BLCoupler (Branch-Line Coupler)

Symbol

Parameters

Name	Description	Unit	Default
Subst	Microstrip substrate name	None	MSub1
F	Center frequency	GHz	1
DeltaF	Total frequency bandwidth	GHz	0.5
Zo	Characteristic impedance	Ohm	50
ResponseType	Type of frequency response	None	Chebyshev
N	Number of coupler sections; set N=0 to compute N	None	0
Rmax	Maximum voltage reflection coefficient at the input	None	0.5
C	Coupling coefficient	dB	3
Delta	Length added to branches for tuning performance	mil	0

Notes

1. A branch-line coupler outputs from the coupled port (pin 3) a fraction of the power presented at the input (pin 1). The remainder of the power is passed through to the output port (pin 2). At the center frequency the phase difference between the outputs is 90 degrees, with the coupled port representing the quadrature (Q) output and the output port representing the in-phase (I) output. The coupling coefficient specifies the ratio of the input power to the coupled power (P ₁ /P ₃ ). Pin 4 represents the isolated port, and it is typically well isolated from the input port near the center frequency.
2. The coupling coefficient must be positive and greater than 3 dB. Best results are obtained for tight couplings of 6 dB or better (C < 6 dB). Choosing the coupling parameter larger than 6 dB often causes width constraint violations to occur on the MTEE components, resulting in warning messages during design and simulation. A coupling coefficient of 3 dB provides an equal power split between the two outputs.
3. For broadband performance, the coupler can have multiple sections. If the number of sections N is set to zero, the Design Assistant chooses N such that the reflection coefficient is less than Rmax over the bandwidth DeltaF (centered at the design center frequency). The resulting bandwidth can be broader than that specified. Otherwise, rmax and DeltaF are ignored.
4. The ResponseType specifies the distribution of the partial reflection coefficients seen at each section interface - Uniform, Binomial, and Chebyshev distributions are available.
5. The optimization minimizes the input reflection coefficient (S11) at the design center frequency by changing the length of the lines forming the four branches. All branches are changed by the same physical length during the optimization.This optimization generally provides very good results but can not guarantee that the specified coupling is attained at the design frequency. More advanced tuning can be performed by changing line width of the branch lines.
6. A SmartComponent subnetwork is empty until the Design Assistant is used to generate the design. Refer to Design Assistant in the Filter DesignGuide.

For a more detailed discussion of this device, see: D. M. Pozar, Microwave Engineering, 2nd Edition, John Wiley & Sons: New York, 1998, pp. 379-383.

Example

A single-section branch-line coupler was designed for a center frequency of 5 GHz with an equal power split between the I and Q ports. Tuning using the Optimization Assistant yielded a value of Delta = 49.532 mil.