Group delay measurements using the FM envelope delay technique with a digital phase detector

The envelope delay technique is proposed as a low cost alternative to the vector network analyser when measuring absolute group delay. Quasi-static FM theory is presented to show there exists a relationship between the phase delay experienced by the modulation applied to an FM test signal and the group delay of the device being tested. A practical group delay measurement system based on this approach and using off-the-shelf equipment is described. A feature of this design is the use of a synchronous, digital phase detector, based upon high speed analogue-to-digital conversion and a correlation algorithm, to accurately measure the modulation phase delay and hence the group delay. A comparison of measurements taken with the new system and a commercial vector network analyser, shows an accuracy of the order of 2ns is feasible. The dynamic range of the measurement system is approximately 40dB.