In recent years the VXIbus architecture has proven to be a versatile interface standard for instruments from many diverse manufacturers to operate within the same mainframe. The VXIbus incorporates the user-friendly features of instruments designed to operate within the General Purpose Interface Bus (GPIB) (for example, using ASCII-level programming) and the high throughput capability of the VME devices, which are programmed and communicate directly in binary code. The VXIbus was not designed to replace any existing standard, but as an additional tool to help in overall test or data acquisition solutions. To this end several methods of communicating with VXIbus devices were defined, enabling VXIbus solutions to be integrated with VMEbus, GPIB or as stand-alone portable solutions. These stand-alone test solutions are comprised of separate instrumentation devices constructed within the framework of a VXIbus card slot and plugged into a mainframe structure similar to a typical computer configuration with card slots inserted in a backplane assembly.

Today many instrument manufacturers are producing VXIbus-compatible instrumentation, necessitating the use of control components that are capable of operating within the VXIbus interface. To meet these requirements, a new series of mechanical microwave switches has been designed that feature miniature size for incorporation in a single VXIbus card slot. These coaxial switches operate from DC to 26.5 GHz and are currently available in three configurations - a dual SPDT, DPDT transfer and SP6T switch. All versions feature an operating life of two million cycles with repeatable performance for each cycle.

Switch Construction

Since the VXI slot spacing is 1.2", the maximum switch body dimension is limited to 1.125". The main mechanical outlines for all three switch types are identical. The only difference between the three is in the area of the SMA RF connectors. Figure 1 shows the mechanical outline of the transfer switch model.

Although all the switches have the same type of control connector, each switch model has a unique pin-out configuration. This feature allows any type of switch to be plugged into the universal control circuitry. This control circuit can recognize which switch has been inserted and reconfigure the control interface accordingly. Thus, any test system utilizing these switches becomes very easy to reconfigure.

The switches are divided into an RF section and a drive section, as shown in Figure 2 . The basic element of the RF section is terminated by two SMA connectors (input and output). Both connectors are inserted into a rectangular cavity with a movable RF contact, commonly called the reed or blade. The cross-section of the movable contact is also rectangular. When the contact is closed, the cavity and movable contact form a 50 transmission line. In the open state the movable contact is terminated against one wall of the RF cavity, thus forming a single ridge waveguide. To provide good isolation, the operating frequency of the switch must be well below the cut-off frequency of the single ridge waveguide.

Typically, the transmission line formed by the closed contact has a number of discontinuities, particularly in the junction areas around the connectors. The discontinuities degrade the RF performance and limit the upper operating frequency range of the switch. To provide a switching solution that operates to 26.5 GHz, the design of the RF section has been optimized using Ansoft HFSS simulation software.

The drive design is based on a linear solenoid. The company has developed a linear solenoid design that provides optimum contact forces over the switches' operating life with almost no noticeable wear of the moving part. This linear solenoid operating characteristic has been achieved by a detailed optimization process that allows the setting of exact clearances between the movable and stationary parts as well as the selection of optimum mechanical finishes, plating schemes and dry lubricant for all key components.

The characteristics of the linear solenoid have been optimized to drastically reduce the level of dynamic forces during switching action. This force reduction results in a substantial increase in the contact's operating life.

Performance

The VXI switches have been designed with a focus on reliability, extended operating life, and phase and insertion loss repeatability. The contact geometry, materials, mechanical finishes and plating schemes have all been carefully selected to provide an operating life well exceeding two million cycles. Table 1 lists the key performance specifications of the VXI transfer switch as an example.

Maximum SWR for all three types range from 1.25 (DC to 4 GHz) up to 1.8 in the 18 to 26.5 GHz range. Insertion loss is typically less than 0.2 dB (DC to 4 GHz) and increases to 0.8 dB max. at 26 GHz. Isolation is 70 dB max. to 4 GHz and degrades to 50 dB at 25 GHz. All of the switches are capable of handling 100 W of CW RF power at 4 GHz and gradually reduces to 30 W at the upper frequency limit. Nominal operating voltage is 24 V DC, while the operating current depends on the configuration. Switching time is 20 ms max. and the operating temperature is specified at -25° to +65°C.

Application for these switches largely centers on use in various test systems and instrumentation designed for use in VXI-type equipment. Any application where the control and routing of microwave signals in a VXI environment is required is appropriate. The VXI family of miniature broadband coaxial switches has pushed the upper limit of operating frequency well beyond the past limit of 18 GHz while maintaining both optimum performance and high reliability.

Dow-Key Microwave Corp., Ventura, CA (805) 650-0260, www.dowkey.com. Circle No. 301