How to Select a RF Switch

Consider the following parameters to select the most suitable switch for a specific application.

1. RF Circuit – The three most common circuit configurations are (1) single-pole-double-throw (SPDT), (2) Transfer (DPDT), and (3) multiposition.

Select the simplest circuit to meet system requirements.

2. Actuator – The Pulse Latching Actuator is recommended. Actuation requires a pulse length equal to or greater than the switching time. This type of switch requires current only during switching and for a very short period of time. A pulse width of 30 milleseconds minimum will more than satisfy current requirements. There is also no need to supply DC power to hold the selected position. However, pulse latching can be operated with continuous DC power supplied to the unit with no degradation or decrease in life expectancy.

A latching switch with internally cut off current circuitry can be obtained from CEI in some models if your system is not equipped with pulse command circuitry. This option will disconnect the drive circuit so that DC current will not be consumed after switching has been accomplished.

The Fail-Safe Actuator is used where system requirements dictate the switch return to the normally closed or fail-safe position when actuator power is removed. Most Charter Engineering fail-safe switches utlize magnetics rather than springs for fail-safe holding power.

The Mometary or normally open Actuator is used where system requirements require all output ports of the switch to be disconnected from the input port until a voltage is applied to a selected position.

4. Actuator Voltage – All standard Charter Engineering switches offer 28.0 Vdc. Other voltages can be supplied depending on the switch model. The following list consists of available voltage options for most CEI switches:

Please contact CEI for available voltage options with Type SC connectors.

5. Polarity – Each CEI switch offers the option of either a positive (+) or negative (-) common. The exception is the SPDT Failsafe series which does not require a common.

6. Power Interface – All standard Charter Engineering switches are provided with solder terminals. The MS connector (MIL-STANDARD) and D’SUB Miniature Series can be supplied instead of terminals on most switches.

Many catalog pictures feature D’SUB connectors and MS Connectors which are readily available alternatives to solder terminals.

Charter Engineering can also supply other sources of power interface.

3. Frequency – Specify the actual frequency to be used as minimum bandwidth. This often results in reduced testing time and associated costs.

Generally, RF Switches are capable of higher-frequencies and greater bandwidths than necessary for the intended usage.

The following table correlates frequency and connector type:

RF Connectors – Type SMA is the recommended connector expect for high power requirements. The normal frequency limit is 26.5 GHz; however, Charter Engineering can supply switches which  function up to 40.0 GHz.

The Type N connector is recommended
for optimum RF performance and operation at
power levels greater than the SMA power rating.
See CW Power Capability vs. Frequency Chart.

The type TNC connector is recommended where  power levels and frequency are high and the  package is small.

The Type SC connector is recommended for high power applications in which large diameter cables  are used for extremely low loss. SC is normally  used up to 1000 MHz, but Charter Engineering has modified the design to operate up to 6.5 GHz and in special cases up to 10.0 GHz.

Type BNC connectors can also be supplied upon request.

7. Options –
Cut off Power Circuit – This applies to latching switches only. A switch that has the ability to disconnect the actuator drive circuit so that D.C. current will not be consumed after switching has been accomplished.

TTL Logic – Transistor-transistor-logic driver circuitry enables the status of the switch to be controlled by the level of the TTL Logic input. Users can apply the power voltage across a pair of designated power terminals and then control switch operation with a 5 volt control circuitry.

CEI’S TTL Logic has a remarkable circuitry which will enable your system to run cooler and will save a substantial power supply to the switches.

8. Intermodulation – Passive Intermodulation (PIM) is a form of signal distortion that occurs whenever signals at two or more frequencies conduct simultaneously in a passive device (electromechanical switch) which contains some non-linear response. This interference phenomenon is attributable to many sources such as dissimilar metals, dirty interconnects, or other anodic effects. PIM is of particular concern whenever:

Two or more transmitter channels share a common antenna
Transmitter signal levels are high
Receiver sensitivity is high
Transmitters and receivers share a common antenna
Two or more base station tranceivers are at a single site

CEI recognizes the PIM problem and offers specially designed electromechanical switches to meet low intermodulation requirements.

PIM measurements will vary depending upon carrier frequencies, power level, and PIM frequency.

For further details, please contact CEI.