Simulation of UHF harmonic generation in logic gates using SPICE

The program SPICE has become widely used in modelling the nonlinear behaviour of devices. Nonlinear characteristic curves may be used at low frequencies, while high frequency devices may be characterised by measurements of the S parameters under varying bias conditions. In the latter case, experience shows that conflict can arise between the need to correctly bias the device at DC and the requirement of providing the correct nonlinear characteristics at high frequencies. This conflict arises because circuit elements in SPICE are not generally frequency dependent. The authors present here a method of overcoming this problem, with particular reference to the nonlinear behaviour of an LSTTL inverter at 500 MHz, when the gate input is held in its high state