I swept the frequency of RF input to the beatbox to calibrate and check linearity range of phase tracker.
Calibration factors are;
  C1:ALS-BEATX_FINE_PHASE_OUT    52.1643 +/- 0.0003 deg/MHz
  C1:ALS-BEATY_FINE_PHASE_OUT    51.4788 +/- 0.0003 deg/MHz

There was systematic error to the linearity check, but at least, calibration factor changes less than 50 % in the frequency range of 10 MHz to more than 500 MHz.


What I did:
  Used network analyzer(Aligent 4395A) to sweep the frequency RF input to the beatbox and getdata of phase tracker signal. I swept from 10 Hz to 500 MHz with 501 points in 50 sec. This sweep is slow enough considering we could lock the 40m arms (typical speed of a mirror is 1 um/s, so bandwidth of the phase tracker should be more than 1 um/sec / 40 m * 3e14 Hz = 75 MHz/s).
  RF amplitude was set to be -3 dBm and splitted into BEATX and BEATY.


Result:
  Plots for BEATX and BEATY are below;



Discussion:
  - Considering delay line length is ~30m, expected calibration factor is;

    2*pi*l/v = 2*pi * 30 m / (2e8 m/s) = 0.94 rad/MHz = 54 deg/MHz

so, this calibration is reasonable.

  - Since frequency sweep of network analyzer is not continuous, phase tracker output is like steps with some ringdown. This makes some systematic error for checking linearity. I'm planning to do slower sweep or continuous sweep. Also, the phase tracker seems like he can exceed 500 MHz.