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RF Electronics Chapter 3: Transformers and Hybrids Page 54 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. To construct a bifilar of trifilar winding, take the required number of wires, in parallel, put one end in a vice and the other in the chuck of a hand drill and turn the operate the drill until the required amount of twisting has been done. The inter-winding capacitance can be controlled by selecting the insulation thickness on the wires. Winding self- capacitance can be minimised by using a single layer winding. Figure 3.5 shows a typical bifilar and pentafilar transformer using single layer windings. Such transformers typically have a 3 decade frequency bandwidth. Figure 3.5. A trifilar wound RF Transformer (left) and a pentafilar transformer (right). For instrumentation grade double balanced mixers a very good balance in the output signals is required, then an additional core, as shown in figure 3.6, can be added [3]. This provides winding 1 with a balanced input, thus causing the impedances at 2S to be exactly the same as those at 3F. Often the cost of the additional core is not justified. Figure 3.6. Balun with improved output balance. Transmission Line Transformers with Ferrite Cores At high frequencies, the primary and secondary windings are sometimes wound using transmission lines, as shown in Figures 3.7. This then uses inter-winding capacitances to use capacitive coupling at high frequency and magnetic coupling at lower frequency thus extending the bandwidth, significantly above what can be obtained using the construction technique of figures 3.4 and 3.5. For the 75 system, shown in figure 3.7, the characteristic impedance for the transmission lines used for the windings are 150 . There is thus no reflection at the junction, where the two lines are connected in parallel. The two transmission lines are in series at the right hand side and the balanced output then form a 300 system. Since the same propagation delay occurs through both the top and RF Electronics: Design and Simulation 54 www.cadence.com/go/awr

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