Design of efficient broadband varactor upconverters

This paper describes a design procedure for optimizing the performance of a varactor upconverter microwave power amplifier with respect to maximum pump efficiency; the procedure gives explicit diode and circuit parameters and operating levels. An optimum diode is selected by inclusion of an empirical relation between diode breakdown voltage and cutoff frequency. A fully driven abrupt junction diode is assumed. The bandwidth of an upconverter is limited by the intermediate frequency input circuit which typically has a 3 dB bandwidth of about 10 percent. We describe a method of obtaining broadband operation where the interface between the intermediate frequency source and the varactor diode is mismatched in an optimum way. Analysis shows that the frequency variation in mismatch loss just compensates for the frequency variation in upconverter gain predicted by the Rowe-Manley relations. The design procedure is illustrated by a 300 MHz to 10.960 GHz varactor upconverter built for use in the transmitter of the short hop radio system experiment. A bandwidth of 120 MHz between 1.4 dB points represents a bandwidth of more than 40 percent at the intermediate frequency. An output power of +16 dBm was obtained using a pump power of +20 dBm giving a pump efficiency of 40 percent. The normal input to the driver amplifier is +3 dBm giving an overall gain of +13 dB. The upconverter operates over a temperature range of −40°F to +140°F with only a small change in bandshape and output power.