In the transmission path, RF Switches can efficiently convey paths.  Four basic electrical parameters can be used to describe the function of this sort of RF  switch. Despite the fact that numerous parameters affect the performance of RF switches,  the following four are crucial due to their significant correlation: 

1. Isolation Isolation is an index that measures the effectiveness of the RF switch cut-off by  attenuating the signal between the input and output of the circuit. 

2. Insertion loss When the RF switch is turned ON, insertion loss (also known as transmission loss)  is the total power loss. Because insertion loss can immediately contribute in system noise  figure, it is the most important metric for the RF designers. 

3. Switching time The time it takes for the RF switch to convert from ON state to the OFF state, and  vice versa is referred to as switching time. For high-power switching, this time can be  measured in microseconds, and for low-power high-speed switching, it can be measured  in nanoseconds. The most typical definition of switching time is the amount of time it takes  for the input control voltage to go from 50% to 90% of its ultimate power. 

4. Power handling capacity The power handling capability of a switch is defined as the maximum RF input  power that the switch can withstand without deteriorating its electrical performance  permanently. 

Electromechanical RF switches 

First RF Switches that were used in wireless applications were mechanical  switches (keys, aerial switches, and electro-mechanical relays).  

Those mechanical or electro-mechanical switches generally switch DC and low  frequencies, and relative high voltage and currents.  

They require having good electrical contacts and to use high isolation materials. 

         • There are two types of electromechanical RF switches: terminated and nonterminated.• When all ports of an RF switch are terminated with a 50 ohms load, the selected  port is closed, cutting off or isolating all currents. The incident signal energy will be  absorbed by the termination resistor and will not be reflected back, to the RF source  in this way. 

         • In a non-terminated RF switch, the system must accomplish external impedance  matching to reduce energy reflection. The non-terminated RF switch has the  advantage of having a low insertion loss. 

         • Electro-mechanical (EM) RF switches provides: 

                       - low insertion loss (<0.1dB) 

                       - high isolation (>100dB) 

                       - high power handling 

                       - no video leakage 

                       - very high ESD immunity 

                       - their frequency range starts from DC 

Electro-mechanical RF switches have lower operating lifetime than Solid-State switches.  

         • The operating life of an electro-mechanical switch can be defined as the number of  cycles the switch will complete while meeting all the RF and repeatability  specifications.  

         • The operating life refers to the electrical life and RF properties of the switch, and not  to the mechanical life (which is much longer than the electrical life). There are some high-quality coaxial relays that use electro-mechanical switches referred  as “frictionless switching” (since there is no friction produced between the jumper contact  and center conductor), and this configuration produces switches that can mechanically  actuate for tens of millions of cycles. 

The drawback is, they might not fail mechanical, but their insertion loss gets higher due to  increasing in time the resistance of the contacts. 

Solid-State RF Switches 

Solid-State RF switches have smaller package thickness and physical dimension  than Electromechanical RF switches since their circuit assembly is relatively flat and does  not contain big components.  

         • High speed silicon PIN diodes or Field-Effect Transistors (FET), or integrated silicon  or FET monolithic microwave integrate circuits, are switching elements used in  solid-state RF switches.  Other chip components like capacitors, inductors, and resistors are independently  integrated on the same circuit board as these switching parts. 

         • PIN diode-based switches have higher power handling capabilities, but FET-based  switch devices have faster switching speeds. 

         • Solid-state switches, have an infinite service life since they do not contain moving  parts. The solid-state RF switch has high isolation (60>80dB), a quick switching  speed (100 nanoseconds), and is stress and vibration resistant. 

         • In terms of insertion loss, solid-state RF switches are inferior of electromechanical  switches. 

         • Solid-state RF switches have limits in low-frequency applications, the lowest limit of  its operating frequency is merely kilohertz, not DC, due to the semiconductor diode's  inherent carrier lifetime characteristics.  

         • Solid-state RF switches are also more susceptible to electrostatic discharge, and  their power handling capabilities are influenced by the connector type, operating  frequency, and ambient temperature. Certain PIN diode RF switch topologies can  withstand peak power of several kilowatts, but this comes to the cost of slower  switching rates. 

         • There are two types of solid-state RF switches: absorptive and reflecting.  

         • In order to obtain a lower voltage standing wave ratio (VSWR) in both ON and OFF  states, the absorption switch has a 50-ohm terminal matching resistance at each  output port, which can absorb the incident signal energy, but the port that is not  linked to the terminating matching resistor will reflect it. Absorptive switches are  ideal for applications where the RF source’s echo reflection must be minimized. 

         • The reflecting RF switch does not have a terminal resistor to reduce the insertion  loss of the open port, and is ideal for applications where reflections does not affect  the system performance. 

         • Emerging micro-electromechanical systems (MEMS) switch technology attempts  to deliver the advantages of traditional electro-mechanical switches, but in a small  form factor.  MEMS switches employ micro-miniaturized mechanical contacts controlled by  electrostatic forces to make RF connections. 

Types and Architectures of Solid-State RF Switches 

There are few main types of Solid-State RF switches: 

         • High-Speed Silicon diodes RF switches 

         • PIN diodes RF switches 

         • Field Effect Transistors (FET) RF switches 

         • Hybrid (FET and PIN diode) RF switches 

Two basic switch architectures that describe the behavior of the unused switch port are  classified as Absorptive or Reflective. 

         • Absorptive switches present a termination (most commonly 50 Ω) to the unselected  arm typically at the expense of increased insertion loss.  Absorptive switch will have a good VSWR on each port regardless the switch mode. 

         • Reflective switches leave the unused port un-terminated.  In a reflective switch, the impedance of the port that is OFF will not be 50 Ω and will  have a very high VSWR. Reflective switches can be further categorized as: either reflective-open or  reflective-short.  

              - Reflective-open architectures do not have a shunt path to ground in the OFF  state; as a result, the loading on the unused port will be minimized.  For example, LNA bypass switches are reflective-open in order not to disturb the  LNA’s functionality when the switch is in the OFF state. 

              - Reflective-short architectures use a shunt path to ground.  This low impedance renders attached circuitry effectively useless. 

         • The rule is to use an absorptive switch when you need a good VSWR looking into  the port that is not switched to the common port, and to use a reflective switch when  high OFF port VSWR does not matter, and when the switch has some other desired  performance feature. 

         • In most cases, an absorptive switch can be used instead of a reflective, but not vice-versa.