Reed relays: from voiceband calls to GHz ATE, now facing MEMS disruption part 2

Reed relays have evolved from their audio roots to handle GHz signals; MEMS technology may push them aside. This section explores the reed relay and its use in ATE applications.

The alternative: the reed relay

Along with the efforts to develop a better central office switching matrix and crossbar, scientists and engineers at the venerable Bell Telephone Laboratories worked on a radically new EMR design during the 1930s. Their goal was to create a small, lightweight, and extremely long-life relay for switching telephone circuits, and they did: the reed relay (sometimes called a reed switch). Perhaps somewhat due to the conceptual simplicity of its concept (if not the actual manufacturing efforts), the 1941 patent is amazingly short, with just one page of three basic drawings and a little over a page of text.

The basic reed-relay design is a model of design elegance. The contacts are in a sealed glass tube that can either have a vacuum or an inert gas, as seen in Figure 1.

Figure 1. The simple concept of the reed relay (left) is seen in its basic construction (right). (Images: Electronics Notes (left), Wikipedia (right))

The activating magnetic field is applied via an external electromagnet or even a permanent magnet (for this reason, reed relays are often used to detect door/window closures). Since the contacts are in a sealed tube, they are immune to dirt/dust build-up or contamination in the air, which might otherwise degrade contact resistance over time or encourage flashover and sparking.

Reed relays were well-suited for handling DC and audio signals. Depending on design specifics, they can also support high-voltage switching as well as sensitive signals with low thermal-EMF implementations (thermal EMF is a source of measurement error in extremely low-level signal paths), and soon found use in instrumentation and automatic test equipment (ATE).

Figure 2. The Pickering Electronics Series 113RF SIL/SIP 3-GHz reed relays begin with the basic “1 Form A” configuration, but they are also offered in other versions. (Image: Pickering Electronics)

But why stop at lower frequencies? Using appropriate design and fabrication, their frequency range has been extended to the gigahertz range needed for RF switching. A good example is the Series 113RF SIL/SIP reed relays from Pickering Electronics, a miniature coaxial reed relay for high-frequency RF systems up to 3 GHz, seen in Figure 2. The basic “1 Form A” (SPST) 5-V version has a coil resistance of 500 Ω and can be driven by TTL-compatible or equivalent outputs.

Each relay is just 12.5 mm long, 3.7 mm wide, and 6.6 mm high, and has a high packing-density stacking on a 0.15 × 0.5-inch pitch. They feature a two-millimeter spacing footprint, enabling them to be stacked at very high densities typical of ATE systems, shown in Figure 3.

Figure 3. Reed relays are often packed densely on ATE system-interface boards, so small size and footprint, but without compromise of electrical specifications, are important design-in factors. (Image: Pickering Electronics)

These reed relays are suitable for switching up to 10 W and 0.5 A. Form A configurations (SPST normally open) are also available with 3 V or 5 V coils with coil resistances of 100 and 300 ohms, respectively. The company maintains that these small, screened reed relays are faster and smaller than conventional electromechanical relays, have a lower insertion loss, better DC capabilities than SSRs, and better hot-switching performance than micro-electromechanical machine systems (MEMS) relays (more on those later).

At low levels, the typical life expectancy of Series 113RF reed relays is greater than 250 million operations. These relays employ the highest quality instrumentation grade reed switches with sputtered ruthenium contacts (think back to high-school chemistry: that’s symbol Ru and atomic number 44, a rare transition metal belonging to the platinum group of the periodic table) and are well-suited for automatic test equipment. Talk about tiny details that make a difference: that type of ruthenium plating ensures stable contact resistance and longer life, while simpler electroplated rhodium plating results in higher, less stable contact resistance.

As these are coaxial, RF-optimized relays, the vendor provides graphs of standard RF parameters such as insertion loss, VSWR, and isolation extending into the multi-GHz range seen in Figure 4.

Figure 4. In accordance with their 3-GHz rating, the performance of these relays in key RF parameters is also called out in the data sheet. (Image: Pickering Electronics)

When multiple relays are widely spaced apart on the system circuit board, the external magnetic field of their activation coils does not cause problems in general. However, when the relays are stacked and packed to high density, as is typical in applications such as ATE boards, the field of one coil may interfere with and even desensitize the activation field of an adjacent relay. For this reason, all Series 113RF reed relays feature an internal mu-metal magnetic screen to enable high-density stacking of relays without the risk of adjacent devices interfering with each other, resulting in inconsistent or faulty operation.

The nine-page data sheet gives details on all versions of the Series 113RF relays. Other collateral includes an informative table comparing the attributes of the Pickering designs with what they refer to as “typical” reed relays; although it is undoubtedly somewhat biased, it nonetheless provides some genuine insight. If you are not as familiar as you’d like to be with reed relays, they have a three-minute tutorial video available here, a three-minute video on how their reed relays are constructed here, and a short video about this new series here.

The final section looks at MEMS-based relays and their function in ATE applications.