I've been measuring a bunch of transfer functions of the FSS related stuffs.
There are a lot to be analyzed yet, but here I put one mystery I'm having now.
Maybe I'm missing something stupid, so your suggestions are welcome.

Here is a conceptual diagram of the FSS control board

                                                          TP3             TP4
                                                           ^               ^
                                                           |               |
RF PD -->--[Mixer]-----[Sum Amp]------>--[Common Gain]--->----[Fast Gain]----[Filter]--> NPRO PZT
              ^     |      ^        |                  |     
              |     V      |        V                  |
LO ---->-------    TP1     IN      TP2                 -->---[Filter]--[High Volt. Amp.] --> Phase Corrector

What I did was first to measure a "normal" openloop transfer function of the FSS servo.
The FSS was operated in the normal gain settings, and a signal was injected from "IN" port.
The open loop gain was measured by TP1/TP2.
Now, I disconnected the BNC cable going to the phase corrector to disable the PC path and locked the ref. cav. 
only using the PZT. This was done by reducing the "Common Gain" and "Fast Gain" by some 80dB.
Then I measured the open loop gain of this configuration. The UGF in this case was about 10kHz.
I also measured the gain difference between the "normal" and "PZT only" configurations by injecting 
a signal from "IN" and measuring TP3/TP2 and TP4/TP3 with both configurations (The signal from the Mixer was
disconnected in this measurement). 

The first attachment shows the normal open loop gain (purple) and the PZT only open loop gain scaled by the 
gain difference (about 80dB). The scaled PZT open loop gain should represent the open loop gain of the PZT
path in the normal configuration. So I expected that, at low frequencies, the scaled PZT loop TF overlaps the normal
open loop TF.
However, it is actually much larger than the normal open loop gain.
When I scale the PZT only TF by -30dB, it looks like the attachment #2.
The PZT loop gain and the total open loop gain match nicely between 20kHz and 70kHz.
Closer look will show you that small structures (e.g. around 30kHz and 200kHz) of the two
TFs also overlap very well. I repeated measurements many times and those small structures are always there (the phase is
also consistently the same). So these are not random noise.

I don't know where this 30dB discrepancy comes from. Is it the PC path eating the PZT gain ?

I have measured many other TFs. I'm analyzing these.
Here is the TO DO list:

* Cavity response plot from AOM excitation measurements.
* Cavity optical gain plot.
* Reconstruct the open loop gain from the electric gain measurements and the optical gain above.
* Using a mixer and SR560(s), make a separate feedback circuit for the PZT lock. Then use the PC path
  to measure the PC path response.
* See the response of the FSS board to large impulse/step inputs to find the cause of the PC path craziness.
etc ...

Summary:
Since we reduced the integration time of the particle counter by a factor of 10, we had to add a gain of 10
to the EPICS channels C1:PEM-count_full and C1:PEM-count_half.
I asked Alex to change it and he did it. I forgot to ask him to change the gain of C1:PEM-count_half. So now only
C1:PEM-count_full has x10 gain.

Detail:
C1:PEM-count_full and C1:PEM-count_half are 'Soft Channel' records in the database (Pcount.db). The values are actually
written into the VAL fields directly by an SNL code Particle.o.
Particle.o reads data from the RS-232C port, to which the particle counter is connected. Then it parses the data and put values
into relevant EPICS channels using channel access. This means we cannot change the gain of the channels by modifying the
database file. For example, ASLO field does not have any effect when the value is directly written into the VAL field.
We had to modify the SNL code. Alex modified Particle.st and the new SNL object file is Particle_x10.o sitting in 
/cvs/cds/caltech/target/c1psl/. I modified seq.load so that c1psl loads Particle_x10.o when rebooted.
The source code for the old Particle.st can be found on lesath.ligo.caltech.edu in
/export/CDS/d/epics/apple/Caltech/40mVac/40mVacScipe/dev/src
I asked Alex to disclose the location of the source of the new code.
In order to compile the SNL code into an object file for Motorola CPU by ourselves, we have to call Dave Barker at LHO.

At the request of Steve, I modified the HIGH value of C1:PSL-FSS_RMTEMP from 21.27 to 23.0.
The HIHI is set to 23.50.

scp Particle.st username@lhocds.ligo-wa.caltech.edu:/cvs/cds/lho/target/t0sandbox0

ssh username@lhocds.ligo-wa.caltech.edu

ssh ops@control0

cd /cvs/cds/lho/target/t0sandbox0

setup epics

make Particle.o

scp Particle.o controls@nodus.ligo.caltech.edu:/cvs/cds/caltech/target/c1psl/