9.2.4 Mixer Stage
The mixer combines the incoming RF signal and the LO to produce new frequencies at the sum and difference of the two inputs. The difference frequency becomes the IF. Ideally only this product is passed, but nonlinearities also produce harmonics and cross-terms. Although the carrier and sideband frequencies are translated from RF to IF, the shape of the waveform remains the same, as shown in Figure 9.12.
Depending on design, a mixer may exhibit conversion loss (passive mixers) or conversion gain (active mixers). Conversion characteristics influence overall noise figure and dynamic range. Mixer performance depends on conversion gain, linearity, port isolation, and suppression of spurious products. Modern designs use double-balanced diode mixers, FET mixers, or active Gilbert-cell mixers to minimize undesired responses. In SDR architectures, mixing is frequently performed digitally after down-conversion by high-speed ADCs.
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