DickG, MarcP

RF Noise    via CER TO ISC RACK CONTROL ELECTRONICS cabling:  RF= 9.100230 MHZ

For a locked working H1 IFO:
We have measured the coupling of a near RF frequency (pseudo noise) 9.100230 on cable ground..  
noise signal to "DARM"   as seen on control Room aLIGO DARM display.  
We here first try the main 9.100230 Mz cable, looking for a nearby beat "interference" as seen in DARM.
We couple an ISR RF signal at -20 dBV ie .1 Vrms and 9.100230 MHZ + 55 Hz, 65 Hz, and 105 Hz successively.

The coupling is inductive, via a ferrite core + single loop BNC to ISR RF Gen. around the main 
CER to ISC Rack 9.100230 MHz cable;  the cable and loop are thus a 1:1 transformer; a new gadget.
The driven loop plus ferrite induces external shield current at ~ 50 ohm cable level at 0 dBV  ie 1V rms.
The ISR freq was at 9.100285, 9.100295, and 9.100335 MHz.

> A clean 1.3 e-18 m/sqrthz level DARM "noise signal" spike  was seen at 55 Hz, 65 Hz, and 105 Hz 
> for the 3 frequencies  consistent with beating somewhere in the IFO 9.100230 MHz control detection systems.

>> The IFO tolerated a 'visible' noise signal...stayed locked.

We expect a ground noise signal on one cable gets shared amoungst all of the CER to ISC cables.
The transformer stratagem for driving or detecting on cable promises to make possible finding and curing 
the sources and leaks in our large  and imperfect system.
 
>>> The ferrite cable xfmr coupled to an RF Anal or source   is a good cable ground current noise diagnostic.

We expect to map out and understand the cable and module cross couplings, sources, and leaks; 
and perhaps control module negative features, 
and propose cures, fixes.