  We present the set-up and test-measurements of a waveguide-integrated
heterodyne diagnostic for coherent transition radiation (CTR) in the AWAKE
experiment. The goal of the proof-of-principle experiment AWAKE is to
accelerate a witness electron bunch in the plasma wakefield of a long proton
bunch that is transformed by Seeded Self-Modulation (SSM) into a train of
proton micro-bunches. The CTR pulse of the self-modulated proton bunch is
expected to have a frequency in the range of 90-300 GHz and a duration of
300-700 ps. The diagnostic set-up, which is designed to precisely measure the
frequency and shape of this CTR-pulse, consists of two waveguide-integrated
receivers that are able to measure simultaneously. They cover a significant
fraction of the available plasma frequencies: the bandwidth 90-140 GHz as well
as the bandwidth 255-270 GHz or 170-260 GHz in an earlier or a latter version
of the set-up, respectively. The two mixers convert the CTR into a signal in
the range of 5-20 GHz that is measured on a fast oscilloscope, with a high
spectral resolution of 1-3 GHz dominated by the pulse length. In this
contribution, we will describe the measurement principle, the experimental
set-up and a benchmarking of the diagnostic in AWAKE.
