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RF Electronics Chapter 5: Frequency Mixers Page 137 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Mixer terminations As shown in figure 5.37, for down conversion, it is normal to remove the RF and LO frequency components at the IF port, as they can cause intermodulation distortion in the subsequent IF circuits. Mini-Circuits application note AN-75-007 [4] shows that mixer improvement can be obtained by using reflectionless filters at all ports of a mixer and thus provide a near ideal termination to the mixer. Figures 5.17 and 5.26 show that using reflectionless filters instead of Butterworth High pass and Low pass filters for the IF/RF diplexer improved the conversion loss of the mixer. To illustrate this for a DBM, different types of filters are added to the basic DBM of figure 5.31, as shown in figure 5.42. The reflectionless filters used are the same as shown in figure 5.7 and provide a better than 120 dB return loss for the mixer ports. Like figures 5.11, 5.13 and 5.22, the corner frequency of the filter was 20 MHz, so that LO and RF signals are severely attenuated at the IF port. A comparison was made between the performances of the different DBM terminations. DBM1: Basic DBM of figure 5.31, no filters at the ports. DBM2: A DBM with reflectionless filters added to all ports, as shown in figure 5.42. DBM3: A DBM with a reflectionless filter only added to the IF port. DBM4: A DBM with a second order low pass filter at the IF port, resulting in a high impedance at the mixer transformer at high frequencies as shown in the left of figure 5.43. DBM5: A similar configuration but resulting in a low impedance at high frequencies as shown in the right of figure 5.43. Figure 5.42. Circuit diagram of a DBM with reflectionless filters (DMB2). Figure 5.43. DBM4 with Inductor First (L) and DDM5 with Capacitor First (R). RF Electronics: Design and Simulation 137 www.cadence.com/go/awr

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