Digital Signal Processing Reference
In-Depth Information
31.3.1.1.1 3dB Coupler as Splitter and Combiner
In principle, a directional coupler consists of two closely adjacent parallel
lines with a length of Λ/4. The spacing between the lines determines the
overcoupling attenuation; if this is 3 dB, it is called a 3 dB coupler. If a
signal is fed into an input of a 3 dB coupler, 3 dB of it are coupled out with
0 degree phase shift at the output opposite the input, and another 3 dB of it
are coupled out with 90 degree phase shift at the output of the Λ/4 line sec-
tion connected electrically to the input.
A 3 dB coupler can be used for adding powers by feeding a signal into
one input with 0 degree phase shift and feeding it with a 90 degree phase
shift into the input of the parallel Λ/4 line section (3 dB combiner). The
signals then cancel at one output and the aggregate power is present at the
other output with 90 degrees phase rotation. The unused output is termi-
nated with 50 Ohms (load matching resistor). In the case of phase errors or
power differences in the signals supplied, power is also absorbed in the
load matching resistor.
Fig. 31.6. shows a simplified circuit symbol of a 3 dB coupler which
will be used in the following sections.
50 Ohm
50 Ohm
0 dB 0
o
P = 0
R
Source
R
Term
/4
50 Ohm
50 Ohm
0 dB -90
o
+3dB -90
o
R
Source
R
Load
Fig. 31.5.
3db combiner
Fig. 31.6.
Simplified circuit symbol of a 3 dB coupler
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