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

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

```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.

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

-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

```