LCoupler (Lange Coupler)

Symbol

Parameters

Name	Description	Unit	Default
Subst	Microstrip substrate name	None	MSub1
F	Center frequency	GHz	1
C	Coupling coefficient	dB	20
N	Number of fingers (4, 6, or 8)	None	4
Zo	Characteristic impedance	Ohm	50
Delta	Length added to fingers for tuning performance	mil	0

Notes

1. A Lange coupler outputs from pin 2 a small fraction of the power presented at the input (pin 1). The remainder of the power is passed through pin 3. The coupling coefficient specifies the power ratio P ₁ /P ₂ . Pin 4 is isolated, and often the isolation is 10 dB better than the coupling coefficient in microstrip realizations.
2. The Lange coupler is best for weak couplings of roughly 10 dB or more (C >10dB). Choosing the coupling coefficient too small can produce an unrealizable design. If the design creates a finger spacing S that is not realizable, increase the value of N.
3. The Design Assistant computes the required even and odd mode impedances to achieve the desired coupling and translates them to finger width and spacing. The length of the fingers is a quarter wavelength at the design frequency.
4. The optimization minimizes the absolute difference between S21 and the specified coupling coefficient at the design center frequency by changing the length of the fingers section.
5. A SmartComponent subnetwork is empty until the Design Assistant is used to generate the design. Refer to Design Assistant.

For a more detailed discussion of this device, see: I. Bahl and P. Bhartia, Microwave Solid State Circuit Design, John Wiley & Sons: New York, 1988, pp. 209-211.

Example

A Lange coupler was designed for a center frequency of 3 GHz with 20 dB of coupling and 6 fingers. Tuning using the Optimization Assistant yielded a value of Delta =-4.57 mil.