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Entry  Tue Nov 2 22:47:35 2010, Zach, Electronics, GYRO, demodulation noise analysis demod_noise.png
    Reply  Wed Nov 3 20:25:01 2010, Zach, Electronics, GYRO, demodulation noise analysis demod_noise.png
Message ID: 1134     Entry time: Wed Nov 3 20:25:01 2010     In reply to: 1128
Author: Zach 
Type: Electronics 
Category: GYRO 
Subject: demodulation noise analysis 

Koji pointed out that the PD noise at DC is irrelevant to the demod noise. I have repeated the measurement the correct way. All of these spectra were generated by looking at the "INMON" port of the primary PDH box. They were then divided by the meaning of life (42) to reflect the true level at the "IN" port. The traces are:

  1. 50-ohm terminator in place of the PD (demod noise only)
  2. PD connected but shutter closed (demod noise + PD dark noise)
  3. PD connected, 100 mW incident power, EOM off (demod noise + PD dark noise + shot noise)
  4. PD connected, 100 mW incident power, EOM on (demod + PD dark + shot + RFAM from EOM)
  5. Measurement noise floor for (1) and (2)
  6. Measurement noise floor for (3) and (4)

demod_noise.png

Two things of note:

  • There is a large amount of RFAM noise (yellow trace) that needs to be minimized. This might explain a lot of our noise coupling.
  • 100 mW is not sufficient for shot noise to surpass the dark PD noise. This is the highest power I was able to put on the diode downstream of the initial beam split even when putting all of the light through one of the two paths.

I agree with Koji's assertion that our first priority should be replacing the REFL PDs we have in place at the moment. We will likely go back to the PDA255 for the primary loop while we wait on the custom RFPDs.

 

Quote:

 NOTE: The plot below is somewhere between misleading and wrong. See reply.


Attached is a plot showing the following (all of these were taken with the laser shutter closed in order to focus on electronics noise):

  • Voltage noise out of the mixer with the PD connected to it
  • Voltage noise out of the mixer with a 50-ohm terminator in place of the PD
  • Noise directly from the PD. Note: the raw data for this was divided by two to account for the change in transimpedance gain when going from Hi-Z (Agilent) to 50-ohm (mixer)
  • Measurement noise floor

The "mixer output" here is actually the output of the lowpass filter after it.

demod_noise.png

The noise from the demodulation seems negligible in comparison to the dark PD noise, with the exception of a broad peak at around 49 kHz that appears to come from the mixer.

 

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