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

[Joseph Gwinn]

On Fri, 25 Mar 2022 16:48:08 -0400, time-nuts-request at lists.febo.com 
wrote:
time-nuts Digest, Vol 215, Issue 33
>
>    7. Coupling between oscillators -- an example (John Ackermann N8UR)
> 
> Date: Fri, 25 Mar 2022 15:41:46 -0400
> From: John Ackermann N8UR <jra at febo.com>
> Subject: [time-nuts] Coupling between oscillators -- an example
> To: Discussion of precise time and frequency measurement
> 	<time-nuts at lists.febo.com>
> Message-ID: <dcda1bf1-6576-a74f-82b3-b01646c966a4 at febo.com>
> Content-Type: multipart/mixed;
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> 
> 
> 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.
> 
...
> 
> 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!

On ships, there is no such thing as ground at audio frequencies, and 
the usual way to keep ground-loop interference (largely from ships 
propulsion and/or any big radars) is by a coupling capacitor that 
blocks audio frequency currents from flowing through the following RF 
isolation transformer, preventing those currents from saturating the 
isolation-transformer core.


> 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. 

It's the ever-present ship-hull vibration (under full steam) that is 
being compensated for operational vibration, which falls largely 
between ~10 Hz and 200 Hz.  Big gun shocks cause rare noise bursts.  
Also, the shock from a naval gun quickly degrades anything as 
delicate as this kind of time hardware, and so the time stuff is 
never mounted close to a gun.

Joe Gwinn