2 GHz Balanced Mixer Using SOT-23 Surface Mount Schottky Diodes - App Note

A series of general purpose RF Schottky diodes has been introduced in the SOT-23 package. One of these diodes, the HSMS-2822, is a matched series pair, an ideal configuration for a balanced mixer. This note describes the procedure for designing the impedance matching circuit for a 2 GHz mixer using the HSMS-2822. A conversion loss of 6.6 dB was achieved. The usual procedure for impedance matching begins with a measurement of the diode impedance. A diode model is often designed so that the calculated model impedance matches the measured impedance over a range of frequencies. A matching circuit is then designed to provide the conjugate impedance to the diode. In this application a novel, more experimental technique was used. The mixer was first assembled with a commercial 3 dB, 90° hybrid coupler and the diode pair. No tuning was used between the diode and the hybrid. Instead, a slide screw tuner was placed after the diode. The tuner was followed by a low pass filter to terminate the circuit at 2 GHz while passing the IF output to the receiver. This technique uses only one impedance matching circuit on the IF side of the diodes rather than the conventional two matching circuits (one for each diode) on the RF input side of the diodes.