RF Switch Selection Guide | RF Switch specifications
This page covers RF switch selection guide and technical specifications. It mentions common rf switch specifications one need to consider while selecting or buying RF switch for their need.
The RF Switches are broadly divided into two types electro-mechanical and solid state. Solid state types are further divided into silicon diode based, PIN diode based, FET based and Hybrid (combination of PIN diode and FET based). This is depicted in figure-1.
Electro-mechanical RF switch relies on mechanical contacts for their operation. They are mainly used for switching DC and low frequency signals. They need good quality electrical contacts and materials with high isolation.
Solid state RF switches rely on solid state devices for their operation viz. PIN diode, GaAs FET, high speed silicon diode or hybrid etc. They are used for switching high frequency signals.
There are two basic switch architectures viz. absorptive and reflective. Refer Absorptive vs Reflective RF Switch>> for comparison between these two architecture types.
RF Switch selection guide
Following guidelines should be followed while selecting RF Switch as per your need.
➤Based on performance requirements as per specifications, choose switch type either electro-mechanical or solid state(PIN diode/FET/Hybrid). Refer table-1 above for detailed comparison of various switch parameters.
➤Determine switch architecture either absorptive or reflective.
➤Determine topology of the switch to be used either multiplexer and relay. Multiplexer will have more than one inputs and single output for example 8x1 MUX will have eight inputs and one single output. The relays can be SPST (Single Pole Single Throw), SPDT (Single Pole Double Throw), SP3T etc. Select based on input and output connectivity requirements. The relays (e.g. SPDT) are scaled down versions of the multiplexer.
➤The important other considerations are frequency of operation, Switching time and settling time of switch, isolation, insertion loss, return loss, handling power, life etc. Let us understand RF switch specifications in table-2 below.
RF Switch Specifications
|RF Switch Specifications||Description|
|Characteristic Impedance||It determines how signals are propagated (i.e. transmitted or reflected) as it travels through the transmission line. It can be either 50 Ohm or 75 Ohm. It is determined based on physical structure of line.|
|Bandwidth or frequency range||The range of frequencies over which RF switch can be used for particular application with acceptable specifications (e.g. loss, speed, life etc.). Bandwidth is the difference between maximum and minimum frequencies supported by RF switch.|
|Topology||This is one of the important parameters in RF switch selection. There are two main topologies viz. Multiplexer (such as 4x1, 8x1 etc.) and SPDT Relay.|
|Insertion Loss||It is measure of power loss or signal attenuation between input and output ports of RF switch. Less is better for the RF switch. Check frequency versus insertion loss graph in the datasheet of the RF switch.|
|Isolation||High isolation is good as it prevents undesired signals to leak in the path of wanted or desired signals. High isolation means higher attenuation in that path.|
|Return Loss or VSWR||These parameters (RL and VSWR) are measure of
reflection at the ports of the RF switch.
RL is ratio of reflected wave to transmitted wave.
VSWR is calculated based on (1+RL)/(1-RL) equation.
RL vs VSWR>>.
|Switching speed and settling time||The speed with which RF switch operates from OFF to ON state and viceversa. It is also the speed with which RF switch operates from one output port to the other output port. GaAs FET based RF switches are fastest with less than 1ns of switching time.|
|Rise time||If it is specified by the manufacturer than it should be less than rise time of highest harmonic (fifth) which needs to be routed through the RF switch with minimum distortion.|
|Operating life or MTBF|| It is measure of minimum number of cycles
before RF switch starts to fail.
Refer MTBF vs MTTF vs MTTR>>
|Repeatability||It assures accurate RF switch test results. It is measure of insertion loss or phase for RF switch matrix from cycle to cycle over time period.|
|Power handling||This is power of the RF signal which need to be switched using RF switch without any damage to the physical and electrical properties of the switch.|
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