Summary:
  We need I-Q frequency deiscriminator to control the arm length fine and continuously.
  I checked the beatbox (LIGO-D1102241-v4; see elog #6302) and it was working.

What I did:
  1. Measured some transferfunctions with a network analyzer (Aligent 4395A) and checked the cabling is correct.

  2. Put 30 m/1.5 m delay line and checked I-Q outputs are actually orthogonal. I did this by sweeping the frequency of RF input to the beatbox. See attached picture. You can see nice circle on the oscilloscope.

Some measurement results:
  - Gains of the transferfunctions(@ 10-100MHz) between;

   RF in -> RF mon: -25 to -20 dB
   RF in -> fine delay out: -50 to -40 dB
   RF in -> coarse delay out: -50 to -40 dB
   RF in -> LO of mixer RMS-1: ~ +4 dB  (RMS-1 needs +7 dB LO)
 
  - 30m delay line(RG-142B/U) had -2 dB loss.

Note:
  - RF input must be larger than about -3 dBm to get enough LO to the mixer. Otherwise, you won't get I-Q outputs.
  - The comparator, whitening filter and differential DAQ outputs are not installed in the current beatbox.
  - Current beatbox only has electronics for the one arm.
  - The print on the board D1102241 says +15V and -15V, but they are actually opposite. Cabling is swapped in order to supply correct power to the ICs.