7 GHz RF Relay

Characterization of Reed Relays capable of handling frequencies up to 10 GHz

Introduction

For years engineers had thought the best way to switch high frequencies and very short fast digital pulses was to use special semiconductors designed to handle the high frequencies âßß namely gallium arsenide mosfets. Today, however, gallium arsenide is not the only option; new semiconductor materials are being developed that are less expensive and good for RF. Also, in fact, the Reed Relay is making a very big impact in the world of high frequency and fast digital pulses.

The Reed Relay by its basic geometry resembles a coaxial cable (see Illustration above). The magnetic reeds make up the center conductor with a glass envelope setting the spacing from the center conductor to the coaxial shield, and therefore, its characteristic impedance (typically 50 ohms). Generally the RF characteristics were not considered significant in the early years of Reed Relays because the Reed Switches were too big and the corresponding Reed Relays were too large having a long signal path length. However, in the 1980âßßs the Reed Switches began to shrink in size offering shorter and shorter signal path lengths. It was here that the all important signal to shield capacitance began to drop below 1.0 picofarad and hence the improved RF performance. Today, with 5 mm or less Reed Switch lengths the signal to shield capacitance has dropped to 0.5 pf when the reed is in the open state.


The Illustration (on the left) shows the similarities of a Reed Relay with a coaxial shield to that of a RF transmission line. Since the coil is effectively screened by the coaxial shield, it has no effect on the transmission of RF signals along the center lead conductor.

When designing in the frequency domain with semiconductors, one has to deal with special added circuitry to reduce or eliminate inter-modulation distortion (also a potential problem with fast digital circuits). By its nature, no inter-modulation distortion exists in a Reed Relay and therefore, no special circuitry is required. This is particularly useful in attenuator networks constructed from Reed Relays.

Form C Reed Relays (single pole double throw) have the potential added advantage when in its normally closed state, they require no external power. In T/R (Transmit/Receive) requirements this can be of real value, particularly if the receive mode represents 99 % of the duty cycle and the device is battery operated. No battery power is drawn 99 % of the time. Here extended battery life is a clear benefit over semiconductor switching devices where power is required all the time.

In test and measurement, particularly IC (Integrated Circuit) testers, with parallel high switch point counts, leakage current becomes a real problem. Reed Relays specially designed to handle fast digital pulses will also offer leakage currents on the order of 0.1 pico-amps or less, a clear requirement and benefit with this technology. No other technology currently offers anything close to this combination.