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RF Electronics Chapter 5: Frequency Mixers Page 148 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Fr to 1611 MHz results in the lowest DC signal in the IF spectrum and gives a good conversion loss. For all the transmission lines, except TL1, the reference frequency label F0=Fr has been hidden to simplify the schematic. The impedance and length of the transmission lines can be tuned to optimise the design and gain a good understanding of their effect on the performance. For example, the Impedance of TL9 and TL10 have a significant effect on the conversion loss. The hardware shown in figure 5.60 was built, based on a design where Zc = 50 Ω and Zo = 36 Ω therefore that is shown in figure 5.56 and used in figure 5.57 and 5.58. However, changing Zc to 70.7 Ω and Zo to 50 Ω, reduces the conversion loss from 6.981 dB to 6.642 dB. Figure 5.58 shows the IF spectrum of the Branchline coupler balanced mixer. The spectral frequencies are shown in figure 5.55. The IF spectral components for the Branchline BM circuit are comparable to those of figure 5.55 for the transmission line balanced mixer. It is easy to optimise the circuit parameters of the circuit in figure 5.56, before a Microstrip realisation is produced. Figures 4.38 and 4.39 show that the amplitude and phase shift between the two output ports of the Branchline coupler is nearly constant over a 20% bandwidth. As a result, the bandwidth of the Branchline coupler mixer of figure 5.56 is wider than the bandwidth or the transmission line mixer of figure 5.53. Hardware Realisation Figure 5.59. Microstrip realization of a Branchline coupler balanced mixer. The basic block diagram of figure 5.56 is realised as a Microstrip circuit, as shown in figure 5.59. This circuit was based on a long-standing mixer design and produced by the author before AWRDE was available, and as a result, the impedance of lines TL7 to TL19 of this layout, do not result in the optimum conversion loss, that can be obtained by tuning the basic circuit of figure 5.56. Figure 5.60 shows the corresponding layout. Figure 5.60 shows the PCB layout corresponding to the circuit diagram of figure 5.59. The vertical transmission lines of the Branchline coupler are folded using bends, to reduce the size of the PCB. The green pads are the locations for the diodes and the capacitor. The short circuited quarter wave stubs are thin, corresponding to a high characteristic impedance to ensure as high an impedance over as wide a bandwidth around the LO and RF frequencies. The open circuited quarter wavelength stubs are wide transmission lines, providing a low shunt impedance for as wide a bandwidth as possible corresponding to as wide a bandwidth around the LO and RF frequencies, so that as much of the RF energy is converted to IF signals as possible. The layout is designed so that the position of C1 can be moved closer to or further away from the IF port, to optimise the performance. RF Electronics: Design and Simulation 148 www.cadence.com/go/awr

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