Modern RF/microwave switching systems can improve the accuracy of production testing, while greatly increasing measurement throughput.
Switching systems are vital parts of automated-test-equipment (ATE) systems for evaluating communications components. Since a typical test system must route a wide range of signals, including RF, microwave, and DC bias signals, to a wide range of instruments (including network and spectrum analyzers and power meters), a switching system must perform reliably, accurately, and efficiently in support of a high-throughput ATE system. Properly configuring an RF/microwave switching system can improve the performance of an ATE system as a whole.
The purpose of an RF/microwave switch is to route signals between measurement instruments and the device under test (DUT). With the help of a switch, an instrument can measure or source multiple DUTs without the need to change cabling for each one. Multiple tests with different instruments can be run on the same DUT or multiple instruments can test multiple DUTs. With the help of a switch system, the test process can also be automated. For example, in the typical lifetime test of Fig. 1, the DUT (in this case, a mobile phone) can be stressed at an elevated level for a specified period, then its electrical characteristics can be measured. The DUT can be then stressed even further and the electrical characteristics measured again. Automated switching allows this process to be performed very efficiently.
Switch systems can be quite simple or quite elaborate. For example, a single-pole, double-throw (SPDT) switch can be used to route signals to two different DUTs (Fig. 2a). It can be expanded further into a "multiplexer" configuration, so that a single instrument can be routed to many different DUTs (Fig. 2b). As with the SPDT switch, a multiplexer employs a "blocking" arrangement in which only one signal path is active at any time. For improved flexibility, a series of switches can be arranged in a matrix to connect multiple instruments to multiple DUTs (Fig. 3). In order to switch any signal to any DUT at any time, a "nonblocking" matrix, like the 4 × 4 matrix of Fig. 4, can be used. While the nonblocking configuration has the highest flexibility, it increases the cost of the switching system by a factor of 5 to 10.
Many tests require more components than just switches. For example, testing mobile-telephone receivers involves switching gain and attenuation in and out, to simulate varying receiving ranges and multipath effects. This means adding both active components, like amplifiers, and passive components, like attenuators, splitters/combiners, circulators, and directional couplers, to the test setup (Fig. 5). Rather than connecting these components externally with a patchwork of cables, it is often preferable to include them within the chassis of the switching system. Not only is this an uncluttered arrangement, it will give more consistent and repeatable results than an ad hoc arrangement.
The use of a switch will inevitably degrade the performance of the measurement system, so it is important to consider several critical parameters that may affect system performance. Both electrical and mechanical specifications are very important when configuring a switch. These switching systems are complex to design and manufacture, so they tend to be significantly more expensive than lower frequency switch systems. During the design phase, the costs and benefits are often weighed against each other to achieve an optimal solution.
Some key specifications that are critical in selecting an RF/microwave switch system include impedance matching, insertion loss, isolation, and VSWR. Since a switch will be positioned between the measurement instruments and the DUT, it is critical to match the impedance levels of all three system elements. For optimal signal transfer, everything in the signal path—the source, the switch, the DUT, and any terminations used—must all have the same impedance. The most commonly used impedance level is 50 Ω, although 75-Ω switching systems (commonly used for cable-television systems) are also available. Impedance mismatches increase VSWR and can contribute to measurement errors. In high-power systems, they can even lead to equipment damage.
In a system, any passive component added to the signal path will cause some degree of loss. The amount of loss is especially severe at higher frequencies. When signal level is low or noise is high, insertion loss is particularly important. The insertion loss is reflected as a decrease in the available power on the DUT as compared to the test instrument source value. Normally, it is specified as the ratio of output power over the input power in decibels (dB) at a certain frequency or over a frequency range:
Insertion loss = −10log(Pout/Pin)
Pout = the output power (in W) and
Pin = the input power (in W).
At higher frequencies, signals traveling on different paths can interfere with each other due to capacitive coupling between the paths or through electromagnetic radiation. Sometimes referred to as "crosstalk," it is especially severe when signal paths are not properly shielded or decoupled from each other. Crosstalk is particularly problematic when a weak signal is physically adjacent to a very strong signal. When maintaining signal-path isolation is critical, additional isolation measures should be used.
Any component added to the high-frequency signal path will not only cause insertion loss, but will also cause an increase in the standing wave in the signal path. This standing wave is formed by the interference of the transmitting electromagnetic wave with the reflected wave. This interference is often the result of mismatched impedances in different parts of the system or connecting points in the system, such as connectors. The voltage-standing-wave ratio (VSWR) is specified as the ratio of the standing wave's highest voltage amplitude to the lowest voltage amplitude in the signal path. It can be calculated as:
VSWR = Zline/Zload or
whichever is greater,
Zline = the characteristic impedance of the line and
Zload = the characteristic impedance of the load.
VSWR can also be expressed as return loss:
Return loss (dB) = −20log
Figure 6 shows a system with a VSWR of 1.50:1. This is equivalent to a return loss of 14 dB. As a result, with 50 W forward power, reflected power would be 2 W.
Filters connected to switches can be useful in a number of circumstances, such as when spurious noise or harmonics need to be eliminated from test signals. In these cases, filters can be added into the switch path to eliminate spurious signals at unwanted frequencies.
Phase distortion (or propagation delay) is another key switch electrical parameter. As a test system expands in size, signals from the same source may travel to the DUT via different paths of different lengths. For a given conducting medium, the delay is proportional to the length of the signal path. Different signal-path lengths will cause the signal phase to shift. In digital testing where differential signal testing is key, this phase shift may cause erroneous measurement results. Techniques to ensure the same phase or path electrical length should be used to compensate for such effects.
Reliability is a major concern when designing a switch system, since ATE system downtime is expensive and unproductive. Typically, an electromechanical switch relay should provide a lifetime of at least one million closures; some electromechanical relays offer rated lifetimes of five million closures or more.
The repeatability of the switch performance is an equally important issue. Repeatability is the measure of the changes in the insertion loss or phase change from repeated use of the switch system. In RF measurement, it is not easy to eliminate the effects from the cycle-to-cycle change in the switch relay closure.
Mechanical specifications should also be considered when specifying a switching system. For example, depending on the application involved, the switch's physical form factor may limit the choice of the switch system. For a small switch, a simple box will do the job. For example, the model 7999-4 microwave switch (Fig. 7) from Keithley Instruments (Cleveland, OH) is an RS-232-controllable SPDT switch in a small metal enclosure. For a switch system with a medium number of signal pathways, a system based on the Model 7001 two-slot switch mainframe would be appropriate. For a large-scale switch system, a system based on the company's model 7002 ten-slot mainframe (Fig. 8) would be needed.
Many different types of connectors and cables can be used in RF/microwave switch systems. The signal frequency, the system impedance, power rating, and test fixture/handler compatibility, etc. should all be taken into consideration when choosing connectors and cables.
In some cases, status-reporting functions are useful features in switching systems. Switch mainframes that provide a light-emitting-diode (LED) display to indicate the open/closed status of the switch relays are very useful during system setup and troubleshooting.
In addition to electrical and mechanical specifications for an RF/microwave switch system, several factors need to be considered carefully. These factors can easily degrade the system performance even when the best parts are used. For example, most users would like as much switching bandwidth as possible. However, a switching system that can handle 40-GHz signals is more costly than one rated to 18 GHz. If the equipment to be tested involves no frequencies higher than 18 GHz, it is overkill to use a 40-GHz switch. In general, it is more economical to match the bandwidth of the switch to that of the DUT. It is also important to remember that as bandwidth increases the selection of connectors and cables becomes more important, because both attenuation and VSWR tend to increase with frequency. Once again, cost increases with bandwidth.
Another important consideration is the system's ability to transfer the RF power from instrument to DUT. Due to insertion loss, the signal may require amplification. In some applications, it may be necessary to reduce the signal power to the DUT. The use of either an amplifier or attenuator may be needed to ensure that the accurate level of power is transmitted through the switch.
When specifying a switch system, the first step is to consider the system configuration. In order to achieve an optimal, yet cost-effective system, system designers must weigh a variety of electrical and mechanical parameters. Understanding these parameters makes it possible to make informed trade-offs between switch flexibility and system cost.