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Entry  Thu Jul 17 01:24:35 2014, rana, Update, IOO, MC / EOM Stability Mystery Solved! MC_OLG.pdf
    Reply  Fri Jul 18 14:59:07 2014, Evan, Update, IOO, MC servo TFs mcServoTFSettings.pngMCtfs.pdf
       Reply  Tue Jul 22 08:36:08 2014, Evan, Update, IOO, MC servo TFs MCtfExpectations.pdf
Message ID: 10251     Entry time: Tue Jul 22 08:36:08 2014     In reply to: 10235
Author: Evan 
Type: Update 
Category: IOO 
Subject: MC servo TFs 

Quote:

[Rana, Evan]

This morning we took several TFs of the MC servo board using the HP4395A.

The 4395 source was teed, with one output of the tee going to 4395 R and the other output going to the board's IN1. We then took TFs of (4395 A) / (4395 R), where 4395 A was one of the following four points on the servo board:

  • OUT2
  • A TEST1
  • B TEST1
  • SERVO

For each of these points, we took a TF at two gain settings: IN1 and VCO gains both at 0 dB, and then IN1 and VCO gains both at 20 dB.

Before doing these measurements, we calibrated out the cable delay. Additionally, SERVO was always loaded with 50 Ω—either from the 4395 or from a terminator.

The attached png shows the servo board settings when these TFs were taken with the 0 dB gain settings. The settings for the 20 dB measurements are identical, except for the higher IN1 and VCO gains.

Using the modified schematic (40m:10250), I've made a plot of the TFs I expect for GIN1 = GVCO = 0 dB, taking into account our 50 Ω loading of the board.

Evidently I'm somehow missing a factor of 2 in the gain of the overall TF, but the shapes of the expected vs. measured magnitudes agree quite well.

At 1 MHz, I expect we should have accumulated about 80 degrees of phase going through the servo board. In reality, we appear to have lost more like 105 degrees.

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MCtfExpectations.pdf
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