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Entry  Mon Apr 8 19:19:02 2019, anchal, DailyProgress, FSS, South FSS problems 
    Reply  Tue Apr 9 14:35:30 2019, anchal, DailyProgress, FSS, South FSS problems 
       Reply  Wed Apr 10 16:34:55 2019, anchal, DailyProgress, FSS, South FSS problems South_EOM_Driver_TF_Measurements.jpgSouth_EOM_TF_Analysis.pdfNorth_EOM_TF_Analysis.pdf
          Reply  Wed Apr 10 19:05:29 2019, Ultraman, DailyProgress, FSS, South FSS problems 
Message ID: 2318     Entry time: Tue Apr 9 14:35:30 2019     In reply to: 2317     Reply to this: 2319
Author: anchal 
Type: DailyProgress 
Category: FSS 
Subject: South FSS problems 

This apparent problem is clear now. The test path has a 100 ohm resistor to ground (RF Board R10) which with rest of the resistances is creating a voltage divider at DC. The numbers make sense and FSS board is fine. Seeing this, the only option to get better PDH error signal and boost our discriminant is to increase modulation depth. Maybe the modulation depth is indeed not good enough. I'll make the "Apparent Modulation Depth" (Modulation depth calculated by PDH error signal and incident light power) equal to 0.3.

PDH Error Signal

We have been experiencing an extraordinarily low PDH error signal on the South FSS. I think I have found the source of the problem here but not the cause.

  • I used an external mixer, ZFM-2-S+ and directly fed the downconverted signal into test port Test1. I wanted to check if the onboard JMS-1H is faulty.
  • But the error signal directly from the external mixer was the same as the mixer output channel on FSS box.
  • But the mixer output channel is supposed to have a gain of 2.61, which is not happening.
  • The problem is, I tried taking transfer functions of parts from the test port to the mixer output channel, and everything looks like design spec (Elliptical filter and inverting amplifier). But when I compare the error signal at the start of gain stage and at the input to the FSS box, Test1, I see that somehow after going through elliptical filter, the signal is suffering a loss of 1/3 and hence the later gain stage is just making it same again instead of actually amplifying the signal.
  • I'll look into this more tomorrow. I don't understand how the transfer functions are from SR785 look ok but the actual ratio of error signals seen on the oscilloscope is wrong. Do we really need the elliptical filter?


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