a) Normalized detector response as a function of HWP angle for a detector pair with detectors that are sensitive to orthogonal polarizations. The blue points and line correspond to polarization A and the green to polarization B. The data is for a single incident frequency at 147 GHz. The plotted data points have been filtered to show only the DC and fourth harmonic components. b) Detector response as a function of HWP angle and MMW source frequency for the same two detectors. The responses of the polarization A and B detectors are shown in the left and right panels, respectively. The response is peak normalized for each frequency and the colorbar scale goes from blue to red. Fine-scale frequency structure is assumed to be caused by multipath reflections in the experimental setup. c) Polarization fraction and angle as a function of frequency for the two detectors. The left panel shows the polarization fraction (PF) in blue and polarization angle (PA) in gray for the polarization A detector. The right panel shows the PF in green and PA in gray for the polarization B detector. The PF is computed by fitting the fourth harmonic and DC components to the data in (b), as described in the text. The relative phase between the HWP response of the two detectors is 179.8°, corresponding to 89.9° in polarization angle. d) Spectral response of the two detectors at the HWP angle giving maximum response for each detector. The rolloff above 160 GHz is due to a filter in the MMW source. Much of the frequency structure is likely due to the MMW source and reflections in the experimental setup. The frequency structure is very similar between the two detectors.