Microwave test systems often include one or more RF/microwave switches to allow automated testing as well as measurements on multiple devices under test (DUTs). When configuring a highfrequency switching system for test applications, it is important to understand the type of switch and multiplexer options available as well as the critical switch specifications when choosing a switch.

Switch systems for RF and microwave applications can range from simple to quite elaborate. For example, a single-pole, doublethrow (SPDT) switch can be used to route signals to two different devices under test. It can be expanded further into a multiplexer configuration so that a single instrument can be routed to many different DUTs. In addition, multiple instruments can be routed to multiple DUTs. In this case, the switch system is known either as a multiplexer-demultiplexer or a blocking matrix—with only one signal path active at any given time. Figure 1 shows the application of an SPDT and a 1-to-16 multiplexer.

A switch system with a “blocking configuration” (Fig. 2) can connect multiple instruments to multiple DUTs. Any instrument can be connected to only one output at a time. To switch any signal to any DUT at any time, a “non-blocking” configuration (sometimes called a switch matrix) can be used. Although this switch configuration has the highest flexibility, it is also the most expensive. The minimum number of interconnecting cables in a non-blocking matrix is a product of the number of inputs and outputs. Therefore, a 10 × 10 nonblocking matrix requires 100 cables and single-pole, 10-throw (SP10T) relays. Assembling such a system is a major challenge. Furthermore, the use of a large number of components opens the way for potentially reduced reliability. Figure 3 shows a 4 × 4 nonblocking switch configuration, and Fig. 4 shows an expanded 4 × 6 switch configuration.

Any kind of switch can be thought of as an “interruption” in a signal path through a measurement system and, as such, it is impossible for the switch to have no effect on the system. In fact, the use of a switch will inevitably degrade the performance of a measurement system, so it is important to consider several critical parameters that may affect system performance significantly. Two types of specifications are very important when considering a switch for a measurement application: electrical and mechanical characteristics.

In terms of electrical specifications, such switch characteristics as impedance matching, insertion loss, isolation, and return loss can impact the overall performance of an RF/microwave measurement system. Because a switch will be positioned between the measurement instruments and the DUT, it’s critical to match the impedance levels of all three system elements. For optimal signal transfer, the impedance of the source must match that of the switch and the DUT. In RF testing, different impedance levels are used to achieve different purposes. The most common impedance in high-frequency systems is 50 O and all RF/microwave relay manufacturers make relays with characteristic impedance of 50 O. This impedance represents a trade off between the theoretical impedance at which maximum power transfer occurs, 30 O (30- components are not available in the marketplace), and minimum theoretical attenuation at 75 O (75- components are used in cable television). Also, 50- components are easily constructed.

Insertion loss is a measure of unintended signal attenuation that occurs due to dissipative losses in dielectric materials, impedance mismatches in connectors and circuit leads, and a variety of other contributing factors. Any component added to the signal path will cause some degree of loss, but the amount of loss is especially severe at higher or resonant frequencies. When the 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 to 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 dB at a certain frequency or over a frequency range:

Insertion loss (dB) = –10log(P_out/P_in)

Each component in the signal path will have its own insertion loss specification with some, such as couplers and power dividers, contributing an amount of loss above and beyond the decrease in signal level from a power coupling or power division.

Isolation is an important consideration since, at higher frequencies, signals traveling on different paths can interfere with each other or create “crosstalk” due to capacitive coupling between the paths or through electromagnetic (EM) radiation. This is especially severe when signal paths are not properly shielded or decoupled from each other. Crosstalk is problematic when a weak signal is physically adjacent to a strong signal. In such cases, the highest possible isolation is desired to minimize interference.

Any component added to the highfrequency signal path will not only cause insertion loss, but will affect the voltage standing wave ratio (VSWR) of 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 VSWR is specified as the ratio of the standing wave’s highest voltage amplitude to the lowest voltage amplitude in the signal. VSWR is often also expressed as return loss:

Return loss (dB) = –20 log[(VSWR–1)/ (VSWR+1)]

As the size of a test system is expanded, signals from the same source may travel to the DUT via different paths of different lengths. When the lengths of the signal paths differ the result is phase distortion or propagation delay. 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. This phase shift may cause erroneous measurement results. Therefore, techniques to ensure the same phase or path length can be used to compensate for such effects.

Reliability and repeatability are important considerations for switches in test systems. Typically, a switch relay should provide a lifetime of at least one million closures; many relays offer rated lifetimes of five million closures or greater. 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 making RF/microwave measurements, the effects of cycle-to-cycle changes in a switch relay closure can impact the accuracy of the system.

In some applications, particularly when low-power signals are being switched, solid-state PIN diodes can be used as the switching elements. With no moving parts, they offer a much longer life than electromechanical switches with their moving parts. Where high volume testing is being performed, and test tolerances are not that tight, solid-state switches can be very cost-effective. Their disadvantages are much higher insertion loss at a given frequency (typically more than 1 dB at 4 GHz for a PIN diode switch compared to 0.3 dB for an electromechanical switch). The isolation is much lower for a PIN diode switch compared to an electromechanical switch. An electromechanical relay can provide 100 dB or more isolation through 6 GHz, where a PIN diode switch may achieve less than 30 dB isolation below 3 GHz.

In terms of a switch’s mechanical specifications, the switch’s physical form factor may limit the choice for a particular test system application. For a switch system with a medium number of signal pathways, a system such as the Model S46 RF/ Microwave Switch System from Keithley Instruments can offer numerous flexible configurations with up to 32 control ports. For a large-scale switch system, the firm’s System 41 RF/Microwave Signal Routing System offers dual 1 × 36 multiplexers or a single 1 × 72 multiplexer. The System 41 also can be configured as a 6 × 6 or a 10 × 10 nonblocking multiplexer. When both low frequency and microwave switching are required, the company’s model 7116-MWS combines a 1 × 16 microwave multiplexer with 40 channels of lowfrequency DC switching and control capability.

Configuring a measurement system with multithrow switches also involves selection of cables and connectors, and the number of choices is many. The bandwidth of the test system, for example, will determine the type of coaxial connector used on the cable assemblies, with standard formats such as Type N connectors operating to about 12 GHz and SMA connectors to about 26.5 GHz. Smaller-dimensioned connectors, such as 2.4-mm connectors, can support a continuous bandwidth of DC to 40 GHz. The choice of cables must be made based on whether a test installation is fixed, and can use semi-rigid cables with their excellent loss characteristics, or can sacrifice some additional loss for the versatility provided by flexible coaxial cable assemblies. 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.

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