[time-nuts] I am looking to measure phase jitter between two synchronized DDS signals

Jim Potter jpotter at jpaw.com
Wed Nov 30 04:47:00 EST 2016


I just posted this on the HP Equipment forum. Someone reading it 
there suggested I contact timenuts because what I want to do is 
effectively an accurate time stability measurement.

I am attempting to look at the phase jitter between two synchronized 
DDS rf sources, one at 3 Ghz and one at 750 GHz. My 10 Gs/s scope has 
way to much jitter to be useful. A vector voltmeter might be just the 
thing. I have had an HP 8508A VVM and would buy another one if it was 
useful, but it is rated only to 2 GHz. I also have an old PRD 2020 
with the 2021 head that is in working condition. It is rated to 2.4 
Ghz, but syncs on the 750 MHz signal and apparently passes the 3 GHz 
signal somewhat. I get a reading of about -40 dBm for +10 dBm in at 3 
Ghz. However, I do get a phase measurement that varies with the phase 
of the 3 Ghz signal corresponding to the changes I make in the output 
phase. The issue is the noise. I see about +/- 1 degree jitter in the 
phase reading on the panel meter. I also see a drift of 2 to 3 
degrees. I don't think the DDS units are the source of the drift, and 
I'd like to show that the phase noise is lower than what I am observing.

I don't know of any VVM that works at 3 GHz, beyond the out of spec 
use of my PRD unit. Using an oscilloscope is problematic because I 
have measured the trigger jitter on a 100 Gs/s Tektronix unit when it 
was brand new. It's about 3.5 ps RMS. A 3 GHz signal has a period of 
333 ps so that corresponds to 10 degrees. I see that Tek has a new 
scope with 70 GHz bandwidth and 200 Gs/s, but I don't think that is a 
big enough improvement over the 100 Gs/s unit. In any case even 
renting one of these is probably too expensive.

I would appreciate any suggestions for how to accurately measure the 
phase jitter. I have a Tektronix RSA 306A which has been handy, but 
I'm uncertain about how to use it to measure phase jitter. In any 
case, what really matters is not the jitter of the individual signals 
but the jitter in the phase difference between the two signals which 
are all derived from the same time base.

One obvious question is, if I can't measure it how can it matter? The 
answer is that each rf unit is a signal source for a section of 
linear accelerator. If the phase jitter is excessive, it will show up 
as loss of beam quality and current. It's a bit hard to set up a 
proton accelerator to see if the jitter is acceptable. I really need 
able to make a bench measurement to qualify the performance.

Thanks in advance for any suggestions.

Jim


James M. Potter PhD, Pres.
JP Accelerator Works, Inc.
2245 47th Street
Los Alamos, NM 87544

TEL:505-690-8701
FAX: 888-301-2833

mailto:jpotter at jpaw.com
http://www.jpaw.com




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