Directional couplers are implemented using a variety of construction techniques
(stripline, coaxial, lumped element) and packages
(connectorized, solder pins, substrate carrier). What they all
have in common is the ability to tap a sample of the input power without significantly affecting
the original signal. Generally, for well-designed devices the smaller the portion of the input
power that the coupled power port extracts, the less profoundly the input signal is affected.
For instance, a 30 dB coupler takes only 0.1% of the input power as a sample to Port 3 and
passes the remaining 99.9% out Port 2 (assuming no mainline resistive
loss). The opposite extreme is a hybrid 3 dB coupler that evenly divides the input
power between Port 2 and Port 3 (see illustrations below).
Directional couplers are fundamentally 4-port devices, although many packaged designs bury
the isolated port and its associated termination inside the package
(or have it permanently attached to the outside and covered over with a heavy layer of epoxy
paint). The following image illustrates the fundamental configuration for a directional
coupler. Note that the coupled path is ¼ wavelength long, so there is a 90° phase shift between
Port 1 and Port 4, and between Port 2 and Port 3. In physical devices, the phase shift can
be realized either with transmission line (for higher frequencies)
or with lumped element inductors and capacitors.
Directional (Bidirectional) Coupler
(alternate symbol to right)
An online calculator for power at the ports of a directional coupler is provided
here. The same
calculator is included in the
RF Cafe Calculator Workbook and is available as a free download.
The directional coupler calculations presented below are textbook equations, where all
four ports are assumed to be terminated in impedances that are exactly matched to the characteristic
impedance of the coupler, and perfectly linear operation is also assumed. The "Directivity"
term accounts for non-ideal isolation between the coupler input (Port
1) and the isolated port (Port 4). The "Mainline Loss"
term exists to account for resistive losses in the coupler (a specification
often provided by the manufacturer).
This page has a list of directional coupler manufacturers.
|Main system signal input, PInputPort
Note: In the following equations a lower case 'p' indicates
power in units of W, mW, etc., and an upper case 'P' indicates power in units of dBW, dBm,
|Main system signal output, POutputPort
Forward Cpl'd Port
|Power sampled from Port 1, PCoupledPort
Reverse Cpl'd Port
|Power sampled from Port 2, PIsolatedPort
||Power transferred to the coupled port relative to Port 1.
This is S13=S31 in the forward direction and S24=S42
in the reverse direction, assuming all ports are terminated without reflection. It will be
a negative number expressed in dB.
||Amount of power lost to the coupled port (3)
and to the isolated port (4). Assuming a reasonable directivity,
the power transferred unintentionally to the isolated port will be negligible compared to
that transferred intentionally to coupled port.
(mainline loss not included)
Example (using easy numbers):
Assume that the coupling factor is -10 dB and the directivity is 20 dB. Intuitively, we
know that -10 dB is equal to 1/10 in terms of power. Therefore, 10% of the input power is
transferred to the coupled port and 90% is passed to the transmitted port. 20 dB of directivity
means the power that leaks to the isolated port is 20 dB lower than what is present at the
coupled port, so it is 30 dB lower than at the input port. 30 dB represents 1/1000 of the
input power (0.1%), so the total lost due to coupling and non-infinite
directivity is 10.1%. Therefore, 89.9% of the input power arrives at the transmitted port.
Coupling Loss = 10 * log10 (0.899)
= -0.462 [dB]
||Resistive loss due to heating (separate from coupling
loss). This value is added to the theoretical reduction in power that is transferred
to the coupled and isolated ports (coupling loss).
||Power level difference between Port 3 and Port 4 (related
to isolation). This is a measure of how independent the coupled and isolated ports
are. Because it is impossible to build a perfect coupler, there will always be some amount
of unintended coupling between all the signal paths.
||Power level difference between Port 1 and Port 4 (related to directivity).
This is S14=S41 in the forward direction and S23=S32
in the reverse direction, assuming all ports are terminated without reflection.
Typical Uses for a Directional Coupler
The most common use for a directional coupler is to tap off a sample of the input power
for use in signal monitoring circuits. That sample can be measured to determine the power
level, frequency, and/or signal shape (modulation) and either
presented for human viewing or can be integrated as part of a feedback loop that adjusts the
output to stay within system specifications.
When monitoring the voltage standing wave ratio (VSWR) of
the load at the transmitted port, both the coupled port and the isolated port outputs are
used to sample the incident and reflected power, respectively. This configuration is referred
to as a bidirectional coupler.
A special configuration of the directional coupler is referred to as a hybrid coupler,
or a 3 dB coupler or a hybrid coupler or a "quadrature coupler," or a 3 dB hybrid coupler
amongst other names. Regardless of what you call it, this coupler has the characteristics
of dividing the input power into two paths that have equal powers when terminated properly.
3 dB Quadrature Hybrid Coupler
The quadrature hybrid coupler has a unique property that makes it very useful for amplifier
design, and that is when two equal, but not necessarily 50 Ω (or 75
Ω ,or whatever the system impedance) loads are connected to Port 2 and Port 3, Port
1 will "see" the system impedance (50 Ω, eg.) as long as that
same 50 Ω is connected to Port 4. So, what that means is if you are designing a power amplifier
with transistors that operate at, for example, 4.0 - j3.5 Ω input impedance and 1.5 - j2.5
Ω output impedance, a pair of them can be inserted between a set of quadrature couplers
(Ports 2 and 3), with 50 Ω impedances at Port 1 and Port 4.
Doing so eliminates the need to transform the transistor impedances, and the accompanying
matching losses, to 50 Ω. The image below illustrates the hookup. Theoretically, an infinite
number of these connections can be used. An added benefit of the balanced amplifier configuration
is that both the IP3 and the P1dB is 3 dB greater than that of the individual devices. See
this app note by Thomas Shafer.
Quadrature Hybrid Coupler Ports
Quadrature Hybrid Coupler Hookup for an Amplifier
The same characteristic permits the hybrid coupler to be used as an attenuator. In this
configuration, the signal input is on Port 1, and the output is taken on Port 4, the isolated
port. Identical PIN diodes (and bias circuits) are connected
to Port 2 and 3. When the diodes are turned off (usually requires a reverse bias to offset
the negative portion of the RF voltage swing), Ports 2 & 3 'see' 50 Ω, and no power is
reflected into the isolated port (Port 4). When the diodes are
shorted (fully biased on), all of the signal is reflected back
into the coupler and exits out of the isolated port (Port 4).
Attenuation is therefore maximum when the PIN diodes are reverse biased and minimum when they
are forward biased. The attenuator can also be realized using a pair of couplers a la the
amplifier circuit above, where the PIN diode circuits are in the place of the transistors.
Quadrature Hybrid Hookup for an Attenuator
Thanks to Christian D. for providing additional content to this article.