AWR eBooks

RF Electronics Chapter4: Transmission Line Transformers and Hybrids Page 89 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. made using conventional Microstrip lines using double sided PCB with the bottom side a ground-plane. This is easy to manufacture, but places restrictions on the amount of coupling that can be obtained. Typically, the spacing between the lines is very small and this spacing must be accurately controlled along the entire line length to obtain the correct amount of coupling. Edge coupled lines are very suitable for sampling the forward and reverse power from transmitters and for filters. The design process of a backward-coupled hybrid using edge-coupled lines is illustrated as follows: Figure 4.40. Calculated geometry for a /4 20 dB coupler. Example 4.3: 20 dB Coupler A 20 dB coupler at 900 MHz is required. The coupled output is thus 20 dB below the input. The coupled voltage is one tenth of the input voltage, i.e. C = 0.1. From equations 4.21 and 4.22 one obtains Z OO = 45.23 and Z OE = 55.28 . The required physical spacing can now be determined iteratively using either the old TXLine program, accessible from Tools TXLine or using the Transmission Line Calculator, which can be obtained by right clicking on a MLIN, MCLIN, SLIN, SCLIN, S1LIN, CPW1LINE, RWG_TEmn, COAX, or COAXC element and selecting synthesize. That then opens up the Transmission Line Calculator, as shown in figure 4.40. Entering the above even and odd mode impedances and other physical parameters, then checking the boxes next to the variables that need to be calculated. Clicking on the directional arrows, calculates the target values. Note that for the same line length, the electrical length is different for the even and odd mode, since the propagation velocity of these modes is different. These calculated values are substituted in the coupled-line circuit diagram shown in figure 4.41. Figure 4.42 shows the corresponding PCB layout including 50 lines needed to connect the input and output ports to the coupler. Figure 4.43 shows the simulated performance. The coupling is 20 dB as expected. Note that the coupled port is in a different location compared with the Branchline coupler. The coupler was designed for a centre frequency of 1 GHz. Coupled lines have a null in coupling at multiples of a half wavelength length. This was used to tune the length of the coupled line to give a null at 2 GHz, thereby giving maximum coupling at the required 1 GHz. Note that the bandwidth of the coupling and the isolation is very wide compared with the other hybrids described before in these notes. At 1 GHz, the isolation (S 31 ) is only 5 dB less than the coupled output (S 41 ), so that further design changes are required to improve its performance. RF Electronics: Design and Simulation 89 www.cadence.com/go/awr

• Cover