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Sun Oct 28 03:03:42 2007
Three Way Excitement
We've been trying to measure the MC mirror internal mode frequencies so that we can measure
their absorption before and after drag wiping.
It looked nearly impossible to see these modes
as driven by their thermal excitation level;
we're looking at the "MC_F" or 'servo' output directly on the MC servo board.
Today, I set up a band limited
noise drive into the 'Fast POS' inputs of the 3 MC
driver boards (turns out you can do this with either the old HP or the SR785).
MC1 28.21625 kHz MC2 28.036 kHz MC3 28.21637 kHz
I don't really have this kind of absolute accuracy. These are just numbers read off of the SR785.
The other side of the setup is that the same "MC_F" signal is going into the SR830 Lock-In which
is set to 'lock-in' at 27.8 kHz. The resulting demodulated 'R" signal (magnitude) is going into
our MC_AO channel (110B ADC).
As you can see from the above table, MC1 and MC3 are astonishingly and annoyingly very close in
frequency. I identified mirrors with peaks by driving one at a time and measuring on the spectrum
analyzer. I repeated it several times to make sure I wasn't fooling myself;
it seems like they
are really very close
but distinct peaks. I really wish we had chipped one of these mirrors
before installing them.
Because of the closeness of these drumhead modes, we will have to measure the absorption by making long
measurements of this channel.