RF directional coupler

Saturday, 06 January 2007

An RF coupler is a passive device that may be used to control the amplitude and direction of radio frequency signals in a transmission path between circuit modules. There are many known microwave couplers. They are consisted of appropriate coupling structures between a main transmission line and an auxiliary transmission line. An RF coupler may commonly be configured as a stripline coupler, a microstrip coupler or the like. Stripline couplers consist generally of a pair of adjacent transmission line conductors located within one or more substrates positioned between one or more ground planes. The transmission line conductors may be coplanar or non-coplanar. A stripline coupler ordinarily functions as an RF signal attenuator which is a device for generating a controlled amount of signal power transfer from one transmission path to another to provide one or more reduced amplitude RF signals. The most well-known coupler for radio frequency equipment is a directional coupler. This device allows a sample of a radio frequency signal, which is input at an input terminal and output at an output terminal, to be extracted from the input signal. Directional couplers can utilize different waveguiding media, for example waveguides, coaxial lines, printed transmission lines (like microstrip, strip-lines, coplanar lines, etc). Directional couplers are widely used in microwave and RF circuits as separate components, or as parts of other devices such as directional filters, mixers, phase shifters, attenuators, balanced amplifiers, magic-tees, modulators, beam-forming networks for array antennas, etc. Most directional couplers used in conjunction with radio frequency power amplifiers are designed to measure both forward and reflected power. These directional couplers are useful in measuring load conditions, in adjusting the matching between the amplifier output stage and the load, and in protecting the output devices of an amplifier from damage resulting from mismatch.

A directional coupler is a circuit element which is adapted to provide an output which is proportional to only unidirectional power from a source of microwave power flowing through a transmission line without reference to reverse power. A directional coupler measures the power in a wave traveling in a particular direction in a transmission line. In radio frequency (RF) systems, it is often desirable to determine the amplitude of a signal input into the system, a signal present in the system, or a signal output from the system. Directional couplers are placed in RF systems at locations where a determination of the amplitude of a signal is desired. A directional coupler allows a sample of a radio frequency or microwave signal, which is input at an input terminal and output at an output terminal, to be extracted from the input signal. A directional coupler samples a small portion of radio frequency and microwave energy traveling through a coaxial cable. The directional coupler divides an input signal into a coupled signal and an output signal; the amplitude of the coupled signal being proportional to the amplitude of the input and output signals. The amplitude of the input and output signals can thus be determined by measuring the amplitude of the coupled signal. Properly designed, the directional coupler can distinguish between a signal input at the input terminal and a signal input at the output terminal. This characteristic is of particular use in a radio frequency transmitter in which both the input signal and a signal which is reflected from a mismatched antenna can be independently monitored. A directional coupler couples a certain amount of power input to a first transmission line to a second transmission line. If a directional coupler is not properly terminated, reflected waves travel back from the load to the input or source of the line. The ratio of the power input to the first transmission line to the power coupled to the second transmission line is referred to as the coupling factor. The directivity of a directional coupler refers to the ratio of the power measured at the forward-wave sampling terminals, with only a forward wave present in the transmission line, to the power measured at the same terminals when the direction of the forward wave in the line is reversed. Directivity is usually expressed in decibels (dB). High directivity in directional couplers is usually attained by manufacturing the transmission line to have a predetermined characteristic impedance that matches the source impedance and/or load impedance. These reflected waves cause degradation in the performance of the system. In a typical radio frequency (RF) application, the directional coupler is used in the RF transmitter automatic level control (ALC) circuit. The transmitter ALC senses the power level being produced by the RF power amplifier and adjusts the bias of the RF amplifier to set the RF power output to a desired level.

A basic directional coupler is a linear, passive, four port network, incorporating two parallel coupled transmission lines. A first transmission line extends between an input port and a through port, and a second transmission line extends between a coupled port and an isolated port. Within a frequency band, a typical directional coupler will divide incident power from a source into two outputs at phase quadrature. The ratio of each output power to the input power will be known for an arbitrary set of impedances connected to the four port device. A directional coupler has a through arm through which a signal passes and at least one coupled arm that samples the signal. A high-power directional coupler causes a sample of an electromagnetic wave propagating on the through arm to propagate on the coupled arm. The coupled arm serves to sample the signal on the through arm. A directional coupler is capable of sampling signals propagating in two different directions. A signal flowing in a first direction on the through arm is sampled on one port of the coupled arm, while a signal flowing in the opposite direction is sampled on the other port of the coupled arm. Microstrip directional couplers typically use structures that obtain flatter coupling versus frequency characteristics at the expense of using greater dimensions to accomplish such characteristics. Directional couplers for high frequencies of a quarter-wavelength coupling line type employing microstrip lines include a dielectric substrate formed with a grounding conductor on its entire bottom surface and a pair of quarter-wavelength coupling lines made of microstrip lines formed on the top surface of this substrate in a mutually parallel relationship with a predetermined separation between them and with their ends connected to different ports. Quadrature directional couplers consisting of parallel-coupled microstrip transmission lines are used extensively in microwave and millimeter-wave integrated hybrid monolithic circuits. Quadrature directional couplers can be used in any microwave or millimeter-wave subsystem with applications which include, among others, power sensing, combining, dividing, balanced mixing, amplifying, and antenna feed networks. Printed directional couplers use pieces of single or coupled lines placed on, or between, planar dielectric substrates. Directional couplers made of coupled lines have wider frequency bandwidth.

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