Jenne put the MC board on extender today - its still that way but everything is probably connected (check AO).
We measured the TFs of the DAQ section for MC_F because of how everything looked wrong in the plots Jenne
put in the log earlier. Everything we measured today seemed to jive with the schematic. We also looked up
the original traveler for this board which Betsy filled in years ago: it also is in spec for the DAQ filtering.
So then I looked at the power spectrum of the output signal to the VCO. It had lots of HFC (high frequency crap).
I adjusted the parameters of the FSS (common gain, fast gain, RF phase  ) and lowered the MC common gain. This
produced a global minimum in that 4D parameter space.
I think that basically, the FSS gain is too low even with the common gain slider maxed. Having the fast gain up
at 19 dB like I had left it was bad - even though it minimizes the PC control signal, it produces a lot of HFC
up around 100 kHz in MC_F. After John (finally) gets around to measuring the FSS loop we can figure out how to
better tune this. The MC gain then has to be tuned so as to best minimize the HFC given the new FSS gain; there's
basically no coupling from the MC gain to the FSS loop shape so its always best to tune the FSS first. 
The RF phase of the FSS was a mystery - I have no idea why it should do anything and I have never heard of this
and I don't know why I tried it today. But...changing it by ~0.6-0.7 slider units reduced the HFC by another factor
of ~3. Somebody should put this slider into units of degrees.8-)
Here's a table of the changes. Please make these the new nominals:you asked for: diff 2008/07/21,13:00 2008/07/22,2:44:16 utc '.*FSS.*|.*MC.*'
LIGO controls: differences, 2008 07/21 13:00:00 utc vs. 2008 07/22 02:44:16 utc
__Epics_Channel_Name______ __Description__________ __value1____ __value2____
C1:IOO-MC_REFL_GAIN 22.000000 19.000000
C1:IOO-MC_REFL_OFFSET 0.818380 0.818100
C1:PSL-FSS_MGAIN 10.000000 30.000000
C1:PSL-FSS_PHCON 2.073170 1.413170
The attached plot shows the "SERVO" TNC output of the board; this is supposed to be the same as the voltage going to the
VCO box. So its V/Hz transfer function is flat above 40 Hz. Tomorrow Jenne will post more data and remove the extender
board.
Since I only used an SR785, I only saw noise up to 100 kHz. Its key to use an RF spectrum analyzer when checking out
the FSS and the MC systems. |