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RF Electronics Chapter 3: Transformers and Hybrids Page 55 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. bottom transmission lines, the + and – signals are perfectly balanced at all frequencies. The corresponding hardware is shown in figure 3.8 Figure 3.7. Diagram of transformers with ferrites and transmission lines. The determination of the number of turns required for this transformer is the same as that for the bifilar or trifilar winding outlined above. The lower corner frequency is determined by the magnetising inductance. The upper corner frequency is determined by the losses in the ferrite. For the transmission-line transformer the winding length is no longer limited to be less than 10% of the wavelength, as both outputs have the same propagation delay through the transmission line. Typically, these cores are wound on low loss ferrites, giving them very high upper cut-off frequencies. Figure 3.8. TV Balun using Ferrite RF transformer and transmission line windings. Figure 3.8 shows a TV balun (Balanced to Unbalanced) transformer, corresponding to the circuit diagram in figure 3.7. This allows a typical dipole antenna, which has a balanced output of 300 impedance to be connected to a 75 coaxial cable, for a low loss connection to a TV. The transmission line balun is an ideal match at high frequencies and the ferrite provides the isolation at low frequency. By simply wrapping a coaxial lead around a ferrite or steel former, a non-inverting transformer, like figure 3.9, is obtained. This will provide isolation between both ends of the coaxial cable. Such a lead is useful in preventing earth loops for V measurements. A transformer, using transmission lines for the windings, can be used as an isolating transformer, by connecting opposite sides of the transmission line to ground as indicated in figure 3.10. The lower frequency of this transformer is determined by the magnetising inductance of the former. This can be increased by simply adding more cores or having the transmission line go through the hole in the core many times. RF Electronics: Design and Simulation 55 www.cadence.com/go/awr

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