Magnus described with his HP5370B 
anomaly, even when using buffers,
by coupling thru the air, PS, between cables, grounds, etc.

Yes, have to be careful not to add analog dead time between readings.
One easy way to achive "Zero" dead time between tau readings is to use 
oversampling techniques with proper filtering.

ws

*********************
Bruce Griffiths bruce.griffiths at xtra.co.nz  wrote:

The tight PLL isn't a cure all for injection locking which creates a
frequency offset despite the PLL (see p102 of Wolaver).
If the isolation is inadequate instability in the injection level and
phase shift due to environmental fluctuations will cause a measurable
effect.
The isolation achieved depends both on the OCXO reverse isolation and
that of the measurement setup.

If the frequency offset due to injection is to be less than 1E-12 then
the reverse isolation needs to be greater than 120dB for a loaded
crystal Q of around 1E6.
Alternatively with a crystal Q of 100,000 a reverse isolation of 120dB
coupled with an injection amplitude stability of 10% or so may suffice.

Another trap when using some DVM's to measure the frequency fluctuations
in the tight PLL method is the finite dead time between measurements.

Oscillators that are not deliberately phase locked also exhibit
injection locking effects if the frequencies of the 2 oscillators are
close enough (within the capture range of the equivalent first order
PLL) and the inherent instability of the  2 oscillators isnt large
enough to preclude locking due a too narrow equivalent PLL bandwidth.

Bruce

*******************
ws at Yahoo wrote:
> Others said:
>> There is a downside to this approach which should be understood, it
>> will also averaging out the white noise of the DUTs.
>> The time interval counter method severely undersamples the phase
>> noise spectrum leading to aliasing effects.
>> The measured ADEV depends on the associated filter bandwidth
>> Filtering is tricky since you will both reduce the measurement
>> systems noise as well as the the DUTs noise.
>> The aliasing effect is definitively there.
>> Question is how to remove the system noise from the DUT noise
>
> One way to avoid those trade-offs is to do the frequency difference
> averaging with a TPLL (Tight Phase Lock Loop) using a "proper"
> integration of it's output over the tau time period.
> example at:     http://www.thegleam.com/ke5fx/tpll.htm
>
> ws
>
> ******************
> Magnus Danielson magnus at rubidium.dyndns.org
>
> Hej Bruce,
>
> On 12/27/2010 08:13 PM, Bruce Griffiths wrote:
>> Tom Van Baak wrote:
>>>> 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.
>
> True, but it is tricky since you will both reduce the measurement
> systems noise as well as the the DUTs noise (which is what you intend to
> measure).
>
> The aliasing effect is definitively there.
>
> Question is how to remove the system noise from the DUT noise, and I
> know of only approach which really avoids it is cross-correlation, but
> otherwise it is only various measures to remove and filter out the
> signal from noise before it is folded in, i.e. conservative design
> measure.
>
> Anyway, I wanted to play around with averaging to see how the filtering
> effect behaves.
>
> Cheers,
> Magnus
>
>