  We study the performance of achromatic half-wave plates (AHWP) as a function
of their construction parameters, the detection bandwidth of a power detector
operating in the millimeter wave band, and the spectral shape of the incident
radiation. We focus particular attention on the extraction of the degree of
incident polarization and its orientation angle from the intensity measured as
a function of AHWP rotation angle, which we call the IVA (intensity versus
angle). We quantify the phase offset of the IVA and point to potential
systematic errors in the extraction of this offset in cases where the incident
spectrum is not sufficiently well known. We show how the phase offset and
modulation efficiency of the AHWP depend on the relative angles between the
plates in the stack and find that high modulation efficiency can be achieved
with alignment accuracy of few degrees.
