R. Savage, J. Oberling, E. Merilh

Work performed on 4/9/2015.

Summary

We are investigating the channels in the PSL MEDM screens to determine what they do and if they do it correctly, and making changes when needed.  We started with the LO MON on the PSL FSS MEDM screen.  It is a monitor of the LO signal used in the PDH locking of the FSS servo.  We found the gain to be set incorrectly to -0.00666, so we changed it to 0.0044033.  We also changed the readout so that it will read 100% when the LO signal is equal to its nominal value (13.86V), >100% when larger and <100% when smaller.

Details

We have begun an effort to go through the more vaguely labeled outputs on the PSL MEDM screens to figure out what exactly they do and if they are doing it right, and making changes when necessary.  We are starting with the FSS, as that is the hot item in PSL land at the moment, with the LO MON readout located at the bottom left of the FSS MEDM screen.  This signal is read in percentage, but no indication what it is a percentage of.

Looking at page 1 of the FSS schematic in D040105-B shows the LO Mon as the output LOM from an ADC 10-4 directional coupler.  A portion of the LO signal (used as part of the PDH locking for the FSS servo) is diverted and output so we can monitor the LO signal.  This LOM is read into EPICS and sent to a filter with the simple math of: output = (signal – offset) * gain.

The offset is set to -22710.0 counts and the gain is -0.00666.   As this is a monitor signal and reported as a percentage, it follows that the LO MON is reporting the percent error of the LO signal as compared to some nominal value: % error = ((signal – nominal)/nominal) * 100.  This nominal value is the offset in the filter, 22710.0 counts (or 13.86V).  The gain is then equal to 100 divided by the nominal value, or 100/22710.0 = 0.0044033; the gain was found to be -0.00666, so it seems the LO MON readout on the FSS MEDM screen has been misreporting the % error of the LO signal.  We therefore changed the filter gain to 0.0044033.

Also, as the MEDM screen does not properly indicate that the LO MON is a % error, we turned off the offset in the filter.  The math is now: output = signal * gain = signal * 100/nominal.  With this change the LO MON readout will now read 100% when the LOM signal is equal to the offset, >100% when larger and <100% when smaller.