Usually the RF system has many input and output ports, microwave switches can be multi-input multi-output signal to switch, although a number of parameters and RF and microwave switch performance, but the following four because of their strong correlation between each other But is considered a critical parameter: isolation, insertion loss, switching time, power handling capability.
The degree of isolation between the circuit input and the output is the measure of the effectiveness of the cut-off.
The insertion loss (also called transmission loss) is the total power dissipation when the switch is in the on state. Since the insertion loss can directly lead to an increase in the system noise figure,RF Switch the insertion loss is the most critical parameter for the designer.
The switching time is the time required for the switch to transition from the "on" state to the "off" state and from the "off" state to the "on" state. The most common definition of switching time is the time required for the input control voltage to reach 50% of its final RF output power to 90%. In addition, the power handling capability is defined as the maximum RF input power that the switch can withstand without any permanent electrical performance degradation.
RF and microwave switches can be divided into electromechanical relay switches and solid state switches two categories. These switches can be designed for a variety of different configurations - from single-pole single-throw to single-pole,RF Switch six-throw, or more-throw configurations that can convert a single input into 16 different output states. The switch is a double pole and double throw configuration switch. This type of switch has four ports and two possible switch states, which allow the load to switch between two sources.
As the solid-state RF switch circuit is relatively flat and does not contain large components, so the package thickness is small and the physical size is usually smaller than the electromechanical switch. Solid-state RF switches use high-speed silicon PIN diodes or field-effect transistors (FETs), or integrated silicon or FET monolithic microwave integrated circuits. These switching elements are integrated with other chip components such as capacitors, inductors and resistors on the same circuit board.
Switch products that use PIN diode circuits have higher power handling capabilities, while FET-type switching products typically have faster switching speeds. Of course, because the solid state switch does not contain moving parts, so its life is infinite. In addition, the isolation of the solid state switch is high (60 ~> 80dB), the switching speed is very fast (<< 100 nanoseconds), the circuit is better impact resistance / vibration.
Other noteworthy performance of solid-state RF switches includes their insertion losses. Solid-state RF switches are inferior to electromechanical switches in terms of insertion loss. In addition, solid-state RF switches have limitations in low-frequency applications. This is because its operating frequency limit can only be to kilohertz, rather than dc. This limitation arises from the carrier lifetime properties inherent in the semiconductor diodes it uses.
The designer should be aware of other features of the various switch products described above, such as a 50 ohm resistive load. In the switching circuit, any idle open transmission lines are likely to resonate at frequencies within the microwave range. This resonance can reflect the energy back to the working state of the RF source, and thus cause damage. For systems operating at 26 GHz or higher, the damage will be more severe due to a significant reduction in isolation. Therefore, most transmission lines are designed with 50 ohm impedance, making the RF switch in the built-in 50 ohm resistive load, the reflected energy is minimal.
Electromechanical RF switch is divided into two types of termination and non-termination. In a terminating switch, when all channels are terminated with 50 ohm load, the selected channel is turned off to turn off or isolate all current. Thus, the energy of the incident signal will be absorbed by the terminating resistor without reflecting back to the RF source. Non-terminating switches do not have a 50 ohm load, so the impedance matching of the energy reflection must be achieved by the rest of the system. The advantage of the non-terminating switch is that its insertion loss is small.