Understanding the “Black Art” of RF and Microwave Switching
Understanding RF, especially microwave switching, can be a challenge. In many cases, test systems have been compromised because a test engineer did not consider all the components of their test system: instruments, switching, cabling, Mass Interconnect (test fixture interface), fixture design, and so on. Each component has its own set of characteristics. With RF and microwave components, effects including insertion loss, VSWR, bandwidth, and so on all need to be considered as well.
Higher bandwidth is a constant demand. The most obvious current example is the emergence of 5G systems that will be hitting the market in the near future, but there are many defense systems – often with their own protocols – that are now operating at RF and high-frequency bandwidths. In short, the need to employ repeatable, accurate RF and microwave switching systems is only going to increase, and while this may appear to be a “sexy” subject, without a proper appreciation of the challenges involved, test systems designers could find themselves facing a lengthy process involving a great many iterations while inconsistencies and failures are ironed out.
The important point to remember is that parameters including insertion loss, VSWR, etc., across instruments, switching, cabling, connectors, fixtures, and probes can be additive. For example, if you have a switch with 1 dB of loss going through a 3-meter cable that has a loss of about 0.1 dB per meter, then via a connector with 0.2 dB loss and into another cable, you can very soon accrue almost 2 dB of insertion loss. This may be allowable if you have planned for the loss and can calibrate it out, but depending on the test specifications, it may be unacceptable.
Designing a Test Strategy
A test strategy is driven by many factors including the test specification from engineering, expected accuracy, the test budget, available hardware, time to market, and the list goes on. In any case, proper switching system design is essential, so it's important to make the right choices if you want your automated test system to be successful. If forced to make compromises, keep in mind that in an RF test, the accuracy is the sum of all components in the measurement/stimulus chain.
Before designing a functional test system, it is best to ask “what are we testing?” Understand the DUT (Device Under Test) and the parameters that need to be tested. Some questions to consider include:
What is the frequency range of the signals to be switched, and how fast must the switching system operate?
How much power will the system have to handle?
What kind of cabling and connectors does the DUT use and are there any preexisting connectors and cabling to which the switching system must connect?
How will the switching system be controlled?
What parameters need to be sourced and measured? What tolerances are included in the Engineering Test Plan?
The heart of the switching system is the switch itself — usually a module with electromechanical relays or solid-state relays (see photo). The table summarizes the advantages and disadvantages of both. The connector chosen for the test system is another critical component, and there are many different types available including:
SMB connectors suit systems to 4GHz and are available in 50Ω and 75Ω versions. Their small size suits them for use on PXI modules; however, they can only be used with relatively thin coaxial cables that can increase losses at higher operating frequencies.
MCX connectors operate up to 6 GHz and are available in both 50Ω and 75Ω versions.
SMA connectors are suitable for use up to 18 GHz or even higher and mate well with semi-rigid and larger cables, ensuring systems that use this connector have high performance and low loss. They are, however, bigger than SMB and MCX connectors and the use of a torque wrench is advisable to tighten the connector nut.
QMA connectors were specifically developed for telecommunications systems, small cellular systems, and Wi-Fi applications, where high-performance, tool-free connections must be made.
Type N connectors are popular on bench instruments because they are large and robust.
Type F connectors are the preferred connector for broadcast applications. To be safe, they should only be used up to 1 GHz.
Multipole RF connectors pack a relatively large number of connections in a small amount of front panel space, but the construction severely limits the bandwidth of connection.
The choice of cabling will also impact the performance of the test system. The frequency of the system will largely determine the choice of cable and the losses can be calculated and allowed for. With semi-rigid and large cables that have significant metal shielding, the arrangement of the cables — how they lie on the test bench, the degree and the positioning of bends, sometimes referred to as plumbing — can cause significant differences in measurements, so care should be taken in fixture design to ensure consistency.
Terminating the ports of an RF switch is advantageous in some applications. Although RF switches with terminated inputs and outputs cost more than non-terminated switches, terminated outputs improve system performance by reducing the amount of signal that leaks to other connections (crosstalk and isolation). Where there is some leakage, the RF switch characteristics will be more consistent. A disadvantage of using switches with built-in terminations is that the termination limits the RF power that the switch can handle.
The impact of crosstalk and isolation on RF switching systems can vary widely, depending on the application. In some cases, it barely matters at all; for example, when the products under test use different frequencies or different time slots to transmit and the measuring device is either frequency-selective or time-selective. In other applications, though, the RF switch might be asked to handle a variety of signals and it's important to keep crosstalk to a minimum. If you are multiplexing four signals with similar frequencies and levels, the selected channel will see interfering signals from the other three channels.
Finally, you need to consider the platform for your RF switching system. All systems have advantages and disadvantages. PXI systems, for example, tend to be physically smaller than LXI systems and are a good choice for systems that use relatively diverse and compact switching and instrumentation from multiple vendors. The LXI platform, on the other hand, may be the better choice for systems that need large switching architectures, the highest parametric performance, or control at a distance. Testing cable runs in an airframe, for example, might require you to locate instrumentation, and the RF switching system to support it, at one end of a very long cable. Keep in mind that your system needn't be entirely PXI or entirely LXI but can be a hybrid of both.
This article was written By Bob Stasonis, Technical Product Specialist at Pickering Interfaces (Chelmsford, MA) and President of the PXI Systems Alliance. For more information, visit here .