[time-nuts] Coupling between oscillators -- an example

[John Ackermann N8UR]

One of the things that needs attention as we measure oscillators more 
carefully is ensuring that there isn't mixing or injection locking 
between multiple sources.  I just encountered a doozy of a case where 
that happened.

I have 3 FTS 9110 5 MHz OCXOs that came out of (dead) FTS military 
cesium units.  They run on 24V and have 4 SMA outputs.  They are pretty 
loud: +17 dBm.

I powered up all three off a common 28V battery supply and let them warm 
up for a day or so (I connected only DC power and not any of the EFC 
connections).  Then I hooked them to the three DUT inputs of the 
PhaseStation using well-shielded cables and started logging.  The 
attached "initial_adev" plot attached threw me for a loop.  That's quite 
a roller coaster ride, particularly on #3019.

I thought that maybe the three +17 dBm signals going into the test set 
were causing some overload, or at least allowed for cross-talk.  So I 
put a 9dB attenuator right at the output of each unit.  That didn't 
really help things -- in fact things look worse on the first run after 
doing that (see the "worst_ripple_adev" plot).

The frequency counter plots show that #3019 is about 0.168 Hz high, 
#3028 is about .012 Hz high, and #9025 is about 0.588 Hz high.  Those 
differences seem about right to create beat notes in the 1 to 10 
second-ish range.  And then there are beat notes beating against each 
other... it gets complicated!   The "worst_ripple_freq" plot is really 
interesting; you can easily see the beat notes.

My next step was to put each oscillator on a separate power supply. 
That helped a lot, but there were still numerous but smaller ripples 
left.  (And I saw that these oscillators are very sensitive to noise or 
variations on their power input; there may not be much regulation inside 
the box.)

Finally, I put an isolation transformer at each oscillator output (just 
after the 9 dB attenuator) so the signal cables had no DC path except at 
the measurement inputs.  That cleared things up almost completely.  The 
"isolated_ripple_adev" plot shows the results -- #3019 still shows a 
possible ripple at around 10 seconds, but that's a small dip on a line 
that's in the low 13s.  I'll take that!

This exercise shows that sometimes you need to use some tricks to 
eliminate coupling, but it can be done.

John

PS -- an interesting feature of these oscillators is that they are an 
"S12" variant that has a little accelerometer bolted on and hooked to 
the EFC circuit to compensate for G forces.  These were ship-board Cs 
units, so I wonder if the accelerometers were intended to compensate for 
ordinary ship vibrations, or maybe the firing of the big guns. :-)
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