Low Cost, Ultra-high Isolation SPDT Switches
This month's cover features a DC to 5 GHz high isolation switch that can be used with a personal computer to produce a low cost, high speed, automated test station
Low Cost, Ultra-high Isolation SPDT Switches
Wideband test instrumentation often requires signal routing. This function is usually accomplished with mechanical switches, which are expensive and slow. When isolation requirements are not high, electronic switches may be utilized. However, most high isolation electronic switches available in today's market are very expensive. A pair of low cost, high isolation, fast-switching, TTL-driven, connectorized switches have been developed that have typical isolation of 85 dB at 1 GHz and 60 dB at 5 GHz. The new switches have been designed in both absorptive and reflective versions. This article describes these DC to 5 GHz high isolation switches as well as a typical application.
Signal routing and automated testing are two major applications of mechanical switches because of their high isolation and low insertion loss. However, mechanical switches have very long switching times and limited life. Solid-state switches using GaAs FETs overcome these problems and are extremely popular in switching applications. The switches have many advantages such as small size, extremely fast switching, low cost and long life. When these switches are integrated with TTL drivers, they can be easily driven with other logic circuits or with a personal computer that has TTL driver cards.
The main disadvantage of the GaAs switch is its moderate isolation. As a result, circuit designers cascade the switches and integrate the drivers, which consumes large areas of circuit board space. To solve this problem, a pair of solid-state, high isolation switches1 with integral drivers has been developed. The models ASW-2-50DR (reflective) and ASWA-2-50DR (absorptive) switches provide an isolation of 53 dB (typ) at 1 GHz and 35 dB at 5 GHz. The reflective switch provides a short circuit termination at the output port in the off state and the absorptive switch provides a 50 W termination at the same port. The choice of the switch depends on the application.
For applications requiring even higher isolation, switch models ZASW-2-50DR (reflective) and ZASWA-2-50DR (absorptive) have been introduced. These switches were developed by cascading the ASW-2-50DR and ASWA-2-50DR switches. Careful layout and internal shielding were utilized to preserve the isolation performance. As a result, the ZASW-series switches have extremely high isolation - typically 100 dB to 100 MHz, 90 dB to 1 GHz, 80 dB to 2 GHz and 60 dB to 5 GHz. These high isolations result in an extremely low level of leakage from the unselected port. Figure 1 shows the typical isolation of the reflective and absorptive ZASW-series switches.
These building block switches are solid state and, hence, have extremely fast rise and fall times of 5 ns (typ) and a switching time of 10 ns (typ). This speed is extremely useful in automated test environments. For example, typical test times vary from a few milliseconds to a few seconds, and ZASW-series switches have a switching time of only a few nanoseconds. Hence, the overhead added by these switches is very low.
Although the configuration is a cascade of several switches, the insertion loss of these switches is moderate, typically 1.8 dB to 2 GHz and 3 dB at 5 GHz. Figure 2 shows the insertion loss of the reflective and absorptive switches; Figure 3 shows the SWR. Note that the SWR of the reflective switch output port in the off state is high.
A Typical Application
Figure 4 shows the ZASWA-2-50DR absorptive switch in a low cost, high accuracy, automated test application. The application comprises a vector network analyzer, device under test (DUT), attenuators and the high isolation switch. The attenuators are added at all ports of the switch to improve return loss. For this purpose, the return loss of the precision 6 dB attenuators must be extremely good.
Initially, the analyzer is calibrated by connecting the cable going to port 1 of the DUT to port 1 of the switch. At this time, the switch is turned on to provide low insertion loss from the S port to port 1. This state of calibration (state 1) is saved in an instrument register. The calibration is then repeated with the cable going to port 1 of the DUT connected to port 2 of the switch. At this time, the switch is in the on position from the S port to port 2 and the calibration data are saved in a different register (state 2). The DUT is then connected as in the original diagram. By keeping the switch in an on state from the S port to port 1 and recalling calibration state 1, insertion loss of the DUT from port 1 to port 2 can be measured. Similarly, by turning the switch on from the S port to port 2 and recalling calibration state 2, the insertion loss of the DUT from port 1 to port 3 can be measured. It is also possible to measure the return loss at port 1 of the DUT in any of the previously described switch states as well as the return loss of ports 2 and 3 when the insertion losses from 1 to 2 and 2 to 3, respectively, are measured. Connecting a personal computer to the test setup allows these measurements to be automated.
The disadvantage of this setup is that only two of the possible three insertion loss states can be measured. This disadvantage can be overcome by using three switches instead of one. Figure 5 shows the connections with three switches. The state of each of the switches determines the three DUT insertion loss measurements. For nonreciprocal devices, there can be twice as many insertion loss states. By using a network analyzer with a two-port S-parameter set along with this three-switch setup, all of the states can be measured by interchanging the stimulus and response states of the analyzer.
A pair of high isolation switches has been introduced. These switches cover most of the frequency bands of today's commercial, high volume markets. The switches can be used with a personal computer to produce a low cost, high speed, automated test station. In addition, the switches can find application in signal routing in communication equipment.
1. "High Isolation Switches with Integral Drivers," Microwave Journal, Vol. 41, No. 9, September 1998, pp. 164-168.