UHF intermodulation product prediction in digital systems using SPICE: the need for nonlinear macromodels

The interaction of an interfering UHF signal with a transmission line link between two LSTTL gates is investigated. The interaction results in the formation of intermodulation products (IPs) on the line, which increase in intensity as the interfering signal level is increased. The gates are characterised by measurements of their s-parameters under varying DC bias conditions, and this information is used in a SPICE model to simulate the levels of the IPs. The low order IPs centred on the interference frequency are reasonably well predicted (to within ±5 dB of experiment), whereas the higher order IPs centred on harmonics of the interference frequency are less well predicted. Improved, nonlinear macromodels are required to describe the formation of IPs with sufficient accuracy for realistic simulation of interference phenomena that arise as a result of poor EMC design. The paper describes some of the features that such macromodels will need to include