Quadrature Coupler Design: Components or Traces?

Key Takeaways

• Quadrature coupler use two pairs of input and output ports to split or combine signals with a  90° offset at the output.

• Discrete components or quarter-wavelength traces are available for building the coupler.

• Power combining lower-quality amplifiers with a quadrature coupler greatly improves performance, provided the reflection coefficients are similar.

A pair of tightly coupled traces is one method of implementing a quadrature coupler design.

Often in design, signals need to combine (or, conversely, split) to support the various logic requirements of the schematic. It’s not as simple as merging or splitting traces: design has to use tuned trace lengths or discrete components to manage the transition effectively. One method to split or combine lines is a quadrature coupler design.

Implementation Methods for Quadrature Coupler

The Basics of Quadrature Coupler Design and Variant Types

A quadrature coupler is a four-port design where the input splits between two outputs separated by 90° in phase angle. The ports are identical in construction but differ in functionality:

• The input port accepts the signal.
• The through port carries the in-phase portion of the signal.
• The coupled port carries the 90° phase-shifted portion of the signal.
• The isolated port connects the second device to the input.

With a pinout numbered clockwise starting with the top left port, the difference between pins 2 and 3 is a positive 90° for input signals on port 1, while the difference between pins 2 and 3 is a negative 90° for input signals on port 4. Typical operations use port 1 as the input with an impedance-matched load on port 4: the advantage of this design is that the input ports (1 and 4) are isolated from each other, as are the output ports (2 and 3). Ideally, the structure improves power delivery to the load by canceling out reflections that would otherwise cause dissipative energy losses, resulting in a loss of power delivery while generating heat. During signal transmission, the split between the coupled and through lines can ensure a high reduction of reflection coefficient within a connected amplifier network, up to a 4x reduction at the center frequency (the reduction effect diminishes considerably towards the edges of large bandwidths).

Advantageously, branchline quadrature couplers are exceptionally easy to implement at the system level of design, consisting of two parallel traces for the input/output and two perpendicular traces of length λ/4 (with lambda corresponding to the center frequency) for the 90° phase shift. A coupled-line variant does away with these perpendicular traces, removing any direct connections between the adjacent I/O pairs. The lengths of the I/O pair traces in the coupled-line coupler are instead the λ/4 length to achieve the 90° phase shift. Other quadrature coupler designs are available, and while they may not be as quick to realize as these base branchline structures, their unique advantages may make them more suitable for certain applications:

• Two-stage - The branchline coupler is iterable using a common arm of the couplers. A two-stage coupler improves the bandwidth of the standard branchline coupler but incurs greater loss.

• Lumped elements - For appreciably high frequencies, the characteristic λ/4 length for the branchline coupler arms is trivial regarding design space. However, this length may be unwieldy at lower frequencies; in those cases, designers can use pi filter networks (shunt capacitors and series inductors) for the arms of the coupler. This method is also compatible with multi-stage design.

• Unequal-split - Instead of equivalent impedances in the four branches of the quadrature coupler, designers can group the branches into two equal impedance pairs (opposite to each other) and vary the impedance of the pairs to divide the power proportionally, similar to a voltage divider.

Other coupler designs like Lange or waveguides offer varying tradeoffs between isolation and bandwidth. The defining feature of these (and any quadrature coupler) is the 90° phase shift between the pairs of the input/through ports and the isolated/coupled ports.

Use Cases for Quadrature Coupling

The design of the quadrature coupler offers a few inherent advantages. Theoretically, the coupler is lossless when impedance-matched. While practically this is unachievable, it enables a power transfer from the input to the non-isolated ports that is even and separated by 90° (i.e., sine and cosine). The power division (or combination) grants a high amount of utility:

• Antenna feed - Alternative network methods require delay lines and power combining/impedance matching to achieve the elegant solution of a quadrature coupler.

• Image-reject mixers - Parallel lines of mixers and local oscillator (LO) signals can combine in quadrature to cancel either the sum or difference terms. These undesirable mixing products can experience a reduction of 30 to 50 dB.

• Power amplifier combiners - Power amplifiers combined in quadrature can reduce odd-order harmonics, while a push-pull configuration can also tamp down on even-order harmonics.

The last implementation is especially useful: a power amplifier, or balanced amplifier, can combine amplifiers with similar reflection coefficients and achieve a 50-ohm impedance-matched signal at the output. This feature means lower-quality amplifiers can provide performance far exceeding their normal capabilities with a quadrature coupler, such as improved input/output impedance matches and return loss.

Cadence Solutions Combine Speed, Ease, and Power

Quadrature coupler design supports numerous circuit features due to the ability to split evenly and produce outputs with a 90° offset. Due to its relative ease of implementation at the board level without components, it can be a space- and cost-efficient method to improve amplifier performance with a balanced amplifier topology. Designers can readily simulate quadrature coupler performance without investing in fabrication and assembly with the powerful tools included in Cadence’s  PCB Design and Analysis Software packages. These models readily integrate with the fast and easy-to-use OrCAD PCB Designer for a seamless ECAD experience that accelerates PCB production. 

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