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RF Electronics Chapter 5: Frequency Mixers Page 118 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. at the IF frequency and thus increasing the conversion loss. In the following designs a 20 MHz corner frequency is chosen, allowing baseband signals up to 5 MHz to be used, with <0.02 dB loss. To obtain the best impedance looking into port 1, series elements are required to connect to port 1. A Butterworth high pass and low pass filter design is a good starting point and optimisation to provide a low S11 return loss, as is done in example 2.2 and figures 2.8 to 2.15 can be used to improve the impedance match for the mixer diodes. This S11 optimisation results in a 0.575 dB improvement in conversion loss for the single diode mixer and 0.2365dB improvement in conversion loss for the balanced mixer, compared with the Butterworth filters. These optimised values are used in the single diode and balanced mixer circuits of figures 5.13, 5.22 and 5.25. Figure 5.13. Circuit diagram of a single diode mixer as a down-converter. Figure 5.13 shows the circuit diagram of a single diode mixer. The transformer is normally not included in the circuit, since in this case it is simply a non-inverting transformer. It is included here to illustrate the differences between the single diode mixer of figure 5.13 and the balanced mixer of figure 5.25. The mixing behaviour of a single diode mixer can be demonstrated by considering the current flowing through the diode of figure 5.13. The current can be expressed as: 5 5 4 4 3 3 2 2 1 0 a a a a a a V a V a V a V a V a a I Eqn. 5.7 If two voltages V a and V b are now applied to the diode, due to the LO and the RF signals, then the current will be 5 5 4 4 3 3 2 2 1 0 ) ( ) ( ) ( ) ( ) ( b a b a b a b a b a a V V a V V a V V a V V a V V a a I Eqn. 5.8 The term ) 2 ( ) ( 2 2 2 2 2 b b a a b a V V V V a V V a contains the required b a V V a 2 2 , which results in the sum and difference frequencies. All the other terms cause unwanted frequency components. It is thus desirable to use diodes with an I-V characteristic where a 2 is large in comparison with the other terms in the binomial expansion of the I-V characteristic. Some of the additional frequency components due to the a 3 , a 4 and higher order diode nonlinearities, can fall close to the desired frequency thus causing an interference. RF Electronics: Design and Simulation 118 www.cadence.com/go/awr