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An improved layout for Lange couplers.

The Problem

The Lange coupler is often used in hybrid microwave circuit applications. It has been widely used in balanced amplifiers for signal dividing and combining over a broad frequency bandwidth. The original Lange coupler[1], shown in Figure 1, is not symmetrical with respect to either the X- or Y-axis. Some intrinsic phase deviation from the desired 90 [degrees] between two output ports is inevitable.2 This intrinsic phase deviation is usually ignored where the applications do not require exact amplitude and phase balance between the outputs, such as in broadband amplifier applications.

The original Lange coupler has a phase deviation of 3 [degrees] over the 2 to 4 Ghz frequency range when the input is at port 1. The 3 [degrees] phase deviation plus 1 dB of amplitude imbalance contributes less than 0.05 dB to overall amplifier gain and is usually ignored. Near perfect 90 [degrees] relation between output ports 2' and 3' can be obtained if the electrical lengths between 2 and 2' and 3 and 3' are predesigned to compensate for this intrinsic deviation.

In applications where the amplitude and phase balances between output ports are critical or where inputs are applied simultaneously, phase compensation techniques are invalid. Therefore, the intrinsic phase deviation of the Lange coupler must be minimized. These applications include the use of couplers in a Butler network for a monopulse radar receiver or the use of the coupler to extract the sum ([Sigma]) and delta ([Delta]) information in a direction finding related signal processing receiver, such as the one shown in Figure 2, where the [Sigma]/A ratio is critical.2

The coupler layout of Figure 3 exhibits an intrinsic phase deviation of less than 1.50 over the 2 to 4 Ghz frequency band. The same coupler layout when scaled to the 8 to 10 Ghz frequency range shows an intrinsic phase deviation of 7 [degrees], which is greater than the design can tolerate.

Solution to the Problem

A close examination of coupler layout of Figure 3 reveals that the signal path between ports 1 and 3 has no interconnecting wires, while the path between ports 2 and 4 is wired. The different interconnecting techniques for different ports can result in different phase angles for input ports 1 or 4. The improved layout of the coupler reduces the phase angle difference. Figures 4 and 5 show the improved layout of the coupler. The coupler becomes symmetrical with respect to the Y-axis. The paths between ports 1 and 3 and 2 and 4 have identical construction for interconnections. The measured intrinsic phase deviation of the coupler is less than 2.5 [degrees] over the 8.5 to 9.5 Ghz frequency range with a return loss and isolation better than 15 dB. The same coupler used in Figure 2 for [Sigma]/[Delta] and A extraction shows that the ratio [Sigma]/[Delta] is greater than 25 dB over the 8.5 to 9.5 Ghz frequency range.

References

[1.] J. Lange, "Interdigitated Stripline Quadrature Hybrids," IEEE Trans on MTT, Vol. 17, Dec. 1969, p. 1150--1151. [2.] C. Ho and L Moser, "Symmetrical Coupler Reduces Phase Error," Microwaves, April 1981, pp. 82-84.

Chen Y. Ho received his PhD from Rice University in 1972. Currently, he is president of Ho Microwave Lab, an independent consultant for microwave components, and subsystem design and prototyping. Additionally, he is a faculty associate at Arizona State University, where he teaches courses in microwave circuit design. Ho has been involved in the microwave industry for over 20 years, during which time he has worked for Microelectronics Technology Inc., M/A-COM, Motorola-GEC and Collins Radio.

Frankie Wong received his BA degree from the University of California, Berkeley in 1978 and his MSEE degree from Santa Clara University in 1985. Currently, Wong is a senior design engineer at CTT Inc., where he is engaged in the design of monopulse DF radar receiver front ends. His recent work has been the development of a a5 to 20.Ghz hybrid distributed amplifier Prior to CTT Inc., he was with Ferretec Inc., where he was engaged in the design of YUG-tuned oscillators and 2 to 20 Ghz hybrid distributed amplifier designs.
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Author:Ho, Chen Y.; Wong, Frankie
Publication:Microwave Journal
Date:Jun 1, 1992
Words:697
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