AWR eBooks

RF Electronics Chapter4: Transmission Line Transformers and Hybrids Page 83 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Figure 4.29 shows the performance of the Branchline coupler. This coupler is a convenient structure with low impedance values (50 and 35.36 ) thus allowing low loss hybrids to be made using Microstrip-line circuits. The output at the coupled port has a 90 phase shift compared with the output at the direct port. That is useful for applications such as IQ up or down conversion. To realise the design as a printed circuit board, the required track widths and lengths need to be calculated. The Roger's RO4003 substrate [5] that was used for the circuits in figures 4.9 and 4.10 is also used in this example. The parameters can be obtained by using the Libraries *AWR Web Site Rogers RO RO4003 and select 32 mil, 1 oz. and place that as a component in a circuit. Remember to set Er = 3.55 and ErNom = 3.38 to match Er with the substrate data sheet [5] as described earlier. To determine the length and width required, TX line from the Tools menu in CADENCE AWR DE can be used or measurements on some simple test circuits can be made. Figure 4.25 shows the resulting test circuit. W50, the width of a 50 track and W35, the width of a 35.36 track are made as variables, so that those values can be used in the rest of the design. The line between port 1 and 2 is very long. W50 is determined by observing S 11 and tuning S50 to be less than -60 dB over the required frequency band. Similarly, W35 is obtained by tuning it and minimising S 33 . To determine the length of the quarter wavelength long lines, the circuit between ports 5 and 6 is used. The circuit uses a 50 line between ports 5 and 6 and a 35.36 line as a stub. Thus, TL4 and TL6 in figure 4.30 correspond to TL1 and TL2 in figure 4.28 respectively. (For a 3 dB coupler Zy=50 and Zx=35.36 ). To obtain accurate results, elements incorporating EM models, such as MTEEX and MLEFX are used in this circuit. The MLEFX element, with the length of half a 50 track-width is used to place the end of the quarter wavelength stub on the middle of the 50 track that is to join as shown in figure 4.30, to produce the complete coupler shown in figures 4.32 and 4.33. The length of the stub (L2) is tuned to place the notch at 1 GHz. A similar circuit is made to determine the required length of the 50 quarter-wavelength lines. Figure 4.30. Microstrip test circuits for determining length and width of lines. When the line lengths obtained from figure 4.30 are used in the circuit of figure 4.26, the centre frequency is slightly offset. Fine-tuning is achieved by tuning or optimising the This circuit is to determine the width required for 50 ohm and 35.36 ohm tracks. (port 1 and 3) Ports 5 and 7 allow the track length for a quarter wavelength to be determined. W35: 3.026 W50: 1.788 Z35: 35.36 MLIN ID=TL1 W=W35 mm L=4*L1B mm PORT P=6 Z=50 Ohm PORT P=5 Z=50 Ohm PORT P=4 Z=Z35 Ohm PORT P=3 Z=Z35 Ohm PORT P=2 Z=50 Ohm PORT P=1 Z=50 Ohm MLEFX ID=TL12 W=W50 mm L=W50/2 mm 1 2 3 MTEEX ID=TL9 W1=W50 mm W2=W50 mm W3=W35 mm MSUB Er=3.55 H=0.8128 mm T=0.035 mm Rho=0.7 Tand=0.0027 ErNom=3.38 Name=RO4003C MLIN ID=TL6 W=W35 mm L=L2 mm MLIN ID=TL5 W=W50 mm L=4*L2B mm MLIN ID=TL4 W=W50 mm L=L2B mm RF Electronics: Design and Simulation 83 www.cadence.com/go/awr

• Cover