Learn about the system design of RF microwave switch testing

Jacktang

Learn about the system design of RF microwave switch testing [Copy link]

The tremendous growth of the wireless communications industry has meant an explosion in the testing of components and subassemblies for wireless devices, including the testing of the various RF ICs and microwave monolithic integrated circuits that make up the communications systems. These tests typically require very high frequencies, typically in the GHz range. This article discusses key issues in RF and microwave switch test systems, including the different switch types, RF switch card specifications, and RF switch design considerations that can help test engineers increase test throughput and reduce test costs. The Difference Between RF Switches and Low-Frequency Switches Converting a signal from one frequency to another seems easy, but how do you achieve extremely low signal loss? Designing switching systems for both low-frequency and direct current (DC) signals requires consideration of their own unique parameters, including contact potential, settling time, bias current, and isolation characteristics. High-frequency signals, like low-frequency signals, require consideration of their own unique parameters that affect signal performance during the switching process, including VSWR (voltage standing wave ratio), insertion loss, bandwidth, and channel isolation. In addition, hardware factors such as termination, connector type, and relay type can also greatly affect these parameters. Switch Type and Construction The capacitance within the relay is a common factor that limits the signal frequency that can be switched. The materials and physical properties of the relay determine the internal capacitance that is formed. For example, in RF and microwave switching above 40 GHz, special contact structures are used in electromechanical relays to achieve better performance. Figure 1 shows a typical construction, where the common termination is located between the two switch terminations. All signal connections are made of coaxial wire to ensure the best signal integrity (SI). In this case, the connectors are SMA female. For more complex switch configurations, the common termination is radially surrounded by the individual switch terminations. Figure 1 – High Frequency Electromechanical Relay A range of complex switching topologies are used in RF switches. Matrix switches allow every input to be connected to every output. Two types of matrices are used in microwave switch architectures – blocking and non-blocking. A blocking matrix connects any input to any output, so no other inputs or outputs can be connected simultaneously. This is an effective, low-cost solution for applications where only one signal frequency needs to be switched at a time. Signal integrity is better because there are fewer relay paths, especially phase delay issues are avoided. Non-blocking matrices allow multiple paths to be connected simultaneously. This architecture has more relays and cables, so it is more flexible, but also more expensive.

Figure 2 – Single-channel blocking and non-blocking matrices

The stacked switch architecture is an alternative to multi-position switches. It uses multiple relays to connect one input to multiple outputs. The path length (which also determines the phase delay) is determined by the number of relays the signal passes through.

Figure 3 - Cascaded switch architecture

Tree architecture is an alternative to cascaded switch architecture. Compared to cascade architecture, tree technology requires more relays for the same system size, however, the isolation between the selected path and other unused paths is better, which reduces crosstalk between relays and channels. Tree architecture has some advantages, including no unterminated stubs and similar characteristics of each channel. However, having multiple relays on the selected path means that losses are greater and signal integrity is also concerned.

Figure 4 – Multiple switches (one dual switch shown)

RF Switch Card Architecture In RF switch card applications in test instrument mainframes, there are many electrical specifications that need to be understood to ensure signal integrity.   Crosstalk is the capacitive coupling, inductive coupling, or electromagnetic radiation between signals transmitted on different channels or between a signal on a channel and an output signal. It is usually described in decibels at a specific load impedance and a specific frequency.   Insertion loss is the attenuation of a signal as it travels through a switch card or system, expressed in decibels over a specific frequency range. Insertion loss is a particularly important specification when the signal is low or the noise is high.  Voltage Standing Wave Ratio (VSWR) is a measure of the reflection of a signal on a transmission line and is defined as the ratio of the highest voltage amplitude to the lowest voltage amplitude of a standing wave on the signal path.  The limited frequency range over which a signal is switched, transmitted or amplified is called the bandwidth. For given load conditions, the bandwidth range is defined by the -3dB (half power) point.  Isolation is the ratio of the voltages of adjacent channels and is defined in decibels over a frequency range. RF Switch Design To design an RF switch system, a number of additional key factors need to be considered. Impedance Matching - Assuming the switch is placed between the measurement instrument and the DUT (device under test), all impedances in the system must be matched for several reasons. For optimal signal transmission, the output impedance of the source should be equal to the characteristic impedance of the switch, the cable impedance, and the impedance of the DUT. In RF testing, a common impedance level is 50 or 75 ohms. Regardless of the impedance level required, proper impedance matching will ensure the integrity of the entire system. The input VSWR and the signal path VSWR determine the accuracy of the measurement. Mismatch Uncertainty (dB) = 20 x log (1 +/- Γsig path * Γinst) Where Γ = VSWR-1/VSWR +1 If the signal path output and instrument input have a very good VSWR, such as 1.3:1, the mismatch uncertainty is approximately +/-0.15dB. Termination - At high frequencies, all signals must be properly terminated or the electromagnetic waves will be reflected at the termination points, resulting in an increase in VSWR. An unterminated switch in the off state will increase the VSWR. A switch generally needs to provide a 50 ohm termination resistor to match the on or off state. The increased VSWR may even damage the source if the reflected portion is large enough. Power Delivery - Another important consideration is the system's ability to deliver RF power from the instrument to the DUT. Due to insertion loss, the signal may need to be amplified. In some applications, it may be necessary to reduce the power of the signal to the DUT. Using an amplifier or attenuator can ensure that the exact signal power value is delivered to the switch system. Signal Filters - Signal filters are useful in some cases, such as when noise is accidentally added to the signal transmitted through the switch. Filters are also useful if the original signal frequency is not suitable for the DUT test frequency. In this case, filters can be added to the switch to change the signal bandwidth or filter out the unwanted signal frequency. Phase Distortion - As test systems grow in size, signals from the same source may travel through different paths to the DUT, causing phase distortion. This metric 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, leading to erroneous measurements. To reduce phase distortion, ensure that the signal paths are the same length. Summary Discussing and understanding the various design parameters that go into building an RF/microwave switch system will help ensure signal and system integrity.