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

Martyn Smith martyn at ptsyst.com
Sat Aug 29 07:54:09 EDT 2015



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.


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 

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.


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


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


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!!!


Martyn Smith

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