Microwave switches

Revised March 23, 2008

Here we will discuss both solid-state and mechanical switches, including design considerations as well as laboratory switch operation.

Here is a clickable outline for our Microwaves101 web pages on microwave switches:

Switch considerations and terminology (below)

Solid-state switches (separate page)

Switch device models

Comparison of PIN diodes and FETs for switching

Two-terminals versus three terminals
Geometry lesson
Meandered gate switch FETs
Bandwidth considerations
Figure of merit
Resonant structures for overcoming FET off-capacitance
Lower frequency limit of PIN diode
Dissipation of switch elements

Power handling considerations for switches (separate page)

On-state
Off-state

Driver considerations (separate page)

PIN diode driver examples
FET driver examples
Positive voltage biasing for FET switches

Ferrite switches (separate page)

Mechanical switches (separate page)

Reed-type coax switches

Waveguide switches

MEMS switches (separate page)

The Riddle of MEMS

Switch design (separate page)

SPST switch (switch arm) design
Shunt versus series elements
SPDT switch and multi-throw switch design
Designing high-speed switches
Designing for high power handling

MMIC switch design example (new for March 2008)

RF switch considerations and terminology

There are many things you must consider when you design or buy a microwave switch. Let's try to list all of them all here, before we get into details below:

Reflective versus non-reflective (terminated) switches
A reflective switch does not provide a "good" fifty-ohm termination to the arm(s) that are switched off. A non-reflective switch provides terminations, and is a more complicated network.

Terminations: do the isolated ports of your switch need to present 50 ohms to your system? Can they handle the intended power?

Bandwidth: not just the upper frequency of a switch is limited by the technology you choose. PIN diode switches don't switch down to DC, but FETs do. Capacitive MEMS switches also have a high-pass response.

Insertion loss: will a dB or two of switch loss kill your system performance?

Isolation: how much RF signal can your switch leak before you are in trouble?

Power handling: is your switch going to be positioned on the output of a power amplifier?

Driver requirements: the driver circuits for PIN diode switches and FET switches are quite different, since the former requires a DC current while the latter requires a DC voltage, usually negative polarity.

Switching speed: how fast does your switch need to change state?

Expected lifetime: how many switching cycles do you expect your switch to handle? This is no concern for solid-state switches, but a big consideration for mechanical and MEMS switches.

Size: can you fit a connectorized switch, or do you need to look at a chip or surface mount IC switch?