[time-nuts] Timepod Phase Noise Measurements and 3 corner hat

Magnus Danielson magnus at rubidium.dyndns.org
Sat Aug 29 09:10:23 EDT 2015


Martyn,

Thanks for this report. This is exactly why I invested in the time-pod 
and why I use 2 reference sources for measurements.

It would be interesting to see papers on the validation of these 
measurements.

Cheers,
Magnus

On 08/29/2015 01:54 PM, Martyn Smith wrote:
> Hello,
>
> HISTORY
>
> I make GPS/GNSS  frequency standards (ultra low phase noise types) and I
> always have to prove my results to the customer.
>
> John Miles told me how to make absolute phase noise measurements using
> three sources with the timepod.
>
> MEASUREMENT TECHNIQUE
>
> We have three sources close in frequency (within 1 x 10E-8 of each other).
>
> Two sources are connected to the Ch0 and Ch 2 inputs of the timepod, the
> unit under test to the ref input.
>
> According to John, the result will be the actual phase noise of the ref
> input, even if it is lower than the two other sources.
>
> No further calculations need to be made.
>
> John asked me for results.
>
> First of all I tried three sources very close in frequency (1 x
> 10E-12).  I did measurements at bandwidths of 0.5Hz, 5 Hz and 50 Hz.
>
> The reason I did this is because the actual two reference sources I
> wanted to use, weren't going to be exactly the same frequency, but 1 x
> 10E-8 apart.
>
> My results showed me that the bandwidth made no difference to the
> measurements.
>
> So I now made three sets of measurements with unit A, unit B and using
> references units C and D.
>
> As they weren't exactly on frequency I chose a 50 Hz bandwidth, so the
> frequency differences didn't come into play.
>
> So I measured unit A and unit B using the above technique.
>
> Then I used the normal timepod technique and connected unit A to the
> main input, unit B to the ref input and recoupled the two SMA jumpers.
>
> So I have three results, unit A's phase noise, unit B's phase noise and
> the phase noise of both unit A and B together.
>
> RESULTS
>
> I got the following at offsets of 1/10/100/1k/10k/100kHz:
>
> Unit A made -112.6 / -141.3/ -160.0 / -165.4/ -166.9 / -167.1
> Unit B made -113.7 / -138.9 / -160.0 / -167.2 / -168.7 / -168.9
>
> Both together made:
>
> -110.2 / -137.0 / -156.6 / -163.1 / -164.6 / -164.8
>
> CHECKING RESULTS
>
> So if we combine units A's and unit B's individual phase noise, then
> compare the combined result phase noise, is the combined result 3 dB
> higher, as it should be.
>
> I did the maths and the combined measurements were exactly 3 dB higher
> (plus/minus 0.1 dB) except at 100 Hz offset which differed by 0.4 dB.
>
> But 100 Hz is always difficult because I made these measurements on a 50
> Hz system so there is always a 100 Hz spike to contend with
>
> CONCLUSION
>
> John Miles technique works well.  I now get immediate results of my
> frequency standards as I will always use the 3 source method of
> measurement on the timepod.
>
> John have saved me about 10 hours a week of measurements!!!
>
> Regards
>
> Martyn Smith
>
>
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