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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: 2319     Entry time: Wed Apr 10 16:34:55 2019     In reply to: 2318     Reply to this: 2320
Author: anchal 
Type: DailyProgress 
Category: FSS 
Subject: South FSS problems 

EOM Driver isn't driving hard enough

  • I suspected that the EOM driver circuit isn't driving EOM hard enough to produce a good modulation depth.
  • I was seeing the saturation of PDH error signal as I increase input power to the EOM driver.
  • I made a spice model and as expected, the driver circuit is limited by the power rails used for it. We are using +-18 V power rails which would limit to 36 V peak-to-peak output.
  • I made some transfer function measurements and power sweep measurements of the South EOM driver circuit to have real facts to go ahead as this circuit is highly unpredictable (it relies on the internal resistance of Emitter-Base junction for amplification)

South EOM Driver Measurements

  • I created a 20 pF capacitor by placing multiple BNC connectors together to simulate the load of EOM.
  • I used high impedance probe HP 41800A to measure the other end of this capacitor. I used 60 dB  (1:1000) attenuation for safety.
  • I made three sets of measurements for +_18V, +-21V and +-24V rails. In each set, I took a transfer function from 10MHz to 100MHz at -10 dBm power and a power sweep from -15dBm to 15 dBm at 37 MHz CW frequency.
  • I averaged over 50 times at the instrument.

Conclusions

  • As expected, the output of the driver saturates according to the power rails.
  • Also, the gain of the driver is dependent on the negative rail voltage.
  • So to achieve 0.3 modulation depth, we need t use power rails of +-21 V or above.
  • We have =-24 V power supply available, so might route two cables from rack to the EOM drivers. Only problem with that is the board has TPS71550 5V regulator on it which has input maximum of 24V only. I might need to bring down the +-24 V supply for the drivers.
  • Another thing I found from spice model is that the EOM output always has a DC offset equal to the positive rail value. In fact DC path is directly linked to the positive rail through some passive filtering but no active voltage regulation. This means that power supply low-frequency noise is directly going into the EOM.
  • This DC offset changes the polarization matching and creates RFAM. A quick back of the envelope analysis shows that fluctuations in power supply are directly proportional to RFAM generation with a factor of sin(V_dc*pi/210) or ≈ V_dc/66.84 times the PDH Error Signal, where V_dc is in volts.
  • At 60 Hz, the passive filter on the board is useless with 0 dB attenuation of any fluctuations in power. So change in frequency of light due to change in power supply at 60 Hz is directly coupled with a factor of 8.108 kHz/V.
  • With 10mV ripple in 20Hz-300kHz band (DCS33-33E), which is equivalent to 33.33 nV/rtHz of flat noise spectrum, effect on frequency would come to about 0.27 mHz/rtHz. Phew!
  • I think we should be fine. I'll do more rigorous calculations later.

Edit Thu Apr 11 15:58:47 2019: Corrected legend and a plot title.

Edit Thu Apr 11 16:16:03 2019: Added same measurements for North EOM Driver

Edit Thu Apr 11 16:31:30 2019: References:

https://doi.org/10.1364/OE.25.032985

https://doi.org/10.1364/OL.39.001980

https://doi.org/10.1364/JOSAB.2.001527

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