- Buyers Guide
Electromechanical distribution systems (switch matrices) are used in both commercial applications and military systems. Computer control of these switch matrices facilitates their use in automated testing and in those systems requiring remote control of switching functions.
The newest product in Dow-Key's line of electromechanical switches is a single-pole 12-throw internally terminated switch that forms the basis for a line of N x 12 switch matrix assemblies and provides a wide range of interface formats such as Ethernet, IEEE 488, RS232 and RS485 for remote control. The Ethernet (802.3) capability means the switch can be accessed from anywhere in the world via TCP/IP protocols on 10 base-T networks. The switch can also be controlled using the CANBus interface.
The model 13D12-480822T SP12T internally terminated switch is an ideal building block for N x 12 matrices when used with a single SP12T (or an additional 13D12 switch). Thus, a 2 x 12 matrices can be created using just 12 semi-rigid cables and two 13D12-series switches, as shown in Figure 1 . The new switch includes access to the termination ports. This access to the termination ports allows for implementation of high power terminations that can be installed remotely and connected to the unit using RF cables. Using the termination ports as outputs, 12 lines can be routed through the switch. A spare device can then be connected to the center port and the switch matrix can now replace a defective device in one of the 12 lines with the spare connected to the center port.
The 13D12-480822T switch is designed to perform up to 18 GHz and operates from a 24 V nominal DC supply with a typical operating current of 500 mA. The switch is equipped with a self cut-off latching actuator that switches the device in a maximum of 60 ms. Each unit is supplied with non-contacting indicator circuitry to monitor the movement of the RF contact actuators using photo-sensors. Table 1 lists its major performance characteristics vs. frequency. The standard version has 2 W CW external terminations installed directly on all termination ports.
RF Head Assembly
The basic design of the RF head assembly is shown in Figure 2 . Movement of RF conductor elements within a rectangular transmission line accomplishes RF switching. When the conductor element is actuated to the "on" position, it forms a 50 Ω transmission line. Each end of the conductor elements contacts the top portion of the RF connector center conductors that extend into the RF cavity. The "off" position conductor elements are forced back into intimate contact with the top side of its respective ground conductor to form single ridge waveguides. Due to its geometry, the cut-off frequency of two waveguides is far above the operating frequency of the switch. This allows for a high degree of isolation between on/off signal paths.
The movement of each reed is controlled by a mechanical linkage, which transfers the rotary motion of the actuator into the linear motion required to accomplish RF switching. When the actuator is in transition from one position to another, the motion is transmitted to the conductor elements through small dielectric plungers. Each conductor element is firmly captured by the insulator plungers and four guide pins.
The materials and processes used in the construction of the RF portion of the switch have been selected to provide high resistance to environmental conditions and to ensure consistently high RF performance. The RF reeds are manufactured from high quality contact material that has the mechanical properties necessary to withstand the impact of repeated switching. These reeds are gold plated for highly uniform performance and environmental resistance.
The switching mechanism, shown in Figure 3 , is equipped with 12 individual actuators. Each actuator consists of two coils attached to the soft magnetic plate by means of soft magnetic cores and separated by a permanent magnet. When the actuator coils are not energized the soft magnetic rocker is attracted to one of the cores applying the contact force to the movable contact through the dielectric plunger. When energizing the coil on the side the rocker is attracted to, the coil produces a magnetic field opposing the holding flux of the permanent magnet. As this net holding force decreases, the attractive force in the air gap on the opposite side of the rocker becomes great enough to break the rocker free, and snap it into a close position on the opposite side.
The indicator circuitry is based on photo-microsensors. The shutter attached to the rocker triggers the sensor when the contact closure is completed. This innovative design allows detecting the closure of the RF contacts with much higher resolution than the standard mechanical contacts. Also, the reliability of this design is much greater compared with other existing designs.
Applications for this switch matrix focus mainly on the wireless systems and satellite ground stations. Figure 4 shows a typical 10 x 10 switch matrix assembly for use in an automated test system. The computer control interface lends itself well to use in automated testing and systems requiring remote control of switching. The bi-directional switch offers high flexibility and high throughput capability, and features a wide bandwidth with high isolation and low insertion loss.
Dow-Key Microwave Corp.,
Ventura, CA (805) 650-0260,
Circle No. 302
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