Group Delay Caused by Impedance Mismatch

Literature on the subject of group delay variations due to transmission line impedance mismatches is scarce. This paper investigates the causes of impedance mismatches and their effects on the group delay of a wideband signal propagating through a system containing transmission lines between various subassemblies. Due to practical limitations, all subassemblies present non-ideal input and output impedance matches to the transmission lines connecting them together. Therefore, this produces the case of two impedance mismatches separated by a length of transmission line. This, in turn, causes sinusoidal group delay (as well as amplitude) variations to occur. There can also be multiple mismatches between a transmission line source and load due to components such as switches, power splitters, directional couplers, etc., inserted between the source and load. The periodic frequency of these sinusoidal group delay variations as well as their peak-to-valley amplitudes depend on the absolute magnitude of the reflection coefficients of the impedance mismatches, the electrical length of the transmission line between them, and the transmission loss of the transmission line. The group delay variations can cause significant signal distortion even with normally assumed sufficient values of voltage standing wave ratio (VSWR) at the input and output ports of the subassemblies. This paper presents various methods to predict the values of these group delay variations. In addition, practical compensation methods to reduce or eliminate the group delay variations are given.