Characterizing Reed Relays Past 7 GHz

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.

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Most important in the testing of any component for frequency response over 100 MHz is a good vector network analyzer (VNA) and carefully designed test fixtures for calibration as well as for the actual testing. The same is true when testing in the time domain. In the time domain, when measuring rise-time characteristics, one must be aware of overshoot and undershoot of the rise time pulse that may compromise measurements. These overshoots or undershoots if real, may compromise the components function in actual test systems. Care must be taken to determine whether this phenomena is real or related to the fixture. Fixture design starts with suitable SMA connectors on high-frequency printed-circuit-board (PCB) material. Several PCB materials are suitable for this application, including FR-4, G-10, and several material products from Rogers Corp. (Rogers, CT).

Many engineers feel that FR-4 material is suitable for such testing since the fixture zeroing process will eliminate its high-frequency loss characteristics. As a general rule, the use of FR-4 below 6 GHz is fine, but above 6 GHz the use of high-frequency circuit materials such as RO3203 or RO4350 from Rogers Corp. will improve test performance. Rogers has several other materials available depending upon the TCE matching of the component/s or performance requirements. Most of these materials are ceramic filled.

Calibration boards were created for a variety of conditions, including shorted to ground, open-circuit, and a through-line circuit. As many ground points as possible were used along with avoiding and sharp corners. The electrical contributions of all calibration boards were measured and stored in the memory of a model 8720ES VNA from Agilent Technologies (Santa Rosa, CA). The relay under test (RUT) was then measured and its characteristics stored in the VNA's memory. The calibration data was then entered and the losses due to the calibration board under the various configurations was extracted, yielding the results shown in Figs. 5 through 9. This was compared with data extracted using the MIMICAD CAE program from Optotek Ltd. (Kanata, Ontario, Canada) using the equivalent circuit presented and the S parameters, with measured and modeled results agreeing closely.

The test fixtures used for evaluating the reed relays doubled as calibration boards. All of the fixture boards used to test the RUTs used SMA connectors for connection to and from the test equipment and for terminations. The following are the makeup of the boards under test:

1. The RUT calibrated with a 50-Ω line and open termination.
2. The RUT calibrated with a 50-Ω line and shorted termination.
3. The RUT calibrated with a 50-Ω line and 50-Ω termination.
4. The RUT calibrated with a 50-Ω line through line.

The fixtures were made from FR-4 PCB material. The use of higher-performance PCB materials may improve the results shown in Figs. 5 through 9.

Insertion loss is the loss of power going through the relay. Insertion loss is one of the most important RF measurements because it is simply a measure of the loss of the signal going through the component (reed relay). Minimizing this loss is a key interest for most applications. For the RUT, insertion loss is minimal to 7 GHz (Fig. 5, left). Clearly, signals, whether digital or analog, will fare very well when switched and passing through this CRF ceramic reed relay. When using semiconductors as a switching element, IMD may occur, giving rise to distortion in the frequency response. With a passive device such as the reed relay, IMD is nonexistent, resulting in flat insertion loss to 7 GHz. This flat insertion loss allows a user to switch and transfer a multitude of different frequencies or different-width digital pulses without using a variety of switches for different frequency bands.

At higher frequencies, it has been proposed that a reed relay, because it uses nickel/iron as its center conductor, will not have very good performance characteristics. Skin effect is often the proposed culprit, because nickel and iron, being ferromagnetic, have a high magnetic permeability (µ). However, as Fig. 5 (right) indicates, this is not the case. When a pure copper wire replaces the reed switch, there is little or no difference in insertion-loss performance. Under high-power conditions, some differences may be noticeable, but for most lower-power applications, the reed relays provide transmission-line-like performance through 7 GHz.

VSWR represents the effects of the transmission of power due to standing waves. When standing waves are present on a line, some power is being reflected back on the line and re-reflecting again from the source. This back and forth reflection produces standing waves. These standing waves interfere with the transmission of the original signals from the source because they are continuously present and continually absorb power. Figure 6 shows the reed relay's VSWR performance. While still an important RF characteristic for analog CW analysis, insertion loss is considered more critical for RF applications.

Isolation is the ability of a component to isolate the RF signal from propagating further in a circuit. For a reed relay, the isolation is a measure of the ability to halt further progress of the signal when it is in the open state. Of course, given RF energy, an open circuit is not totally open because the capacitance across the contacts represents a leakage path; with high enough frequencies, that's exactly what occurs. Figure 7 shows isolation of 50 dB or more at low frequencies, dropping to 15 dB at 3 GHz and at least 10 dB at 7 GHz. Contributing to this drop off in isolation is the contact gap. Increasing the gap on the reed switch is very difficult to do because it would require a larger capsule, which would increase the package size. Also, a larger gap will make the switch less sensitive for closure, requiring more coil power. If the isolation is a critical parameter in an application, stringing more than one Reed Relay together will help. Also using a 'T' switch or half 'T' switch will yield much higher isolations.

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Figure 8 shows reed relay return loss, a measure of the power of the RF signal being reflected back to the source (with the larger value in dB representing a smaller percentage of reflected signal). The return loss has only 35 dB of the reflected signal at the lower frequencies and about 10 dB reflected signal at 6.5 GHz.

Characteristic-impedance measurements (not shown) on the reed relay were performed by checking a signal at certain points along the relay. Since this measurement is a spatial measurement, the actual impedance at each point of the relay can be measured. The relay was found to be slightly above 50 Ω, indicating a slightly inductive entrance into and out of the relay. Compensating with a little capacitance on each end of the relay will tune the impedance to the desired level. This will, in turn, improve the performance of the relay in a given circuit and increase its performance at higher RF frequencies as well.

Plotting a reed relay's S−parameters on a Smith Chart shows the characteristic impedance over a given frequency range. The Smith Chart of Fig. 9 presents a plot of the response of frequencies every 50 kHz up to 4 GHz with points centered around the 50-Ω real point. To better understand this Smith Chart, the second dotted circle starting from the right center point of the large circle is the 50-Ω impedance circle. The centerline of the circle, which is running horizontally, is the real axis. Plots above this line are inductive while plots below this line are capacitive. The plot of the CRF relay is in a tight circle around the real axis, and centered around the 50-Ω circular axis. If tuning is necessary for a particular frequency, an engineer will know whether capacitance or inductance must be added to further improve performance.

As is evident from the data presented, the CRF reed relay is excellent for switching and carrying RF signals to 7 GHz and beyond. Current efforts are underway to improve the design's characteristics up to 10 GHz and beyond, part of an ongoing effort to continually develop new RF relays, pushing the current bandwidth and current state of the art. As higher and higher frequencies are used and components are needed to develop these circuits, the need for reed relays like the CRF series and subsequent improvements on performance over existing data will be needed.