Summary:

Today I tried debugging the mysterious increase in REFL55 signal levels in the DRMI configuration. I focused on the demod board, because last week, I had tried routing these signals through different channels on the whitening board, and saw the same effect. 

Based on my tests, everything on the Demod board seems to work as expected. I need to think more about what else could be happening here - specifically do a more direct test on the whitening board.

Details:

• The demod board is a modified D990511 (marked up schematic + high-res photo to follow).
• Initially, I tried probing the LO signal levels at various points with the board in the eurocrate itself, with the help of an extender card.
• But this wasn't very convenient, so I pulled the board out to the office area for more testing.
• The 55MHz LO signal going into the board is ~0dBm (measured with Agilent network analyzer)
• I used the active probe to check the LO levels at various points along the signal chain, which mostly consists of attenuators, ERA-5SM amplifiers, and some splitters/phase rotators.
• Everything seemed consistent with the expected levels based on "typical" numbers for gains and insertion losses cited in the datasheets for these devices.
• I couldn't directly measure the level at the LO input to the mixer, but measuring the input to the ERA-5SM immediately before the mixer, barring problems with this amplifier, the LO input of the mixer is being driven at >17dBm which is what it wants.
• Next, I decided to check the gain, gain imbalance and orthogonality of the demodulation.
• For this purpose, I restored the board to the Eurocrate, reconnected the LO input to the board, and used a second Marconi at a slightly offset frequency to drive the PD input at ~0dBm.
• Attachment #1 - The measured outputs look pretty balanced and orthogonal. The gain is consistent with an earlier measurement I made some months ago, when things were "normal". More bullets added after Rana's questions:
  • 300 MHz bandwidth oscilloscope used to acquire the data
  • I and Q outputs were from the daughter board
  • Data was acquired via ethernet data download utility
  • 20 MHz low-pass filter turned on on the Oscilloscope while downloading the data

All connections have been restored untill further debugging later in the evening.