[time-nuts] Precision Freq Measurement in Short Time

lymex at vip.sina.com lymex at vip.sina.com
Fri Jul 29 09:40:30 EDT 2005

Hi everyone who has SR620,

I found a way to measure/compare two frequencies to 1E-12 

in 1 second by SR620.

This is desirable in order to measure the Allan Deviation

for tau = 1 second or adjust frequencies in real time.

By surfing the net I found an article from SRS describing

a way how this can be done at:


This is the PRS10 manual, refer appendix B for details.

The principle is:

If RS620 is used to measure frequency, the RMS jitter is 

is typically 100ps. However, the jitter will be reduced to 

25ps if we measure time interval. If we take 1000 samples 

per second and average the result, the RMS jitter will be

less than 1ps (25/sqrt(1000)).

I tried this method several time before by feeding the A 

and B input the same 10MHz source, and the result is not 

as good as promised. It always jump more than 25ps and 

sometimes more than 50ps. When I look at the Jitter:Allan 

(for every second), it something like 20ps. Until today

I found a way to modify it by:

1) Tee the input again and use this 3rd 10MHz as the

external reference for SR620 (I use SR625 as reference 

before, which is wrong)

2) Make the input A  none 50 Ohm input.

When I look at the Jitter:Allan again it says <9ps!

And the mean reading is very much stable to <2ps!

I'm confident (sigma=2) that the consecutive pair will not

change more than 1ps because about 95% of the reading 

will be changed not more than 1ps. Here is the short video 

to demonstrate this (6.0MB):


Therefore, if we replace input B with another 10MHz, we

can observe the drift of a few parts in 10^12 in 1 sec.

If we can record 100 readings (in 100 seconds) and make

some calculation (in Excel, say), we will get the Allan

deviation down to perhaps 1E-12.

Here is the example of my calculation: 

Video (9.6MB): http://www.dl-car.com/~2038/time/DSCN5104.MOV

Calculation: http://www.dl-car.com/~2038/time/ADEV.XLS

Input A: HP58540A, holdover

Input B: Trimble Thunderbolt locked to GPS

Any comments of corrections?


Lymex, BG2VO


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