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RF Electronics Chapter 5: Frequency Mixers Page 115 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. characteristics more to ensure that the a 2 coefficient in the binomial expansion of the diode I-V characteristic shown as equations 5.1, 5.2, 5.7 and 5.8 is maximized in relation to the other terms, thus minimizing the unwanted components. Figure 5.6 by Bert Henderson shows how diodes can be arranged in different ways [3], to obtain these different mixer LO levels. These diode models can be implemented in AWDE to investigate the different mixer LO levels. Example 5.1: Mixer LO Level Calculation A maximum RF input signal of -10 dBm and a level of third order intermodulation products (3IM) of 60 dB below the desired signals is required for a specific application. For a typical 7 level mixer, the IIP3 point is +17 dBm. The RF signal at -10 dBm is thus 27 dB below the IIP3 point. This will result in the 3IM signals being (3-1)*27= 54 dB below the wanted signal. Similarly, a level 10 mixer will result in the 3IM signals being (3-1)*30 = 60 dB below the wanted signals, a level 13 mixer will result in the 3IM signals being (3-1)*33 = 66 dB below the wanted signals. A level 17 mixer will result in the 3IM signals being (3-1)*37 = 74 dB below the wanted signals. A level 23 mixer will result in the 3IM signals being (3-1)*43 = 86 dB below the wanted signals and a level 27 mixer will result in the 3IM signals being (3-1)*47 = 94 dB below the wanted signals. The higher the LO level, the higher the cost of the mixer. A level 10 mixer with thus be the lowest cost device, which will satisfy the specifications. Ideal Multiplier An ideal multiplier is a perfect mixer and its performance is a good measure for the evaluation of the different mixers in the following sections. To enable the comparison to be made, the same input conditions are used for the multipliers and all the mixers in the following sections. Figure 5.7 show the circuit schematics for ideal multipliers for down- conversion (left) and up-conversion (right). Figure 5.7. Schematics for Multipliers as mixers. If V A is the LO voltage, A is ( LO t) , V B is the IF voltage and B is ( IF t), the multiplication process will generate sum and difference frequencies, as shown in equation 5.6. � � � � � � � �� � � � � � � � � � � � � � � Eqn. 5.6 From equation 5.6 the conversion loss is proportional to the LO power level, resulting in LO AM noise appearing in the IF or RF output signals. That is not desirable. A conversion loss that is not dependent on the LO power level will produce cleaner IF or RF signals. RF Electronics: Design and Simulation 115 www.cadence.com/go/awr

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