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How to Quickly and Safely Switch an Antenna or Transducer Between Transmit and Receive Modes

By Art Pini

Contributed By Digi-Key's North American Editors

For many reasons, designers of echo ranging devices—such as radar, sonar, nuclear magnetic resonance (NMR), or ultrasonic ranging—as well as cell phone and satellite communications infrastructure are often in a situation where they must connect a common antenna or transducer to both a high-power transmitter and a sensitive receiver. This requires a method for switching the antenna or transducer between the two devices while at the same time providing adequate attenuation between them to prevent damage to the high-sensitivity receiver components by the high-power transmitter. Additionally, the shared antenna or transducer has to be switched in rapidly after a transmission in order to allow the receiver time to acquire and measure the received RF or ultrasound echo.

To accomplish this, designers can turn to transmit/receive (T/R) switches, also referred to as duplexers. These are designed to handle the task of rapidly switching an antenna or transducer between a transmitter and a receiver, while providing the required isolation between the T/R paths. T/R switches also handle the transmitted power while offering low insertion loss to prevent attenuation of the transmitted signal, and maintain a fixed characteristic impedance to prevent signal reflection and loss. However, to use them effectively, designers must first understand their operation and key characteristics.

There are several technologies available for implementing T/R switches. This article looks at two main types—RF circulators and PIN diode switches—as well as a type used for voltage sensitive applications.

Each technology is matched to specific applications with example devices from Skyworks Solutions Inc. and Microchip Technology.

What does a transmit and receive switch do?

The basic T/R switch connects a common antenna (in RF applications) or transducer (in ultrasonic applications) between a transmitter and receiver (Figure 1).

Diagram of basic T/R single pole, double throw switchFigure 1: A basic T/R switch is a single pole, double throw switch that connects a common antenna or transducer to either a transmitter or receiver. (Image source: Digi-Key Electronics)

The switch is generally a simple single pole, double throw (SPDT) configuration for a single transmitter and receiver. Multi-transmitter/receiver topologies add additional poles to the switch configuration. In the basic configuration there are four key design goal requirements:

  1. First, the power rating of the switch must be sufficient to handle the transmitter output without damage to the switch.
  2. Second, the loss between the transmitter and antenna must be as low as possible.
  3. The third requirement is that when the switch is not connected to the receiver that there be sufficient isolation between the receiver input and the transmitter output to prevent damage to the high sensitivity receiver.
  4. Finally, the switching speed of the T/R switch must be fast enough to match the requirements of the application.

Circulator T/R switches

An RF or microwave circulator is a three-port device used to control the direction of signal flow in RF applications (Figure 2).

Diagram of clockwise (left) and counterclockwise (right) circulatorsFigure 2: The schematic symbols show a clockwise (left) and counterclockwise (right) version of a circulator. There is no significant flow in the reverse direction of each version—a characteristic that makes them ideal as T/R switches. (Image source: Digi-Key Electronics)

In the clockwise version of a circulator shown in Figure 1, a signal input at Port 1 propagates to Port 3; signals from Port 3 propagate to Port 2; and a signal from Port 2 is transmitted to Port 1. Circulators are non-reciprocal devices, meaning there is no significant flow in the reverse direction. For instance, in the example shown, there is little or no signal flow from Port 3 back to Port 1; from Port 2 back to Port 3; or Port 1 back to Port 2. It is this directional property that makes circulators ideal for use as T/R switches (duplexers). In a similar fashion, the counterclockwise circulator version directs signals from Port 1 to Port 2, Port 2 to Port 3, and Port 3 to Port 1. In either case there is very little signal transmission in the reverse direction.

Circulators are passive devices based on ferromagnetic effects, so they are, in part, composed of magnetized ferrite materials. The three-port "Y-junction" circulator is based on cancellation of waves disseminated over two different paths near a magnetized ferrite material (Figure 3).

Diagram of clockwise (left) and counterclockwise (right) circulatorsFigure 3: The physical structure of a Y-junction circulator includes a symmetrical strip line junction of the three ports, ferrite disk, and a magnetic field (HCIR), usually supplied by fixed permanent magnets. (Image source: Skyworks Solutions)

The three-port, Y-junction version of a RF circulator consists of two ferrite disks, one located on each side of a strip line three-port junction. The circulator action is obtained by magnetically biasing the ferrite element in the axial direction with an internal static magnetic field of proper magnitude, shown as “HCIR” in Figure 3. The circulator can operate in two transverse magnetic modes of opposite polarization. Under the circulation condition shown in Figure 3, at a specific applied field these TM modes create a null at Port 3, which is then isolated, and power is transferred from Port 1 to Port 2. The power entering at Port 2 appears at Port 3, and so on, creating the circulator action. In this case the action is counterclockwise. The direction of circulation can be reversed by reversing the polarity and adjusting the strength of the static magnetic field.

The advantage of using a circulator in T/R applications is that no switching is involved; both the transmitter and the receiver are always connected, and isolation is the result of signal phase cancellation.

When implementing a T/R design using a circulator, the transmitter output is applied to Port 1. The antenna is connected to Port 3, and the receiver is attached to Port 2 (Figure 4).

Diagram of clockwise circulator as a T/R switchFigure 4: When connecting a clockwise circulator as a T/R switch, the transmitter output is applied to Port 1, the antenna is connected to Port 3, and the receiver is attached to Port 2. (Image source: Digi-Key Electronics)

An example of a commercial circulator that will fulfill the needs of a T/R switch is the Skyworks Solutions model SKYFR-000736. This 50 ohm (Ω) Y-junction circulator can handle T/R switching operations over the 791 to 821 megahertz (MHz) frequency range. Intended for wireless infrastructure applications and able to handle up to 200 watts (W), the device has a notably low insertion loss of 0.3 decibels (dB) between the transmitter and antenna and a minimum isolation of 22 dB. The SKYFR-000736 circulator is a relatively small, surface mount device measuring 28 millimeters (mm) in diameter with a height of 10 mm. Being a passive device, it does not require any power.

PIN diode switches

PIN diodes are used as switches or attenuators at RF and microwave frequencies. They are formed by sandwiching a high resistivity intrinsic semiconductor layer between the P-type and N-type layers of a conventional diode. As a result, the nomenclature “PIN” reflects the structure of the diode (Figure 5).

Diagram of PIN diode comprises a layer of intrinsic semiconductor materialFigure 5: A PIN diode comprises a layer of intrinsic semiconductor material that is placed between the P and N material of the anode and cathode electrodes, respectively. (Image source: Digi-Key Electronics)

There is no charge stored in the intrinsic layer of the unbiased or reversed biased PIN diode. This represents the “off” condition of switching applications. The insertion of the intrinsic layer increases the effective width of the diode’s depletion layer, resulting in very low capacitance and higher breakdown voltages, both of which are very good features in an RF switch.

The forward biased condition results in holes and electrons being injected into the intrinsic layer. These carriers take some time to recombine with each other. This time is referred to as the carrier lifetime, t. There is an averaged stored charge, which lowers the effective resistance of the intrinsic layer to a minimum resistance, RS. This is the “on” condition in a switching application.

A PIN based T/R switch

The circulator-based T/R switch is a narrowband switch having a restricted frequency range. PIN-based T/R switches can be implemented with quarter wave transmission lines, also resulting in a limited frequency range. An advantage of PIN-based T/R switches is that their designs can be broadband—i.e., using no frequency-sensitive elements. This article will focus on the broadband implementation.

The basic T/R switch is a SPDT configuration and will require a minimum of two PIN diodes for implementation. The switch topology can utilize the diodes in parallel with the transmitter and receiver in a shunt diode connection, or in series with the transmitter and receiver, as well as a combination of both approaches (Figure 6).

Diagram of three T/R switch topologies using PIN diodes in series, shunt, or series-shunt configurationsFigure 6: Shown are three T/R switch topologies using PIN diodes in series (a), shunt (b), or series-shunt configurations (c). (Image source: Skyworks Solutions)

The series diode configuration (a) places the PIN diodes in series between the RF common (antenna) and the transmitter and receiver. The insertion loss between the transmitter and the antenna depends on the series resistance of the forward biased diode. Isolation between the transmitter and receiver is dependent on the residual capacitance of the reverse biased diode.

The shunt arrangement (b) has the diodes in parallel with the transmitter and receiver connections. Isolation is dependent on the resistance of the forward biased diode, while the insertion loss is dependent on the capacitance of the reverse biased diode.

Isolation can be increased by using both series and shunt connected diodes (c). This configuration is the most commonly used. Isolation is governed by the capacitance of the reverse biased series diode and the resistance of the forward biased shunt diode. In addition to greater isolation, it is inherently more protective of the receiver in having two protective diodes. The insertion loss on the transmitter side is a function of the resistance of the forward biased series diode and the capacitance of the reverse biased shunt diode.

A high power version of the high isolation switch could utilize Skyworks Solutions’ SMP1302-085LF as the low capacitance PIN diode and the SMP1352-079LF as the low resistance PIN diode. Both diodes are rated with breakdown voltages of 200 volts. The SMP1302-085LF has a rated power dissipation of 3 W enabling it to handle up to 50 W continuous wave (CW) as the series element in the T/R switch. Its reverse biased capacitance is only 0.3 picofarads (pF). The SMP1352-079LF has a specified power dissipation of 250 milliwatt (mW), which is more than adequate for the shunt diode in this application. Its series forward resistance is slightly less than that of the SMP1302-085LF at 2 Ω at 10 mA and 1 Ω at 100 mA.

The controlling bias signals—Bias 1 and Bias 2—in all topologies must be complementary and change state simultaneously. The switching speeds for both diode types are less than 1 microsecond (µs).

High voltage T/R switches protect low voltage ultrasonic circuits

Ultrasonic applications including non-destructive testing, echo location, and medical ultrasound also require T/R switches. The technique and components used in these applications are different from the previously described RF applications. These applications use a high voltage T/R switch that acts to protect sensitive low voltage electronics from the high voltage pulse signals used to drive an ultrasonic transducer (Figure 7).

Diagram of typical ultrasonic applicationFigure 7: A typical ultrasonic application where a high voltage pulse is applied to one of the piezoelectric transducers. The receiver is protected by a fast T/R switch that senses the voltage increase and opens to protect the receiver inputs. (Image source: Microchip Technology)

The transmitter in an ultrasound application is connected to one of the piezoelectric transducers directly. The transmitter output is a high voltage pulse that drives the transducer. The receiver is connected to the same transducer through a fast two-terminal, voltage-sensitive switch. The switch in this case is a Microchip Technology MD0100N8-G high voltage T/R switch. This is a two-terminal, bi-directional current-limiting protection device. The MD0100 is normally closed, but when the voltage across the device exceeds ±2 volts, the switch opens in about 20 nanoseconds (ns). The open switch can withstand a voltage of up to ±100 volts. In the open state, there is a 200 µA current through the switch used to detect the continued presence of the high voltage. Once the high voltage is no longer applied, the switch will revert back to the closed state. The diodes connected in a back-to-back fashion at terminal B on the receiver side of the MD0100 provide a path for this current through the switch. These diodes also clamp the input to the receiver at ±0.7 volts.

The on resistance of the MD0100 is typically 15 Ω. The capacitance of the open switch is a function of the applied voltage. It varies from 12 pF for a voltage of 10 volts up to 19 pF at 100 volts.

This T/R switch has the advantage of being a simple two-terminal component that does not require a power source.

Conclusion

Switching a single antenna between transmit and receive modes has its challenges, but as shown, the right T/R switch, or duplexer, can solve the problem—assuming the designer understands how the devices work and chooses their T/R architecture appropriately.

Disclaimer: The opinions, beliefs, and viewpoints expressed by the various authors and/or forum participants on this website do not necessarily reflect the opinions, beliefs, and viewpoints of Digi-Key Electronics or official policies of Digi-Key Electronics.

About this author

Art Pini

Arthur (Art) Pini is a contributing author at Digi-Key Electronics. He has a Bachelor of Electrical Engineering degree from City College of New York and a Master of Electrical Engineering degree from the City University of New York. He has over 50 years experience in electronics and has worked in key engineering and marketing roles at Teledyne LeCroy, Summation, Wavetek, and Nicolet Scientific. He has interests in measurement technology and extensive experience with oscilloscopes, spectrum analyzers, arbitrary waveform generators, digitizers, and power meters.

About this publisher

Digi-Key's North American Editors