Microwave Switches

Here we will discuss both solid-state and mechanical switches, including design considerations as well as laboratory switch operation. If you are looking for RF switches, click here.

New for October 2013: it is not often a new switch technology comes around, but chalcogenide switches are an exciting new development. See below.

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

Faking 3-port switch data from two two-port measurements (new for August 2016)

Microwaves101 switch FET model (new for February 2016)

Adding temperature coefficients to measured switch S-parameters (new for December 2015)

Chalcogenide switches

Solid-state switches

MMIC versus hybrid
Switch device models
Comparison of PIN diodes and FETs for switching
Two-terminals versus three terminals
Switching figure of merit (FOM)
DC dissipation of switch elements
Switching speed
Geometry lesson

Switch FETs

Meandered gate switch FETs

Resonating off-capacitance in switch FETs

Power handling considerations for switches


Driver considerations

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

Ferrite switches

Mechanical switches

Reed-type coax switches
Waveguide switches

MEMS switches

The Riddle of MEMS

Switch design

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

Important consideration for designing with shunt switch elements


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?

Chalcogenide switches

At CSICS 2013 the following paper was presented:

"A 7.3 GHz cutoff Frequency, In-line, Chalcogenide Phase-Change RF switch using an Independent Resistive Heater for Activation", by Nabil El-Hinnawy et al, Northrop Grumman Electronic Systems.

Chalcogenide switches use phase-change technology which was originally developed for non-volatile memory for computers. The chalcogenide material has two states: in amorphous state, it has very high resistivity, in conducting state it acts like a metal. The two states are activated thermally. If the heat pulse is quick (amorphizing heat profile), the high loss state is provided, if it is longer and lower temperature (crystallizing heat profile), the metallic state occurs. The heat pulse for either state can be shorter than one nanosecond, making chalcogenide switches faster than MEMS switches. Chalcogenide switches are not technically "solid-state" but have no moving parts, which are the cause of the MEMS Tree of Woe.

The NG switch was partially funded by the Darpa RF-FPGA program. It is amazing how far the technology has come in just one year, and we expect it will continue to improve and set some new benchmarks for low-loss and high frequency response. If you can't think of a dozen ways to use this new technology, you aren't trying very hard. How about a new stab at passive ESAs? Tunable filters? Self-healing SSPAs? Let's move out on this!

The Unknown Editor tweeted that this was the best conference paper, even though he was co-author on a paper in the same session...

More to come.... just remember where you heard about this first!




Author : Unknown Editor