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RF Electronics Chapter 5: Frequency Mixers Page 144 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Transmission Line Balanced Mixer At microwave frequencies, transmission lines are often used to produce two outputs with a 180 phase shift, instead of the transformers used in the balanced mixer of figure 5.25. The circuit for the corresponding microwave mixer is shown in figure 5.53. In this design, the mixer is used for a down-converter for a weather satellite receiver and uses a 1565 MHz Local Oscillator to shift a 1700 MHz RF signal to a 135 MHz IF frequency. The transformer in figure 5.25 provides balanced voltages to drive the diodes. For a 50 Ω impedance at the LO port, the diodes should be 100 Ω each. For an accurate comparison with the balanced mixer of figure 5.25, a 100 Ω impedance is also assumed for the diodes in figure 5.53. The half-wavelength long transmission line TL2 should thus have a 100 Ω impedance for the best impedance match bandwidth. The quarter-wavelength transmission lines TL3 and TL4 provide a DC path for the diodes and a high impedance at 1565 MHz for good mixer operation. Changing the line lengths or impedances of the transmission lines TL2 to TL4 does not have a significant impact on the mixer performance, as can be demonstrated by tuning those parameters in the simulation. For the RF port high pass and the IF port low pass filter, a reflectionless filter similar to the one shown in figures 5.11 and 5.12 is used. A lower crossover frequency gives smaller spectral components at the IF output but a slightly higher conversion loss. For the mixer of figure 5.53, a 480 MHz crossover frequency, being close to the geometric mean of the RF and IF frequency is used. Figure 5.53 Circuit diagram of a transmission line balanced mixer. The conversion loss for this mixer and for the mixer of figure 5.25 both operating at the same RF and LO frequency and with the same RF and IF crossover filter is shown in figure 5.54. The simulation for the transmission line balanced mixer of figure 5.53 shows 0.224 dB less conversion loss than for the corresponding transformer-based mixer. In practice, the transmission lines can be made with very little loss while a transformer operating at 1.5 GHz will have an appreciable loss. Figure 5.55 shows the spectrum for the mixer as a down-converter. The mixer performs well. The RF frequency components in the IF spectrum can be attenuated more by changing the 2 nd order RF-IF diplexer to a 4 th order diplexer similar to the one in figures 2.13 to 2.15. RF Electronics: Design and Simulation 14 4 www.cadence.com/go/awr

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