[time-nuts] Method for comparing oscillators

[John Green]

I have studied the dual mixer approach and the consensus is that it is the
most accurate method. However, it seems pretty difficult to obtain that
accuracy. I do have some DBMs with IF response down to DC. I don't have  a
10811 but do have a pretty good oscillator to use for the offset. The
problem comes in with the time interval counter. The only thing we have is
an old 5328A. I believe, at this time, DMTD is just not possible for me to
do. My oscilloscope method seems to work pretty well. I can't produce graphs
showing frequency stability but that isn't a big deal for me. I just want to
be able to compare a Rb source to a GPSDO and look at several OCXOs either
stand alone or in equipment we have here. If I figure correctly, an error of
1e-12 is 1 Hz every 27.7 hours if comparing 2 10 MHz sources. I don't have
to wait for a full cycle to occur, I can see pretty small phase differences.
Let's say I can see a 10 degree change. That would cut the observation time
down to just over 3 quarters of an hour. Not bad. Most OCXOs will move a lot
more than that so shorter times would work for them. I know from past
experience that this works pretty well for looking at warm up performance.
My first experiment will be the hardest. I am going to check a couple of Rb
sources against a Tbolt. I'll let you know how this works out. Thanks for
all the input.