The Period property must correspond to the lower excitation frequency in the model. In the case of one driving frequency, this is quite trivial: it is usually easiest to define a parameter, f0, for the frequency, and set the Period setting to 1/f0 and the Frequency in any Terminal or Metal Contact feature to f0. In the case of multiple frequencies, things become a little more complicated. Because periodic boundary conditions are used on an extra dimension, frequencies can only be exact integer multiples of each other. It is possible to use the two frequencies of 10 and 20 MHz as presented in Figure 6-2, but it is not possible to use two frequencies of 10 and 18 MHz as presented in Figure 6-3.