An analytic theory is outlined for the operation of avalanche diodes in the circuit first analyzed by Evens. This consists of a low-pass filter situated approximately one-half wavelength from the diode. The magnitude and rate of rise of the voltage for a given charge pulse are estimated, and expressions for the efficiency and diode impedance are developed. It is shown for this model, where the filter acts as a short circuit at all frequencies other than the fundamental, how efficiencies around 30-40 percent can be achieved. The theory is compared with Evans' experimental results and shows fair agreement.