What are the main parameters when selecting a solid state RF Switch?

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Raghav Kapur - everything RF

Nov 28, 2016

The main parameters when selecting a solid state RF switch are as follows:

Configuration: The configuration of a switch is the number of signal paths that it can switch between. 
A single-pole-multiple-throw (SPnT) switch allow a single input to multiple (three or more) output paths. For example a Single-pole-double-throw (SPDT or 1:2) switch routes signals from one input to two output paths.

Double-pole-double-throw (DPDT) switches can be used to switch between two inputs and two outputs, as a drop-out switch, for signal reversal, or to bypass a test component.

Topology: The switch topology tells you about the way switch ports are terminated when that specific path is not active. Reflective switches are the equivalent of a open circuit, so any signal that goes in here will see a high impedance at the port, which can be reflected back in to the system and cause interference. Absoprtive (aka Terminated) Switches incorporate a termination at each of the output ports which result in a low VSWR in both the OFF and ON states.

Power Handling: The Power Handling Capability of a switch is very important, it is the maximum RF input power that the switch can withstand without any permanent degradation in electrical performance.

The most common power parameter which is used to evaluate the power handling capability of a switch is P1dB (1 dB Compression Point), it is either the input power (IP1dB) or output power (OP1dB) at which the insertion loss increases by 1 dB from its small signal value.

The switch’s compression point is influenced by many factors, among which are: the wafer process used for the design, the design topology of the switch itself, the magnitude of the DC bias control voltage applied to the gates of the transistors, the frequency of the input signal, and the thermal properties of the package in which the die is packaged.

Linearity: The linearity of a switch is another very important parameter speciall when it is being used for wireless applications that require minimal signal degridation. The IP3 Level is the best indicator of linearity. You must make sure that they switch meets the system linearity and spurious emission requirements at the actual input signal levels that will be incident upon the switch in a system.

Insertion Loss: The insertion loss is one of the most important parameters of a RF switch. It is a measure of power loss in the signal. The insertion loss usually varies over the frequency band, so if you are using a switch with a wide bandwidth, look at the IL at the frequency of interest. Power loss can be calculated as follows:

Isolation: Isolation is the magnitude of a signal that gets coupled across an open circuit. So if you have an SPDT switch and the signal is passing through path one, there is a certain amount of the signal which will couple through to the other path. The ratio of the two signal levels is known as Isolation, the higher the isolation the better.

Switching Speed: Another key parameter is switching speed – It is the time needed to change the state of a switch port (arm) from “ON’ to “OFF” or from “OFF” to “ON”. The most common definition of switching time is the time measured from 50% of the input control voltage (TTL) to 90% of the final RF output power. This is usually in micro seconds or nano seconds for Solid State Switches.

Package: Solid State switches are usually available as a die, surface mount package or in a module with connectors. The package can be selected based on the application where you plan to use the switch.

These are the main parameters of RF Switches. everything RF has the largest searchable database of RF switch products - Click here to search for RF Switches across multiple manufacturers by specification.