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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
Espresso Engineering 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 coupled port (Port
3) 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.
| Port 1 Input Port, aka Incident Port |
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, etc. |
| Port 2 Output Port, aka Transmitted Port, aka Through Port |
Main system signal output, POutputPort |
| Port 3 Coupled Port, aka Forward Coupled Port |
Power sampled from Port 1, PCoupledPort |
| Port 4 Isolated Port, aka Reverse Coupled Port |
Power sampled from Port 2, PIsolatedPort |
| Coupling Factor |
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.
|
| Coupling Loss |
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. To verify:
Coupling Loss = 10 * log10
(0.899) = -0.462 [dB]
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| Mainline Loss |
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). |
| Directivity |
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.
|
| Isolation |
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.
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