A Multiposition Backup Switch
Dow-Key Microwave Corp.
Ventura, CA
Today's high power RF communication systems often consist of multiple parallel 
amplifier stages to achieve high power and reliability. The use of RF transfer relays to provide automatic backup for a failed amplifier stage, as shown in Figure 1 , is becoming more routine. To accomplish this backup effectively, the relay must operate from simple control logic utilizing a minimum of additional circuitry, and perform the subsystem transfer quickly with a minimum of signal interruption in a cost-effective manner.
The model 3D4J-3X08 multiposition backup switch (a spinoff from a previously developed terminated multiposition switch) meets these challenges. The DC to 3 GHz unit features mechanically linked indicator circuits, TTL control, an immersion seal, multiple control circuit interfaces, optional control voltages and assorted RF connectors.
All moving parts and their corresponding mating parts in the RF path are plated with multiple layers of gold on the mating surfaces, resulting in low losses across these gaps when physical contact is made. Because gold is very stable in extreme environments and resilient to corrosion even in the presence of caustic materials, the relay features an extended lifetime and high reliability, and is capable of operating in a broad range of applications. The relay's nominal dimensions and associated mechanical tolerances are designed to take into consideration varying thermal linear expansion rates, thus preventing binding at low temperatures and excessive clearance at high temperatures. As a result, uniform RF performance and predictable power consumption are maintained over a broad operating temperature range.
The unit is capable of bulkhead mounting, requiring only one hole to provide connector access when the application requires a panel mount. The flange mounting facilitates both through holes or threaded holes for simple installation. The offset surface accommodates flush mounting on panels up to one-eighth-inch thick.
Access to the RF circuit is simplified because the RF connections are located on the opposite side of the same mounting plane where the relay and relay controls are found. Routing of the RF connections is achieved without negotiating other hardware considerations. As a result, the system has more effective routing with minimized line losses and matched phase lengths. An outline drawing of the switch is shown in Figure 2 .
In the event the switch requires service, all connections can be accessed on a common plane. This configuration allows for easier identification, isolation and replacement of suspect units than was possible with more traditional switch configurations. Although the switch is not hermetically sealed due to cost constraints and a lack of industry demand for such a capability, the relay is available with an immersion seal. When used with watertight external mating connections, this seal enables the unit to function in accordance with its specifications while being sprayed with water.
Specifications for the 3D4J-3X08 backup switch include an operating frequency range of DC to 3 GHz with a maximum SWR of 1.3. Maximum insertion loss is 0.2 dB and minimum isolation is 85 dB. The switch is able to handle 100 W CW of RF power at 3 GHz and operates in a pulse latching mode with a maximum 15 ms switching time. The unit features break-before-make RF contacts with a lifetime of one million cycles and an operating temperature range of -20° to +70°C. Figure 3 shows the switch's schematic diagram. The switch currently is available in 12 V (model 3D4J-3208) and 28 V (model 3D4J-3308) versions with typical current drains of 444 and 198 mA, respectively.
Although this switch does not eliminate the need for system backup components, it allows for less complicated RF wiring, simplifies control logic, reduces space requirements and lowers overall system cost. Additional information on pricing and available options can be obtained from the manufacturer.
Dow-Key Microwave Corp.,
Ventura, CA
(805) 650-0260.