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The Basics of RF and Microwave Switching Test Systems

Wed, 03/25/2009 - 5:52am
Understanding the key components of an RF and microwave switching test system will ensure overall signal and system integrity.
By Dale Cigoy, Keithley Instruments, Inc.


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Figure 1. High-frequency electromechanical relay.
The tremendous growth of the wireless communication industry has meant an explosion in the level of testing of the components and subassemblies for mobile wireless devices, including everything from RF ICs and microwave monolithic ICs to complete communications systems. These tests often require very high frequencies, typically in the gigahertz (GHz) range. This article looks at the key components of an RF and microwave switching test system, including different switch types, RF switch card specifications, and considerations in RF switch design that helps test engineers increase test throughput and lower their cost of test.
Difference between RF and Low Frequency Switching
Switching a signal from one point to another seems simple enough. But how is this done with minimal signal loss for the type of signal being routed? Low-frequency and DC signals have their own special parameters that need to be considered when designing a switching system, including contact potential, settling time, offset currents, and isolation, among others.

High-frequency signals, like their low-frequency counterparts, have a different set of parameters that need to be considered. These parameters affect signal performance as they are routed through the switch and include VSWR (voltage standing wave ratio), insertion loss, bandwidth, and isolation. Furthermore, hardware decisions, such as termination, connector type, and type of relay, will greatly impact these parameters.
Switch Types and Configurations

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Figure 2. A single-channel blocking matrix and a non-blocking matrix.
The capacitances within relays are a common factor that limits the frequency of switched signals. Materials and physical configurations of relays determine inherent capacitances among their components. For example, the capacitance between poles degrades AC signal isolation by coupling the signal from pole to pole or relay to relay.

Specialized contacts and architecture are used in electromechanical relays to obtain good performance for RF and microwave switching up to 40GHz. A typical configuration is shown in Figure 1 where the common terminal is between two switched terminals. All signal connections are coaxial to ensure optimal signal integrity. In this case, the connectors are the female SMA type. For more complex switching configurations, the common terminal is surrounded by switched terminals in a radial pattern.

A number of different switch topologies are used for RF switching. 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, so other inputs and outputs cannot be connected at the same time. This is usually a cost-effective solution for applications that only need to switch one signal at a time. Signal integrity is typically better because of the fewer number of relay paths, especially the absence of phase delay issues. A non-blocking matrix allows multiple paths to be connected simultaneously through the matrix. With its greater number of relays and cables, this configuration is more flexible while being a bit more expensive.


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Figure 3. A cascade switching configuration.
A cascade switch configuration is an alternate form of a multi-position switch. It 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.

The tree configuration is an alternative to the cascade configuration. When compared to cascade, the tree technique requires more relays for the same size system. However, the isolation between a given path and any unused paths may be somewhat better, reducing crosstalk between relays and channels. The tree configuration has some advantages, including the absence of unterminated stubs and the fact that the channels have similar characteristics. However, multiple relays in a given path mean there will be more losses and signal integrity could suffer.
RF Switch Card Specifications
RF switch cards in a mainframe have various electrical specifications that need to be understood in order to maintain the best signal integrity: •Cross talk is the coupling of a signal from one channel to another or to the output by stray capacitance, inductive coupling or radiation. It’s typically expressed in decibels at a specified load impedance and a specific frequency. •Insertion loss is the attenuation of a signal being routed through a switching card or system. Specified as a decibel value over a frequency range, insertion loss becomes more important with low signal levels or high noise levels. •VSWR (voltage standing wave ratio) is a measure of signal reflection along a transmission line and is expressed as a ratio of the highest voltage to the lowest voltage found along the signal path. •The range of frequencies that can be switched, conducted, or amplified within certain limits is called the signal bandwidth. Under given load conditions, bandwidth is defined by the –3dB (half power) points. •Isolation is the ratio of the power level between adjacent channels, which is expressed in decibels over a frequency range.
RF Switch Design
A few additional key factors need to be considered when designing an RF switch system.

Figure 4. Multiplex, or two-tier, switching.
Impedance Matching— Given that the switch is positioned between the measurement instruments and a DUT (device under test), 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 commonly used impedance levels are 50 and 75 Ohms. Whichever impedance level is required, proper matching will ensure overall system integrity.

The input VSWR and signal path VSWR determine the limitation on the accuracy of the measurement:

Mismatch Uncertainty(dB) = 20 x log(1 ±Γ sig path * Γinst) Where Γ = VSWR-1/VSWR +1

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 ۪.15 dB.

Termination— At high frequencies, all signals must be properly terminated or the electromagnetic wave will be reflected from the terminating point, causing an increase in VSWR. An unterminated switch increases VSWR in its off condition, while a terminated switch will try to provide a 50-Ohm match on or off. The VSWR increase may even damage the source if the reflected power is large enough.

Power Transmission— Another important consideration is the system’s ability to transfer RF power from instrument to DUT. Due to insertion loss, the signal may often 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 Filters— Signal filters can be useful in a number of circumstances, such as when spurious noise is inadvertently added to a signal as it is routed through the switch. Filters 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, causing erroneous measurement results. To minimize phase distortion, keep the path lengths the same.
Conclusion
Understanding and taking into account all of these design parameters when configuring an RF/microwave switching system will ensure overall signal and system integrity.

Dale Cigoy is a lead applications engineer for Keithley Instruments, Inc., www.keithley.com.

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