RF and microwave switches are used to route signals through transmission paths with a high degree of efficiency. They are used extensively in microwave test systems for signal routing between instruments and the devices being tested.
In this quick guide, we’ll teach you the basics of RF switches, including:
- The electrical parameters of RF and microwave switches
- Types of RF switches
- EM and SS RF coaxial switch details
Electrical parameters of RF and microwave switches
While there are several electrical parameters associated with the performance of RF and microwave switches, there are only four considered fundamental to the design because of their strong interdependence:
- Isolation is a measure of how effectively a switch is turned off – the attenuation between the input and output ports of the circuit.
- Insertion loss, also known as transmission loss, is the total power lost through the switch when it is “on”. This is the most critical parameter because it may add to the system’s noise figure.
- Switching time is the period a switch needs to change from “on” to “off” and “off” to “on”. This time can range from several microseconds in high-power switches to a few nanoseconds in low-power, high-speed devices.
- Power handling is the maximum RF input power RF switches can withstand without any permanent degradation in electrical performance.
Types of RF switches
RF switches can be divided into two primary groups:
- Electromechanical (EM) relay switches: EM switches are usually larger since they incorporate a series of coils and mechanical contacts. They generally have a low insertion loss (< 0.1 dB), high isolation (>85 dB, and can switch signals in milliseconds. The benefit of such RF switches is that they can operate down to dc and up into millimeter frequencies (50+ GHz) and are not at risk of electrostatic discharge. They can handle high power levels up to several thousand watts of peak power and have no video leakage.
- Solid-state (SS) switches: SS switches have lower package profiles and are usually smaller than EM switches because the circuit assembly is planar and contains no bulky components. Their insertion loss is not as good as EM switches, and they are of limited use at low frequencies with operation only down to the kilohertz range. They are also more sensitive to electrostatic discharge. However, they are more reliable, last longer, and switch faster than the EM version. As such, they are preferred in applications where fast switching speed and high reliability are crucial.
EM and SS RF coaxial switches details
RF EM and SS switches come in a wide variety of package sizes and connector configurations. Most RF coaxial switch designs use SMA connectors for operations up to 26 GHz; 40 GHz typically use 2.92 mm or K connectors; 50 GHz use 2.4 mm connectors; 65 GHz employ 1.85 mm connectors.
RF switches with waveguide ports are often used for high-power communication signals for microwave and millimeter-wave frequency bands as they give the lowest possible insertion loss. RF coaxial switch designs that handle more power might use larger N-Type or TNC connectors.