Applications for RF components continue to grow, not only in traditional military markets, but in commercial, industrial, medical, and automotive applications. With the rising frequencies of analog signals and increasing speeds of digital signals comes the need to switch these signals along different transmission paths. While GaAs-based switches have handled high-speed signals for many years, there is now another component option for switching fast signals through 10 GHz: the reed relay.

In its geometry, a reed relay resembles a coaxial cable (Fig. 1). The magnetic reeds make up the center conductor with a glass envelope setting the spacing from the center conductor to the coaxial shield, and establishing the characteristic impedance (typically 50 Ω). Early reed relays were large and not considered for RF applications. But as designs began to shrink in the 1980s, their signal paths decreased to dimensions that were more practical for the short wavelengths of RF signals. During this period, the all-important signal-to-shield capacitance began to drop below 1.0 pF and the RF performance improved. In modern reed relays with reed switch lengths of 5 mm or less, the signal-to-shield capacitance has dropped to 0.5 pF when the reed is in the open state (Fig. 2).

By their nature, reed relays do not suffer the intermodulation distortion (IMD) common to high-frequency electronic switches. The 3-dB bandwidth of reed relays in the CRF series from MEDER Electronic (Mashpee, MA) is currently DC to 7 GHz and rising. Form C (single-pole, double-throw) reed relays require no external power in their normally closed state, making them well suited for critical low-power applications.

In test and measurement, particularly integrated-circuit (IC) testers and wafer testers, with parallel high switch point counts, leakage current becomes a real problem. Reed relays designed to handle fast digital pulses will exhibit extremely low leakage currents on the order of 0.1 pA or less. No other technology currently offers anything close to this combination.

S-parameters, which represent the magnitude and phase of incident and reflected signals through a component, provide a suitable means of measuring and modeling reed relays. Using a vector network analyzer (VNA), it is possible to characterize the frequency-domain performance of a reed relay at microwave frequencies, and then develop equivalent-circuit models such as those shown in Figs. 3 and 4. By using the S-parameter representations of a reed relay in a computer-aided-engineering (CAE) software program, an engineer can study how the reed relay will interact with other components in a high-frequency circuit.

A time-domain reflectometer (TDR) is used to evaluate the time-domain performance of a reed relay. Time-domain reflectometry characterizes a transmission line or series of components by the reflections or discontinuities occurring from a pulse of known amplitude and rise time traveling through the line or components. A transmission line terminated in its characteristic impedance appears as an infinitely long line (with no reflections). A transmission line with no termination (an open circuit) causes reflections due to impedance mismatches. Detection of relative position of discontinuities, whether inductive or capacitive, depends upon the polarity of the reflected signal. However, by knowing the polarity of the reflection, it is possible to redesign a component to eliminate that capacitive or inductive point and yield smoother signal-transmission characteristics.

In time-domain characterization, rise time is a key parameter for determining a component's effects on signal fidelity. Rise time is usually defined as the time between 10 and 90 percent of the full amplitude of the leading edge of a pulse (although values of 20 and 80 percent are also used). A pulse incident upon a relay with a perfect rise time (0 ps) will be altered once it exits the relay with a rise time stated as the relay rise time. Any system dealing with fast digital pulses must consider the rise time through the components where rounding off and/or distortion of the square wave can occur.

The characteristic impedance represents the distributed impedance at any instantaneous point at the entry, through and exiting the relay. A pulse or signal traveling through the path of the relay seeing any impedance changes will reflect some of its signal strength. Standing waves can occur at these reflection points.

The mechanical features that make reed relays attractive for many designs include the following:

1. Small size.
2. Minimum path length improving the RF and fast digital pulse characteristics.
3. Gold-plated signal path for high conductivity and minimizing any RF loss.
4. No lead frame design eliminating any skewing of leads and coplanarity issues.
5. No internal solder connections eliminating potential internal solder reflow during the solder process, with the capability of withstanding immersion in 260°C during solder reflow process.
6. Internal magnetic shield, eliminating magnetic coupling in tight two and three axes matrices.
7. Rugged ceramic/thermoset molded package, eliminating susceptibility to changing environmental conditions.
8. Matched thermal coefficient of expansion (TCE) in packaging reed switches, eliminating potential stress with temperature variations.
9. Available in ball-grid-array (BGA) packaging.

A reed relay's electrical features include:

1. Capability of switching and passing 7 GHz and higher frequencies.
2. Capability of passing digital pulses in the order of 40 ps without degradation of leading and trailing edges.
3. Characteristic impedance of 50 W.
4. Low switch-to-shield capacitance (0.7 pF).
5. High insulation resistance between all points typically greater than 10¹⁴ W.
6. Thermal offset voltages of typically less than 1 µV.
7. Input (coil and shield) to output (switch) dielectric strength of 1500 VDC minimum.

Miniature reed relays are well suited for a variety of applications, including in integrated circuit testers, wafer testers, functional PCB testers, the front ends of multimeters where low voltage offsets of less than 1 µV and leakage current of less than 1 pA are required, in feedback loops where high-frequency, low leakage, and high voltage isolation are a requirement, for high-speed switching in oscilloscopes, for high-frequency attenuators, in portable devices such as cellular telephones, pagers, and PDAs, and for transmitter/receiver switching.