Improving group delay measurement accuracy using the FM envelope delay technique

The FM envelope delay technique of measuring group delay is an attractive alternative to standard techniques using equipment like the network analyzer. The theoretical basis for this technique is more complicated though leading to some confusion about how to optimize the measurement accuracy. This paper discusses methods of improving accuracy when making group delay measurements using the FM envelope delay technique. The paper concentrates on two effects which are known to limit accuracy in this approach; the finite bandwidth of the test signal and device mismatch. The potential severity of each source of uncertainty is modeled from theoretically derived expressions and computer simulations. Practical measurements made using a FM envelope delay, group delay measurement system are also provided to verify the predicted trends. Adjusting the FM signal modulation index is shown to be an effective method of minimizing error depending on the characteristics of the device being measured