Fundamental limitations in RF switching using semiconductor diodes

Abstract

Semiconductor diodes have been used for many years as RF switching elements. The RF impedance (for limited signal strength) may be switched from one value (usually small) in a forward-biased or conducting state to another (usually larger) in a reverse-biased, non-conducting state. In RF switching networks, the diodes are incorporated into coaxial or waveguide transmission paths in such a manner that a change in diode impedance causes a change in RF signal transmission. The impedance changes are brought about by changes in the dc bias which is applied through leads which are isolated from the signal by RF chokes and by-pass capacitors. Recent developments in PIN type diodes have permitted large increases in the power capacity of these switches and significant reduction in signal losses. These diodes have low capacitance and very high impedance when reverse-biased and can also withstand large RF voltages. In forward-bias, ac resistance is sufficiently low that many amperes of RF current can be permitted. Because of these desirable characteristics, PIN diodes are greatly superior to varactor or other microwave diodes as switching elements. The paper describes the limitations with regard to power and attenuation for several classes of RF switches including single pole, multiple pole, duplexer switches, and phase shifters. Basic figures of merit are presented which express these limitations concisely.