  Full control of the ellipticity of broadband pulses of THz radiation, from
linear to left- or right-handed circular polarization, was demonstrated via a
4-pixel photoconductive emitter with an integrated achromatic waveplate.
Excellent polarization purity and accuracy was achieved, with Stokes parameters
exceeding 97% for linear and circular polarization, via a robust scheme that
corrected electrically for polarization changes caused by imperfect optical
elements. Further, to assess the speed and precision of measurements of the THz
polarization we introduced a new figure of merit, the standard error after one
second of measurement, found to be 0.047$^{\circ}$ for the polarization angle.
