Given the explosive growth of the communications industry, a tremendous amount of testing is being performed on the various components that make up different communications systems. These components range from active components such as Radio Frequency Integrated Circuits (RFICs) and Microwave Monolithic Integrated Circuits (MMICs) to complete communication systems. While the testing requirements and procedures for these components differ widely, all are tested at very high frequencies, typically in the gigahertz range. The main components in a test system may include DC bias, DC measurement, RF power meter, network analyzer, RF sources, and other instruments. Automating the test process and improving test efficiency demands integrating RF/microwave and low frequency switching systems into the test system.
Microwave Switch Types
Available microwave switch configurations include a simple single-pole double throw (SPDT) switch, multi-position switch, matrix, and cascade.
An SPDT switch has one input port, which can be connected to one of two output ports. A multi-position switch connects one input port to one of several output ports. Keithley’s System 46 can accommodate eight SPDT coaxial microwave relays and four multi-pole coaxial microwave relays.
A matrix switch can connect any input to any output. Two types of matrices are used in microwave switching: blocking and non-blocking. A blocking matrix connects any one input to any one output. Other inputs and outputs cannot be connected at the same time. A non-blocking matrix allows multiple paths to be connected simultaneously through the matrix.
The cascade switch configuration is an alternate form of multi-position switch. The cascade switch connects one input to one of many outputs using multiple relays. The path length (and therefore, the phase delay) varies, depending on the number of relays that the signal must go through.
RF Switch Card Specifications
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 significantly. During the design phase, the costs and benefits are often weighed against each other to achieve an optimal solution. When choosing an RF switch system, some of the critical electrical specifications to review include crosstalk (path isolation), insertion loss, voltage standing wave ratio (VSWR), and bandwidth.
RF Switch Design Considerations
When designing an RF switch system, additional factors that may affect switch system performance include impedance matching, termination, power transmission, signal filters, phase distortion, and cabling.
Impedance Matching: Given that the switch is positioned between the measurement instruments and the DUT, matching the impedance levels of all elements in the system is critical. For optimal signal transfer, the output impedance of the source should be equal to the characteristic impedance of the switch, the cables, and the DUT. In RF testing, the most commonly used impedance levels are 50Ω and 75Ω. Whichever impedance level is required, proper matching will ensure the overall system integrity.
The input VSWR and signal path VSWR determine the limitation on the accuracy of the measurement:
Mismatch Uncertainty (dB) = 20 × log (1 ± Гsig pathГinstr)
If both the signal path output and the instrument input have good VSWRs of 1.3:1 at a frequency, then the uncertainty due to mismatch alone is ±0.15dB.
Termination: At high frequencies, all signals must be properly terminated or the electromagnetic wave will be reflected from the terminating point. This, in turn, will cause an increase in VSWR. An unterminated switch increases VSWR in its off condition, while a terminated switch will try to provide a 50Ω match on or off. The VSWR increase may even damage the source if the reflected power is large enough. All paths through a system
must be terminated with their characteristic impedance.
Power Transmission: 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 other applications, it may be necessary to reduce the signal power to the DUT. An amplifier or attenuator may be needed to ensure that the required level of power is transmitted through the switch.
Signal Filter: Signal filters can be useful in a number of circumstances, such as when spurious noise is inadvertently added to the signal as it travels through the switch. They can also be helpful if the original signal frequency does not fit in the DUT testing frequency. In these cases, filters can be added to the switch to modify the signal frequency bandwidth, or spurious signals at unwanted frequencies can be eliminated from the signal to the DUT.
Phase Distortion: As the size of a test system expands, signals from the same source may travel to the DUT via paths of different lengths, resulting in phase distortion. This specification is often referred to as 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. To minimize phase distortion, keep the path lengths the same.
Taking all of these design considerations into account when configuring an RF/microwave switch system can be simplified by ordering a package solution, such as the System 46 RF/Microwave Switch System. It can be configured with up to 32 channels for controlling microwave switches. It also tracks contact closures for proactive maintenance of relays, and performance parameters, such as VSWR or insertion loss, for trend analysis
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Content produced by Keithley, A Tektronix Company: Test and Measurement Equipment | Tektronix
