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The theoretical relationship for a rat-race coupler is that the outputs should be -3dB relative to the input power, and separated by 180of phase. To accomplish this, the four feed lines are separated by 90electrically, with a 270segment closing the loop. This is illustrated in the layout shown.

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Microwave Office does not have a layout associated with the MRRCOUP element, also known as a rat-race coupler. The difficulty with building this layout using regular circuit elements is in "closing the loop", or getting the ring to close with a good layout connection. This example shows a symmetrical loop with a parameterized approach that involves only one variable for tuning the frequency. The user specifies the width of the 50W lines feeding the structure and the lines that make up the ring in Global Definitions. These are easily calculated as a simple function of the substrate parameters, or using a transmission line tool like TXLine. In this example, we are working on 0.010" Al2O3.

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Once the impedances are set, all that remains is to set the frequency. The only adjustable parameter is R90, or the radius of the 90segments. Once this is set, the coupler is fully defined, because the 270segment is a mirror image of the three 60segments. The results indicate the phase difference in red, calculated using the SDeltaP measurement.

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The W and R90 global variables are setup to optimize. You can change the simulation frequencies and optimization goal ranges to optimize this coupler to a different frequency range.

Schematic - Rat_Race

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Schematic Layout - Rat_Race

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Graph - Magnitude

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