Today Andrew and I tried to understand our FSS control loop.

First, we took OLGTFs of both paths.  Then, we took Crossover TFs of both paths.  Finally, we did a little loop math and tried to isolate the PZT (Fast) and EOM control paths.

OLGTFs are taken through the FSS box Common Out1/Common Out2.  These give the overall open loop gain  G = G_PZT + G_EOM.

Crossover TFs are taken through the FSS box Fast Out1/Fast Out2.  These give the "crossover"  C = G_PZT / (1 - G_EOM).

Doing a little loop math, we can isolate G_PZT and G_EOM:

G_PZT = C (G - 1) / (C - 1)

G_EOM = (C - G) / (C - 1)

All of these measurements and calculations are posted below in this order:

1. North OLGTF (two plots on here because we increased the Common and Fast gains after the orange measurement)

2. North Crossover

3. North PZT and EOM

4. South OLGTF

5. South Crossover

6. South PZT and EOM

The results seem sensible, with the EOM dominating at higher frequencies.  Around and above 1 MHz I wouldn't trust our Crossover TFs.  The South path PZT and EOM seem to hug each other from 1kHz to 10kHz.  Unclear why this would be, could be an indicator for why the South path loves to ring while the North is more stable.

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The AD602 used as the variable gain stage in common and fast paths in the FSS box has a gain of 32 dB/V with a range of -12 to 32 dB (log space). The voltage rail limits of the acromags was previously set to -5 V to 5 V, which doesn't cover the full range of possible gains.

The gain through the interface pannel subD25 control interface to the AD602 gain->voltage input is 1/10.

Andrew changed the range of the acromag slow voltage inputs so they go the full -3.5 to 10 V range.