Communication systems are defined in detail by specification standards but traceability, ‘an unbroken chain of measurements from the device under test to the relevant primary SI standards’, is essential to specify, buy, and test components and systems. Above 110 GHz, two or more near identical mixers can be used together to estimate conversion loss and group delay. Here we investigate a free-space approach, based on a photoconductive sampling detector and a frequency comb generated by a femtosecond mode-locked laser. The technique has been demonstrated experimentally at 100 GHz using a pre-existing device and optical pulses from the PTB primary standard electro-optic sampling system. The existing antenna and higher-frequency designs (100 GHz/300 GHz) have been modelled for verification and to identify potential design issues. In operation, the high-frequency signal components are down-converted to a 38 MHz frequency space. We have developed selection rules to allow direct down-conversion and identification of all the signal components of a pseudo random-number QAM waveform. This will allow traceable characterisation of free-space complex modulated THz waveforms.