8.8 OPTIMUM DESIGN TECHNIQUES
8.8.1 Empirical Approach
For optimum oscillator design with maximum output power, generally it is required extensive small- and large-signal measurements. The small-signal S-parameter measurements can be made at several frequencies, along with estimated device equivalent circuit parameters, including package parasitics. Then, a computer optimization program is used to match the measured S-parameters to the S-parameters computed for the active device from its equivalent circuit. The next step is to vary those elements of the equivalent circuit, which can vary under large signals. By varying the active device nonlinear equivalent circuit parameters, the set of large-signal S-parameters can be obtained corresponding to the saturation condition where maximum oscillator output power can be achieved.
The simplified design approach assumes that all of the S-parameters except the magnitude of S21 are constant under large signals. For example, for a metal-semiconductor field-effect transistor (MESFET) device up to saturation with small limitation in accuracy, it is possible to derive the large-signal behavior of the main device nonlinear elements as functions of the device transconductance gm . Consequently, the S-parameters become functions of gm only and, at each incremental reduction of gm, are recomputed and optimized along with power gain. If one were interested in the 1-dB compression point, the S-parameters used would be those at that point.