Having a reference of -30 dBm (-43 dBV in a 50 Ohm system) on one of the inputs is the optimum for maximum measurement range. For example if one signal is at -30 dBm, the other signal can range from 0 dBm (-13 dBV) to -60 dBm (-73 dBV), to give a gain measurement of +30 dB to -30 dB, respectively.
BTW, dBV is defined as: X dBV = 20*log10(Vrms/1Vrms)
The phase detector is a little less sensitive with amplitude, but the VPHS output does vary a bit with different amplitude. See Figures 42 through 47 in the datasheet.
You will probably need either a DMM or a scope to sample the outputs, VMAG and VPHS, to interface with Labview.
Regards,
Joel
