Microwave Varactor Tuned Transistor Oscillator Design

An analysis is made of the common base microwave transistor oscillator circuit which uses a varactor in series with the colIector to tune over octave bandwidths. Equations are derived giving the required feedback capacitances and resonating elements required for octave tuning. Normally, the collector-emitter capacitance C_ce is made approximately equal to the transistor collector capacitance C_c. The emitter-base capacitance C_eb is important only at very high frequencies. It is shown that a high-Q varactor must be used and that only a limited amount of collector-base capacitance C_cb may be added if the circuit is to be resonated over an octave. The output power for such a circuit is normally about 1/5 the maximum power available from the transistor. Experimental oscillators were made from 0.5 to 1 GHz and 1 to 2 GHz which substantially verified the analysis. Using the TIXS13 transistor, an output power of 200 mW was obtained from 430 to 860 MHz tuning from -2 to -115 volts. In the 1 to 2 GHz range a TIXS13 transistor oscillator was tuned from 1.09-1.96 GHz with about 40 mW power tuning from -2 to -115 volts. By use of a lower case capacitance varactor, the 1 to 2 GHz oscillator could be made to tune over the full octave.