Use RF Switches to Test Your Frequency-Dependent Circuits

RF switches are used primarily in automatic test equipment (ATE); though, RF Switches may be used in any device with operating frequencies in the UHF or higher radio bands that need to route signals among multiple paths. Specific applications are discussed in the paragraphs below. The ATE uses the RF switch, or switches, to control movement of signals from one circuit block to another. By choosing when to move a signal to another block, a design may be utilized or verified, one state at a time.

RF switches are used in varying configurations within ATE systems. They can be used alone to switch one signal between two outputs, used in multiple configurations for delivery to various ports, used to transfer multiple inputs to multiple outputs in parallel, and as bypass switches to move a signal around, rather than through, a component or circuit block.

Defining Characteristics of RF Switches

Use of RF switches to control the movement of signals within a design under test (DUT) is applicable from DC to microwave frequencies. Use across frequency bands allows application for many circuit validations. As circuit validation is the intent for circuit block operating frequencies, the RF switches are designed specifically for use with the circuit’s operational characteristics. Some of the more critical parameters of RF and microwave circuits are:

Frequency range: The frequency of the circuit and/or device under test. Insertion loss: Power loss of a signal through the circuit block from input to output. VSWR. Voltage standing wave ratio is an indicator of reflected waves within the transmission line. Return loss: Ratio of the wave reflected backward and forward, aka VSWR, measured in dB. Repeatability: Indicates low insertion loss each time the switch activates and is an indicator of stability and accuracy of the ATE over time. Isolation: The ratio of the power level when the switch’s path is “off” to the power level when the switch is “on.” Switching Speed: The time it takes to change state, in terms of either rise/fall time or on/off time. Rise/fall does not include switch driver delay time whereas on/off time does. Settling Time: The time to settle, as close to 100% from switching. Gallium Arsenide (GaAs) FET switches usually call this out on their datasheets as there is a lag due to the material. Power Handling: The power rating of the RF switch. Termination: Specified for use in matching impedance for low losses. Video Leakage: Spurious signals present at the RF ports when the switch is activated without an RF signal present.

Types of RF Switches for Consideration

To use RF switches in ATEs, you’ll need to determine the frequency of the circuits you’ll be passing with the switches you choose. Below, we discuss three possible selections for use in unique bands. The first is able to operate in the microwave band, the second in applications from DC into the UHF band at 1 GHz, and the third for use in both the VHF and UHF bands from 30 MHz to 3 GHz.

Skyworks AS193-73LF

This part is great for use in the L-band (1 to 2 GHz) microwave frequency range. The L-band is frequently used for military telemetry, GPS, mobile phones (GSM), and amateur radio. Some acronyms unique to this category of RF switch are:

• GSP: Global Satellite Positioning
• GSM: Global System for Mobile Communication
• PHEMT: p-High Electron Mobility Transistors
• T/R: Transmit-Receive
• UMTS: Universal Mobile Telecommunications Service

It employs a high-electron-mobility transistor (HEMT). The switch is made from GaAs and is frequently used in microwave applications for its ability to operate reliably at high frequencies where low voltage is required.

If you are designing satellite receivers or low power amplifiers, take a look at the RF switch offered by Skyworks, the AS193-73LF:

The AS193-73 is a PHEMT GaAs FET IC high-linearity SPDT switch in a SOT-6 plastic package. This switch has been designed for use where extremely high linearity, low control voltage, high isolation, low insertion loss and ultra-miniature package size are required. It can be controlled with positive, negative or a combination of both voltages. Some standard implementations include antenna changeover, T/R and diversity switching over 3 W. The AS193-73 switch can be used in many analog and digital wireless communication systems including cellular, GSM and UMTS applications.

Pinout of Skyworks AS193-73LF SPDT RF switch

Philips / NXP SA630D-01

Some RF switches are able to handle a large span of frequencies, and this part is able to operate from DC to 1 GHz. In addition, using CMOS/TTL technology allows integration with both CMOS and with TTL logic enabling a test of new to old designs. Some terms unique to this category of RF switch are:

• BiCMOS: Semiconductor material that integrates bipolar and CMOS transistors together to take advantage of the
• High input impedance of the CMOS, and
• Low output impedance, together with high gain, of the bipolar
• CMOS: Complementary Metal Oxide Semiconductor
• FSK: Frequency-Shift Key
• TTL: Transistor-Transistor Logic

It is suitable for a variety of applications but some good uses within your ATE would be as a digital transceiver front-end switch, an antenna switch, a filter selection switch, a video switch, or an FSK transmitter. Take a look at the Single-Pole Double-Throw (SPDT) switch offered by NXP Semiconductors, the SA630:

*The SA630 is a wideband RF switch fabricated in BiCMOS technology and incorporating on-chip CMOS/TTL compatible drivers. Its primary function is to switch signals in the frequency range DC - 1GHz from one 50Ω channel to another. The switch is activated by a CMOS/TTL compatible signal applied to the enable channel 1 pin (ENCH1).

The extremely low current consumption makes the SA630 ideal for portable applications. The excellent isolation and low loss make this a suitable replacement for PIN diodes.

The SA630 is available in an 8-pin dual in-line plastic package and an 8-pin SO (surface mounted miniature) package.*

An evaluation board with specific layout instructions is available to research your application. The below diagram shows the evaluation board schematic, along with recommended layout specifications for maximum performance.

SA630 evaluation board schematic and optimum PCB layout

Infineon BGS12AL74E6327TXTSA1

This RF switch is a general purpose switch compatible with circuits operating in the VHF and UHF radio frequency bands. This makes it useful in a variety of applications including FM, television broadcasts, line-of-sight ground-to-aircraft and aircraft-to-aircraft communications, land mobile and maritime mobile communications, amateur radio, and weather radio.

If your design is testing circuits operating in either the VHF or UHF band, take a look at the RF switch offered by Infineon, the BGS12AL7-4:

*The BGS12AL7-4 General Purpose RF MOS switch is designed to cover a broad range of applications from 30 MHz to 3 GHz. The symmetric design of its single pole double throw configuration, as shown below, offers high design flexibility. This single supply chip integrates on-chip CMOS logic driven by a simple, single-pin CMOS or TTL compatible control input signal. The 0.1 dB compression point exceeds the switch’s maximum input power level of 21 dBm, resulting in linear performance at all signal levels. The RF switch has a very low insertion loss of 0.4 dB in the 1 GHz and 0.5 dB in the 2 GHz range.

Unlike GaAs technology, external DC blocking capacitors at the RF ports are only required if DC voltage is applied externally.

The BGS12AL7-4 RF switch is manufactured in Infineon’s patented MOS technology, offering the performance of GaAs with the economy and integration of conventional CMOS including the inherent higher ESD robustness.*

BGS12AL7-4 Functional Diagram

RF switches control movement of frequency signals from one circuit block to another, and the switches are specified for operation within varying bands of the spectrum. Select the switch that complements the frequencies you are testing within the circuits of your device. Unique materials are used in the transistors that are used to switch, each with semiconductor characteristics suitable for your application.

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