Issue link: https://resources.system-analysis.cadence.com/i/1325428
RF Electronics Chapter4: Transmission Line Transformers and Hybrids Page 93 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Figure 4.46 shows a photograph of the Microstrip coupler. To provide suitable shielding and allow for the resistors, capacitors and BNC connectors to be soldered to the PCB, large earth pads are incorporated. Such an earth-plane also allows the circuit to be mounted in a metal box, without affecting the performance. That top ground-plane needs to be solidly connected to the bottom ground-plane, using lots of vias or pins. This is commonly done in commercial designs and figure 7.67 shows a commercial design from Codan [9], using many vias, connecting the top and bottom ground planes. The measured performance closely matches the simulated performance of figure 4.47. The spacing between the vias or pins needs to be a small fraction of a quarter wavelength at the highest operating frequency of the circuit. Figure 4.47. Photograph of the Microstrip coupler. Edge-coupled Backward travelling wave hybrids as shown in figure 4.46, are the best choice if a wide bandwidth and low coupled outputs (-20 dB) and good front to back ratios are required. Lange Coupler For a 3 dB coupler, C = 0.707 and Z OE = 120.7 and Z OO = 20.7 . For edge coupled Microstrip lines, this requires a coupling gap far smaller than 0.1 mm, which is impossible to make. To increase the coupling, many lines can be used in parallel. This is called a Lange Coupler. Figure 4.48 shows a layout of a Lange coupler. At the middle, a bonding- wire is used to connect the upper track end to the middle track and the lower other track end, thus keeping those transmission lines in parallel. Similarly, the track ends at the edges of the coupler are connected to the other appropriate lines. One will thus have the top left port connected by a set of parallel lines to the bottom right port. The top right port is connected by a set of parallel lines to the bottom left port. Typically, the spacing between the tracks and the track widths are of the order of 0.25 mm, which is close to the smallest widths that can conveniently be made using conventional processes. RF Electronics: Design and Simulation 93 www.cadence.com/go/awr