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brief introduction
Directional coupler is a kind of microwave device which is widely used in microwave system. Its essence is to press the microwave signal to a certain directional coupler.
The ratio is allocated to power.
Directional coupler is made up of transmission lines. Coaxial cables, rectangular waveguides, circular waveguides, stripline and microstrip lines can all constitute directional couplers. Therefore, there are various kinds of directional couplers and great differences. But from its coupling mechanism, it is divided into four main types, namely, small hole coupling, parallel coupling, branch coupling and matching double t. Directional coupler
In early 1950s, microwave equipment is used by almost all metal waveguide and coaxial line directional coupler circuit, at that time also for waveguide aperture coupling directional coupler, based on the theory of bethe hole coupling theory, Cohn and levy et al have also done a lot of contributions.
With the development of aviation and aerospace technology, microwave circuits and systems are required to be miniaturized, lightweight and reliable, so stripline and microstrip lines appear. Subsequently, microwave integrated transmission lines, such as fin lines, slot lines, coplanar waveguides and coplanar stripline, have been developed for the needs of microwave circuits and systems. In this way, a variety of transmission line directional couplers appear.
The directional coupler on the first real significance is realized by the H. A. Wheeler design in 1944, energy coupling Wheeler uses a pair of long is 1/4 of the wavelength of the center frequency of cylinder to realize the electric field and magnetic field, it is a pity that this method can achieve an octave bandwidth.
Working principle
The power transmitted in the main line is coupled to the secondary lines in a variety of ways and interferes with each other and is transmitted only in one direction in the secondary line.
Three typical coupling structures of a rectangular waveguide directional coupler. A is a two hole coupling away from the 1/4 guided wavelength. B is coupled by double series branch lines with spacing and length equal to 1/4 leading wavelength. C is the continuous coupling of two propagation modes of TE and Te in the crack area. Taking two structures of a and B as an example, the signal input from port 1 is coupled to the secondary line after two way coupling, and the output direction is equal to phase in the direction of port to fourth direction, and there is output in the direction of travel. In the third direction, there is a difference in the stroke between 1/2 and the leading wavelength, which is counterbalanced and isolated.
Two typical coupling structures of microstrip directional couplers. A is a double parallel branch line coupling with the distance and length equal to the 1/4 guide wavelength, and B is a continuous coupling of two structures in a parallel region. Taking the structure of B as an example, the signal input from port is generated by the electric field coupling on the two ports of the secondary line to generate the in-phase induced voltage, and the magnetic field coupling produces the induced voltage. The results are added at the end of the port and have output, and the third is offset by isolation and no output.
In addition, it can also form a directional coupler for other transmission lines.
Network characteristics
The directional coupler can be seen as a four port network, its characteristics can be expressed by the scattering matrix [S], that is, the calculation formula
The reflection coefficient SII (i=1, 2, 3, 4) of each port is very small (ideal value is zero), indicating the matching of ports. The value of coupling coefficient s13=s31=s24=s42 is determined by the coupling degree of design, s14=s41=s23=s32 is the isolation coefficient and the ideal value is zero.
The main technical indicators of directional coupling are coupling C (DB), directional D (DB) and working frequency band, and the scattering matrix of c=-20lg|s14| (DB) d=20lg|s14/s13| (DB) ideal directional coupler is a formula.
The two output signals have a phase difference of 90 degrees.
The two - or double - Branch coupled single - directional couplers work in a narrow band. If a multi section directional coupler (several single node cascades) with a porous or multi branch coupling structure is adopted, a comprehensive design method can be used to broaden the working frequency band.
main parameter
The main parameters of the directional coupler are as follows:
1, direct parameters: according to the scattering matrix, it is known by S21.
2, coupling parameters: according to the scattering matrix, it can be characterized by S31.
3, isolation parameters: according to the scattering matrix, it is known by S41.
4, reflection parameters: according to the scattering matrix, it is known by S11.
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