Low IF Transceiver and Other Alternative Transceiver Architectures

Chapter Low IF Transceiver and Other Alternative Transceiver Architectures

Radio Frequency Integrated Circuit Design Second Edition Book
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Radio Frequency Integrated Circuit Design Second Edition Book

  • 3.1 Transmitter and Receiver Architectures and Some Design Considerations 473.1.3  Low IF Transceiver and Other Alternative Transceiver ArchitecturesOne alternative architecture is called a low IF transceiver. The low IF architecture sees most of its advantage on the receive side. While this architecture is conceptu-ally the same as a superheterdyne radio, the low IF is chosen so that it is practical to digitize the signal at the IF and thus perform some of the traditionally analog signal processing in the back end of the radio. The main problem with a low IF receiver is that the image frequency will now be very close to the desired sideband. However, if the signal is then digitized, it is easier to employ a high Q digital filter to remove the image or alternatively to make use of an image reject mixer. Image reject mixers will be studied in Chapter 8. An example of a low IF receiver is shown in Figure 3.5. Another architecture that is a compromise between the superheterodyne and the direct-conversion transceiver is called a walking IF architecture shown in Fig-ure 3.6. This architecture derives the IF LO by dividing the RF LO by some fixed number. As a result, the IF frequency is not fixed but “walks” in step with a fraction of the frequency of the RF LO. This transceiver with walking IF still has many of the advantages of the superheterodyne radio (although it is not possible to filter as Figure 3.5  A low IF receiver. Figure 3.6  A walking IF radio architecture.