It appears that the EOM driver in the north path is not amplifying RF signals as we might expect. This is used to drive the broadband Newfocus 4004 EOM at 14.75 MHz. On Friday I had disconnected it from its power supply to re solder a banana plug. I don't have a record of the voltage used so I have used the specified ±18 V in its schematic (see PSL:1090). It was not this value before, I think I remember ~14.5 V
I have now managed to reoptimize the alignment into the north cavity but was unable to lock. It appears that there is no error signal being generated. I checked the RF coming out of the amplfier with it powered up, it was -28 dBm (with 50Ω load of the Aglient 4395A) the 14.75 MHz RF signal going into the amplifier was -14 dBm. It is evident from Tara's posts on testing and returning the EOM driver (see PSL:1092 and others in that sequence) that gain should be on the order of 18.5 dB to 23 dB. Something is not right.
When I power down the EOM driver all the RF goes, so it is actively doing something. I hooked the input and output to the Aglient and attempted a transfer function (attached below). Its not obvious that there is any resonant amplification. Not sure what is going on here. It is clear that that it seems to be attenuating signal going through it and not helpfully amplifying.
I measured the transfer function with the Aglient, strait through from input to output, I realize now that this isn't quite right because the driver is designed to be loaded with the EOM at its output to work properly.
I did this again with the EOM driver connected directly to the Newport 4004 and looking at the response through the monitor port. There is kind of a a peak but nothing like what Tara had here PSL:1317
I realize now, that the EOM driver needs to be connected to the EOM to operate correctly. However, its still no clear why the transfer function has changed shape and what is leading to no error signal being produced at the monitor port of the FSS.