3.1.2 Direct Conversion Transceivers

Chapter 3.1.2 Direct Conversion Transceivers

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 45the IF, and possibly some subset of the original adjacent channels depending on the quality of the filter used. Usually, automatic gain control (AGC) amplifiers are also included at the IF. They adjust the gain of the radio so that its output amplitude is always constant. Once through the AGC, the signals are downconverted a second time to baseband (the signals are now centered around DC or zero frequency). This second downconversion requires a second frequency synthesizer that produces both 0° and 90° output signals at the IF frequency. Two mixers downconvert the signals into an in-phase (I) and quadrature phase (Q) paths. By using two separate phases, both amplitude and phase information can be recovered, and as a result, the incom-ing phase of the RF signal does not need to be synchronized to the phase of the LO tone. The I and Q signals are then passed through baseband filters which removes the rest of the unwanted channels. Finally, the signal is passed through an analog-to-digital converter and into the back end of the radio. There may be additional, possibly programmable, gain stages in the baseband. Further signal processing is performed in the digital signal processing (DSP) circuitry in the back end of the radio.The transmitter works much the same way except in reverse. The DSP circuitry followed by the digital-to-analog converters (DA) produces signals in quadrature. These signals are then filtered and upconverted to an IF frequency. The Tx will usu-ally have some AGC function, which may be either in the baseband or the IF stage. The IF signal is upconverted to the RF frequency by the mixer. If the LO is low-side injected, the mixer is used to generate sum rather than difference products. Thus, for low-side injection, the RF frequency is given by: RFLOIFfff=+ (3.3)If the LO is high-side injected, the frequency of the RF signal is given by: RFLOIFfff=- (3.4)Once upconverted to RF, the signal is passed through a power amplifier to increase the power of the signals and is then radiated by the antenna into the air. In the RF section the PA itself may have a power control function or additional AGC. If the power level is constant, it must be high enough so that the signal can be detected at the maximum distance dictated by the system specifications. The mixer will produce signals on each side of the LO. The RF BPF is needed to filter the signal so that only the sideband at the desired RF transmit frequency is passed to the antenna. The BPF can also be used to remove LO feedthrough from the mixer.3.1.2  Direct Conversion Transceivers A direct downconversion radio architecture is shown in Figure 3.3. In this ar-chitecture, the IF stage is omitted and the signals are converted directly to DC as shown in Figure 3.4. For this reason the architecture is sometimes called a zero-IF radio. The direct conversion transceiver saves the area and power associated with a second synthesizer. No image filter is required and the LO frequency selection becomes trivial. However, generating I and Q signals from a synthesizer at higher