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RF Electronics Chapter4: Transmission Line Transformers and Hybrids Page 91 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. is an interaction between the maximum coupling, its attenuation and the return loss, the line length, line spacing and line width tuning should be repeated until no further changes occur. These values can also be determined using the optimisation routines included in MWO. Note that in figure 4.43, the difference between the coupled port S 41 and the isolated port S 31 is 5 dB at the centre frequency of 900 MHz. By including some very small, sub-pF, capacitors across the terminals 1 and 4 and terminals 2 and 3 the phase velocity is more equalised over a wide frequency range and a lower input return loss (S 11 ) and flatter isolation (S 31 ) is obtained. These capacitors can be obtained as pads on the output transmission lines, or they can be realised using small gaps, using the MGAP2 element. In figures 4.41 and 4.42 the forward signal at the input port 1, passes to the output port 2 with a small loss and couples to port 4 with a desired attenuation. Ideally little power should appear at the isolated port 3. The coupler is symmetrical, so if the output is connected to a load then the reflected signal due to any load mismatch will then appear at the port 2 and couple to port 3. Once can thus also call the coupled port in figure 4.44, the Forward port and the Isolated port the Reverse port. To accurately measure the reflected power, the power at the Reverse port due to the input, must be much smaller than the power from the reflected signal coupled into the Reverse port. One application of the backward travelling wave coupler is in the measurement of power at the output of a device, such as a signal generator or a transmitter. Typically, a coupled signal –20 dB below the forward or reverse power is required. The forward power is the transmitter output power and the reverse power is the reflected signal that occurs if the load for the transmitter is mismatched. At least a 20 dB front to back isolation is required, to accurately measure forward and reflected power. The hybrid will thus require a coupled output of –20 dB and an isolated signal < –40 dB below the input. As can be seen from figure 4.43, the signal at the isolated port (S 31 ) is too large when a quarter wavelength long coupler is used. A suitable coupler with a good forward to reverse ratio can be obtained by making the length of the coupler much smaller than a quarter wavelength and using capacitors shunting the forward and reverse coupled outputs to produce a constant frequency response over the required frequency band. Figure 4.44. Circuit diagram of Microstrip coupler. RF Electronics: Design and Simulation 91 www.cadence.com/go/awr

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