Abstract: This paper presents theoretical analysis circuit design of doubler using two beam-lead Schottky-barrier diodes as varistors. Influence of cavity to the microstrip circuit is fully considered during the CAD process and accurate model is set up. Input frequency is 13~20 GHz. Output frequency is 26~40 GHz. Conversion loss is better than 11.2±1.8 dB. It can extend microwave signal source to Ka-band.