Branchline Coupler
Understanding Branchline Couplers
The branchline coupler is one of the simplest and most widely used 90-degree hybrids in planar microwave circuits. Its rectangular microstrip layout is easy to fabricate and its properties are well-understood.
Branchline Design
- Series arms: Z0/sqrt(2) = 35.35 ohms. Length = lambda/4.
- Shunt arms: Z0 = 50 ohms. Length = lambda/4.
- Splits: Equal power split with 90 degrees between coupled and through ports.
Port Properties
- Port 1 (Input): Matched to Z0.
- Port 2 (Through): -3 dB, 0 degrees (relative).
- Port 3 (Coupled): -3 dB, -90 degrees.
- Port 4 (Isolated): No output (ideally). 20-25 dB isolation.
Frequently Asked Questions
What is a branchline coupler?
A branchline coupler is a four-port 90-degree hybrid using a rectangular ring of quarter-wave transmission lines. It splits input power equally with 90 degrees phase difference between outputs. Used for balanced amplifiers and image-reject mixers.
What is the bandwidth of a branchline coupler?
A single-section branchline has about 10-15% bandwidth for < 1 dB amplitude imbalance. Multi-section designs (two or three sections) extend bandwidth to 30-50%. For wider bandwidth, Lange couplers or broadside-coupled structures are preferred.
Branchline vs Lange coupler?
Branchline: simple rectangular layout, 10% bandwidth, easy to fabricate. Lange: interdigitated fingers, octave bandwidth, requires tight trace spacing. Branchline is preferred for narrowband; Lange for wideband.