A non-destructive bunch length detector has been installed in the CLIC Test Facility (CTF3). Using a series of down-converting mixing stages and filters, the detector analyzes the power spectrum of the electromagnetic field picked-up by a single waveguide. This detector evolved from an earlier system which was regularly used for bunch length measurements in the previous CLIC Test Facility, namely CTF2 [1,2]. Major improvements are increase of frequency reach from 90 GHz to 170 GHz, allowing for sub-ps sensitivity, and single shot measurement capability using FFT analysis from large bandwidth waveform digitisers. The results of the commissioning of the detector in 2006 are presented.