RF & Microwave Electromechanical Switches

Updated: Jan 27

by Sencer OZKURT (Systems Engineer, BUPAT)


This article is also supported by RADIALL and BUPAT GLOBAL LLC


#radiall #rfmw #bupat #switches



An RF switch or microwave switch is a device used to direct RF or microwave signals. Switches are available in a various of configurations that specify the number of input and output signal paths. The signals are oriented in only one way among several possibilities.



Switching technologies are grouped under two main groups.


1- Dynamic

These switches are mostly larger because they incorporate mechanical contacts. They have moving parts under the shell. A dynamic switch can be Electromechanical (axial moving contact) or Mechanical (rotary moving contact). They rely on mechanical contacts as their switching mechanism.


2- Static

These switches have a lower profile than dynamic ones due to their planar assembly. It functions similarly to a dynamic switch except that it has no moving parts. A static switch can be Solid State (change of electronic state) or Ferrite (change of magnetic state).


Parameters of an Electromechanical RF Switch


To select an RF switch it is crucial to know the parameters.


Frequency: It can be between 100 MHz and 67 GHz according to the project to be used.


Configuration: It is the parameter that specifies the number of input and output paths that the switch will regulate.

SPDT Switch (Single Pole Double Throw):





A switch with one input port and two selectable output ports




SPDT Terminated Switch (Single Pole Double Throw terminated):



Same as SPDT, but the unused output port is automatically terminated by a 50 Ohm resistive load.



DP3T Switch (Double Pole Three Throw):



A switch with two input ports and three output ports. Each input (J2 - J4) can be switched between two adjacent outputs with one output being common to both inputs.



DPDT Switch (Double Pole Double Throw):


A four port switch with two independent paths that operate simultaneously in one of two selected positions. In a DPDT / Transfer switch, the two transmission paths are provided as shown left.


SPnT Switch (Single Pole n Throw) (n<13):

A switch with one input port and more than two output ports. The multiposition switch allows direct access to any individual output port by energizing the respective actuator. Radiall SPnT switches provide up to 12 Output ports.


SPnT Terminated Switch (Single Pole n Throw terminated) (n<13):


Same as SPnT, but each unused output port is automatically terminated in an internal 50 Ohm resistive load.


Insertion Loss: The signal loss between the input and the output of the switch after routing is called insertion loss. Losses greater than 1 or 2 dB will attenuate peak signal levels and increase rising and falling edge times. It is measured in decibels.


Isolation: The RF leakage from a connected path to any connector outside that path. Isolation is measured in decibels below the input power. Isolation becomes crucial at higher frequencies. High isolation reduces the influence of signals from other channels, sustains the integrity of the measured signal, and reduces system measurement uncertainties.


Power: The RF power rating is the capability of handling RF power (CW power) through closed contacts. The RF power should be removed during switching.


Peak Power Handling: The maximum peak power, when applied at room temperature under a pulse of one microsecond every millisecond, will not permanently change the specifications of the switch. Power applied over this limit will alter the RF performance of the switch.


Switching Time: The total amount of time between applying voltage to the actuator terminals and the completion of switching (including all contact bounce — if any). Total switching time consists of three parts, namely inductive delay in the actuator coil, transfer time of the RF contacts, and bounce time of the RF contacts.


Latching: A switch with an actuator that contains a mechanism, either mechanical or magnetic, that will maintain a chosen RF contact path (whether voltage is maintained or not) after switching is accomplished. A pulse length of a duration equal to the maximum switching time is enough to change the switch position.


Failsafe: A switch with an actuator that contains a return mechanism, either mechanical or magnetic, that provides RF connection to one selected position when no voltage is applied to the power terminals. This type of switch requires continuous voltage to maintain RF connection to any other position.


Repeatability: The maximun standard deviation in insertion loss specifications on each path over the life of the product.


Interface: The interface of an electronic circuit which enables driving either relays or switches by TTL logic signals. Products equipped with this option have a pin for the voltage of the actuator (12 V or 28 V) as well as a TTL driver pin shared per position. The polarity is not relevant to applications for switches with this option. The logic used is positive, therefore high level nominal +5 V (2.2 to 5.5 V) of TTL signal means logic “1” which enables the corresponding microwave way. Low level i.e logic contacts 0, voltage is 0-0.8 V. Selected position of switches with TTL driver are controlled by a TTL high level.


Failsafe: A switch with an actuator that contains a return mechanism, either mechanical or magnetic, that provides RF connection to one selected position when no voltage is applied to the power terminals. This type of switch requires continuous voltage to maintain RF connection to any other position.


Life: Number of toggles a product is able to carry out. Relays and switches of RADIALL ranges have a life cycle of 2 to 10 million cycles.


Now we can check the differences between dynamic and static switches with the support of RADIALL.


Frequency Range:


RF and microwave applications range in frequency from 100 MHz for semiconductor to 60 GHz for satellite communications. RADIALL Modular System for Electromechanical Switches (RAMSES) can be operational up to 67 GHz.


Switching Time

Switching time is defined as the time needed to change the state of a switch port (arm) from "ON' to "OFF" or from "OFF" to "ON". Solid-state switches are the fastest, while mechanical switches are the slowest. Radiall's electromechanical switches are faster than standart electromechanical switches (10 ms).


Insertion Loss (@ 18 GHz)

Electromechanical and Ferrite switches provide the lowest possible loss along the transmission path.


Isolation

As we mentioned before, isolation becomes more important at higher frequencies. Electromechanical and mechanical switches are better at this point.


To summarize the comparison of the two most commonly used switch types, Electromechanical and Solid-State,


Electromechanical Switches

Pros

Higher frequency and wider band (DC - 40 GHz) (DC-67 GHz for Radiall's RAMSES series)

Higher isolation

Higher Power

Lower insertion loss

Advanced actuation mode (Failsafe / Latching)

More resistant to ESD


Cons

Shorter life cycles (1 to 10 000 000)

Slower switching time

Heavier

Bigger

Sensitive to shock and vibrations


Solid-State Switches

Pros

Longer life cycles (up to 1 000 000 000)

Faster switching time

Smaller

Lighter

Resistant to shock and vibrations


Cons

No DC, narrow frequency band ( 100 KHz - 18 GHz / 18 - 40 GHz)

Higher insertion loss

Lower power handling

Lower isolation

Sensitive to RF power over-stress, temperature and ESD









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