RF and Microwave Coupler is a passive RF device that is used to couple or sample a signal from the main chain. Coupler will take one input signal and provide two outputs signal, one output is a regular signal and the other will be coupled signal with reduced power. The reduction of RF signal power of the coupled port is based on the RF coupling factor design.
RF and Microwave couplers have a number of applications like signal sampling, used to measure VSWR by getting incident/reflected power, etc. RF and Microwave Couplers are very useful components in almost every RF and Microwave system like radars, Communications equipment, Radios, and RF test equipment.
In this article, we will cover the following details about the RF and Microwave coupler.
RF and Microwave couplers are passive devices that divert / couple a fraction of the signal from the main transmission line to another coupled transmission line. The signal output at the main port of the transmission line is called the “direct or through” signal and the other transmission lines (Coupled line) is called the “Coupled signal”. RF signal in the main transmission line remains the same as the input signal and the RF signal at the coupled port is attenuated by the coupled value of the coupler.
Below is the schematic representation of an RF and microwave coupler.
In this symbol, If the signal is feeding on port 1, The line connecting port 1 to port 2 is called the through-line of the coupler. Port 3 and port 4 are called the coupled ports of the coupler. In that port 3 is called forward coupled port and port 4 is called backwards coupled port.
There are mainly two ways RF and Microwave couplers are designed.
To design a coupler, we need to select two transmission lines and arrange them in such a way that some portions of the signal in each line should couple each other (fields of first-line interact with the second line) over some distance along the transmission lines.
Couplers have four-port circuits for both directional couplers and Quadrature couplers. The ports are named input ports, through ports, coupled ports, and Isolated ports. In general, the fourth port ( Isolated port) is terminated internally by the characteristic impedance and it is well covered in the package. There are 2 types of coupler designs that are generally followed.
RF and Microwave couplers are generally designed using the Coaxial, Stripline, Wave Guides, and Microstrip technologies. In general, couplers are designed by a pair of coupled transmission lines, in which RF power on the coupled line and mainline flows in the opposite direction.
The mainline connects ports 1 and 2 and the coupled line is connected between ports 3 and 4. Any port of 1 or 2 can be the input port and the other two ports in the coupled lines are the coupled ports. The port adjacent to the output port is the coupled port, and the other port (diagonal port) will be the isolated port.
The accuracy of coupling of the RF coupler depends on the dimensional tolerances of the spacing between the two coupled lines.
RF and microwave couplers are segregated on the basis of their functionality. In addition, couplers are categorized on the basis of design structure. In general, all the couplers are four-port devices and one port is internally terminated.
For directional coupler, the power that is passing from the input port ( Port 1) to the output port (Port 2) is coupled to the other coupled line and passes to the other two ports ( port 3 and port 4). The coupled signal can get from either port 3 or port 4. Value of coupling based on the distance between the coupled lines.
The coupled output port of the directional coupler is fully isolated from the main output port. Any reflected RF power from the main output will couple to the coupled line and is terminated at the other port internal termination, which results in minimal reflections in the directional coupler.
A dual directional coupler is a four-port device. In which two RF directional couplers are connected in series ( One directional coupler back to back to the other), in opposing directions. In dual directional coupler, these serial connection creates a single mainline and two secondary lines ( one in each) for coupling. Both the isolated ports of each directional couplers are terminated by 50 ohms. Hence we will get the coupled signals at port 3 and port 4.
As there are 2 coupled signals output ports, these couplers are used to monitor forward and reverse power simultaneously in a system.
The 90-degree hybrid couplers are the couplers that split the input signal into two 90 degree phase differential equal signals. 90-degree hybrid couplers are also known as quadrature hybrids couplers because a signal applied to any input port will split into two equal amplitude different quadrant signals (90 degrees phase-separated from each other). Phase shifting is happening on the basis of coupling among the transmission lines.
90-degree hybrid couplers are used for a signal splitter with a phase difference, balanced mixers, Image-rejection mixers, Phase Shifter, etc
180 Degree Hybrid couplers are four-part devices and are similar to 90-degree hybrid couplers, but they split the input signal into two 180 degrees phase differential equal RF and Microwave signals. The 180-degree hybrid couplers are also called the “rat race” couplers. Meandering transmission lines of 180 Degree Hybrid couplers maintain the even 180-degree phase relationship between two coupled lines at all frequencies. 180 Degree Hybrid couplers are used to either add up or split equally a signal with a phase difference.
180 Degree Hybrid couplers are used to merge multiple phase difference signals from an array or to reduce noise in amplifiers by feedback signal combining techniques.
RF couplers have much application in RF and Microwave communication circuits and for the RF testing field. The ability of the RF coupler to sample either the Reverse or Forward or both the signal at a time allows a wide range of applications in Test and Measurement instruments, feedback, and control systems.
RF coupler is used for waveform measurement and monitoring, and other test and measurement application by taping the coupled signal from the mainline. When the RF coupler is connected to the RF chain, the coupled port provides a sample of the forward signal, with attenuated power by the coupling factor, this signal can be used for the above applications.
The RF coupler is used to measure of reflected power (degree of mismatch of the load) by sampling the reflected wave at the coupled port. When the RF coupler is connected to the transmitter output, we can monitor the VSWR of the antenna system. RF coupler is used for adjusting the VSWR by lowering the power in the coupled port.
RF coupler can be used to drive feedback circuitry. The coupled port output of the coupler which is connected to the output of a generator is used for levelling the amplitude of a signal generator by providing a continuous feedback signal to the generator control system.
RF coupler can use for the inter-modulation testing of the receivers by combing the RF signals using a directional coupler, as this method will provide the isolation between the RF signal sources during the combine process.
Bi-directional Coupler is used to simultaneously sample both forward and reflected signals for measuring the VSWR of a system. As a bi-directional coupler, allows monitoring of output power (forward) and reflected power (reverse), it is used in Built-in test (BIT) systems and routine operational monitoring of the RF systems.
Below are the prime specifications which need to consider in the selection of RF and Microwave couplers in the design of the RF and microwave systems.
The frequency range of the RF coupler indicates the frequency over which the coupler can operate with minimal loss and reflection while used in an RF and Microwave circuit.
Coupling of the RF coupler indicates the ratio of the RF power fed to the main port to the power output at the coupled port of the RF and Microwave coupler. Values will be coming as 3 dB, 6dB, 10 dB, 20 dB etc
Coupling variation is the maximum expected peak-to-peak variation in the coupled signal power over the specified frequency range of its operation. It is represented in dB, 0.2 dB over the band of frequency is acceptable.
Insertion loss indicates the reduction in RF power at the output port during the power transmission through the coupled line of the RF coupler. It is expressed in dB, 0.2 dB over the coupler is acceptable.
Directivity of the RF coupler indicates the difference in power level between the coupled and isolated ports. The directivity is measured in dB and it indicates the independence of the isolated and coupled ports of the RF coupler.
Power handling of the coupler is one of the prime features that need to consider in the selection of the RF and Microwave coupler. The power handling capability of the coupler main line should be higher than the maximum power expected to pass through the RF and Microwave coupler. The Average Power in watt indicates the level of RF power that the coupler can handle while maintaining the electrical characteristics of the coupler.
The impedance of the RF and Microwave coupler must match with the remaining circuit to minimize any reflection within the system. Usually, the impedance of the coupler will be 50 Ohms or 75 Ohms. Need to select the coupler accordingly.
The operating range of the RF coupler indicates the temperature over which the RF coupler will work without any variation in its electrical characteristics. In general -40ºC to +85ºC is ideal for the coupler operation. For the specified applications, need to select the RF and microwave coupler as per the required environmental operating temperature range.
RF and Microwave couplers are available will most of the connectors like SMA, BNC, N, TNC, and also waveguide couplers are available. Need to select the coupler as per the application in the circuit.
The physical size of the coupler and the position of the mounting screw need to consider in the selection of the RF coupler for any specific applications.
RF and Microwave Coupler are one of the prime components of any RF and microwave communication system to monitor and bi-pass the signal. The selection of the suitable coupler in the circuit is important for passing and monitoring the RF signal in the transmitter and receiver in the communication systems.
This article help system designers to understand the architecture, function, application, and performance of the RF and microwave coupler, in the communication system.
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