Fundamentals of the hottest RF and microwave switc

RF and microwave switch test system foundation

the great growth of the wireless communication industry means that the testing of components and components of wireless equipment has ushered in a big explosion, including the testing of various RF ICs and microwave monolithic integrated circuits that make up the communication system. These tests usually require very high frequencies, generally in the GHz range. This paper discusses the key problems in RF and microwave switch test systems, including different switch types, RF switch card specifications, and the problems that need to be considered in the design of RF switches that help test engineers improve test throughput and reduce test costs

difference between RF switch and low frequency switch

it seems easy to convert a signal from one frequency point to another, but how to achieve extremely low signal loss? Both low-frequency and direct current (DC) signal switching systems need to consider their unique parameters, including contact potential, establishment time, bias current and isolation characteristics

high frequency signals, similar to low-frequency signals, need to consider their unique parameters, which will affect the signal performance in the switching process. These parameters include VSWR (whether the inflection point can be selected to identify the compliance condition (avoid jaw slip), VSWR, insertion loss, bandwidth and channel isolation, etc. In addition, hardware factors, such as the type of fatigue test of metal materials and relay types, such as termination and connectors, will also greatly affect these parameters

switch type and structure

the capacitance in the relay is a common factor that limits the signal frequency of the switch. The material and physical characteristics of the relay determine its internal capacitance. For example, in RF and microwave switches over 40GHz, special contact structures are used in electromechanical relays to obtain better performance. Figure 1 shows a typical configuration with a common termination between two switch terminations. The connecting lines of all signals are coaxial lines to ensure the best signal integrity (SI). In this case, the connector is SMA female. For more complex switch structures, the common termination is radially surrounded by each switch termination

Figure 1 high frequency electromechanical relay

a series of complex switching topologies are adopted in RF switches. The matrix switch can realize the connection between each input and each output. There are two types of matrices that can be used in the switching architecture in which microwave splines are placed on the two supports of the fixed machine tool of the experimental machine - blocking and non blocking architecture. A blocking matrix can connect any input and any output, so other inputs and outputs cannot be connected at the same time. This is an effective low-cost scheme for applications that only need to switch to one signal frequency at one time, and the signal integrity is better, because there are fewer relay paths, especially to avoid the problem of phase delay. The non blocking matrix allows multiple paths to be connected at the same time. This architecture has more relays and cables, so it is more flexible, but the price is also higher

Figure 2 single channel blocking matrix and non blocking matrix

cascade switch architecture is an alternative form of multi position switch. 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 layered switch architecture

tree architecture is an alternative to layered switch architecture. Compared with the layered architecture, the tree technology requires more relays for systems of the same specification. However, the isolation between the selected routes and other unused routes will be better, which reduces the crosstalk between relays and channels. The tree architecture has some advantages, including unterminated stubs, and the characteristics of each channel will be similar. However, having multiple relays on the selected route means greater loss and worrying signal integrity

Figure 4 multiple switches (the figure shows a double switch)

RF switch card architecture

in the application of RF switch card on the host of test instrument, in order to ensure signal integrity, it is necessary to understand many electrical performance indicators

· crosstalk refers to capacitive coupling, inductive coupling or electromagnetic radiation between signals transmitted on different channels or between signals on channels and output signals. It is generally described by specific load impedance and decibel number at specific frequency

· insertion loss is the attenuation of the signal when it is transmitted in the switch card or system, which is expressed in decibels in a specific frequency range. When the signal is low or the noise is high, the insertion loss is a very important technical index

· voltage standing wave ratio (VSWR) is a measurement of the reflection of a signal on the transmission line, which is defined as the ratio of the highest voltage amplitude of the standing wave on the signal path to the lowest voltage amplitude

· a limited frequency range in which signals are switched, transmitted or amplified is called bandwidth. For a given load condition, the bandwidth range is defined by -3db (half power) points

· isolation is the voltage ratio of adjacent channels, defined as decibels in a frequency range

rf switch design

to design an RF switch system, a series of key factors need to be considered additionally

impedance matching - assuming that the switch is placed between the measuring instrument and the DUT (equipment to be tested), all impedances in several systems must be matched. For the best 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, the common impedance level is 50 or 75 ohms. No matter what impedance level is required, appropriate impedance matching will ensure the integrity of the whole system

input VSWR and signal path through VSWR determine the accuracy of measurement

Mismatch Uncertainty(dB) = 20 x log(1 +/- Γ sig path * Γ inst) Where Γ = Vswr-1/vswr +1

if the signal path output and instrument input have a good VSWR, such as 1.3:1, the mismatch uncertainty is about +/- 0.15dB

termination - in case of high frequency, all signals must be properly terminated, otherwise the electromagnetic wave will be reflected on the termination point, resulting in the increase of VSWR. A switch without termination will increase VSWR in the off state. A switch generally needs to provide a 50 ohm termination resistance to match the on or off state. After VSWR is increased, if the reflection part is large enough, it may even damage the source end

power transmission - another important consideration is the ability of the system to transmit 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. The use of amplifiers or attenuators ensures that accurate signal power values are transmitted to the switching system

signal filter - signal filter is very useful in some cases, such as when noise is inadvertently added to the signal transmitted through the switch. If the original signal frequency is not suitable for the DUT test frequency, the filter is also useful. In this case, the filter can be added to the switch to change the signal bandwidth or filter out the unwanted signal frequency

phase distortion - with the expansion of the size of the test system, signals from the same signal source may be transmitted to the DUT through different ways, resulting in phase distortion. This indicator is often referred to as transmission delay. For a given conducting medium, the delay is proportional to the length of the signal path. Different signal circuits should strictly control the molding temperature, and the diameter length will cause the signal phase to move, resulting in incorrect measurement results. To reduce phase distortion, it is necessary to ensure the same length of signal path

summary

discussing and understanding various design parameters in the purchase and construction of rf/microwave switching system is conducive to ensuring the integrity of signal and system. (the author is Dale cigoy, a senior application engineer of giseley company in the United States) (end)