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RF Electronics Chapter 5: Frequency Mixers Page 146 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. The Branchline coupler can be used to produce the required phase shifts for efficient mixing. If the LO signal is applied at port 2 of the Branchline coupler in figure 5.56 and the RF signal is applied to port 1. The frequency of the Branchline coupler is chosen such that full isolation is obtained at the RF port for the LO signal, since the LO signal is much bigger in power than the RF signal. For a practical down-converting mixer, the LO and RF signals are within 10% of each other, so that reasonable isolation will be obtained for the RF signal at the LO port. The signal at the junction of TL2 and TL3 will then be LO90 + RF180, giving a phase difference between the LO and RF signals of -90. The signal at the junction of TL4 and TL3 will be LO 180 + RF90 giving a phase difference +90. These are the correct conditions for obtaining balanced mixing in a down-converter. Basic Operation Figure 5.56. Basic circuit diagram of a Branchline coupler balanced mixer. To illustrate the principles of operation, a basic Branchline balanced mixer is shown in figure 5.56. The Branchline hybrid consists transmission lines of TL1 to TL4. In figure 5.56, all electrical lengths are in degrees at the reference frequency Fr. Initially the frequency of Fr is the same as the LO frequency. Fr can be tuned to optimise the performance of the mixer. All the lines are of the Branchline coupler are one-quarter wavelength long at Fr. Correct biasing for the diodes must be provided, such that all the RF and LO energy is dissipated in the diodes and all the resulting low frequency energy is passed to the IF port and is not reflected back into the RF or LO ports. TL5 and TL6 are short circuited transmission lines, providing no load to the RF and IF signals, but providing a low impedance to ground so that DC and IF currents can flow easily. Transmission lines TL7 and TL8 are open circuited lines, one quarter wavelength long at the Fr frequency, and will this provide a low impedance to ground at the LO and RF frequencies and a high impedance at IF frequencies. All the RF and LO energy coming from the Branchline coupler is thus dissipated in the diodes. The size of capacitor C1 is chosen to act as a short circuit to the RF and LO signals but have little effect at the IF frequency. Line TL11 is a quarter wavelength long at Fr, so that C1 causes a high impedance at the LO/RF frequencies and little RF energy goes to the IF port. Lines TL9 and TL10, with the high impedance at the junction with TL11, are half a wavelength long. The short circuit at the Fr frequency due to TL7 will reflect as a short circuit and reinforce the short circuit due to TL8 at diode 2 and vice versa. TL9 to TL11 will this ensure that the IF energy from the diodes is passed to the IF port as much as RF Electronics: Design and Simulation 146 www.cadence.com/go/awr

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