The characteristics of bias-voltage-controlled AM and FM of a CW Gunn oscillator are reported. It is found that the loaded Q of the microwave oscillator circuit and the microwave impedance level largely govern these characteristics. Modulation mechanisms are discussed. A particular mechanism, due to the nonlinear I-V curve of the device, is shown to lower the oscillating frequency as the bias voltage is increased. Performance of an automatic frequency control loop (AFC) is also described.