ChaptER 3 System Level Architecture and Design Considerations

Chapter ChaptER 3 System Level Architecture and Design Considerations

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

  • 4343c h a p t e r 3System Level Architecture and Design Considerations3.1   Transmitter and Receiver Architectures and Some Design Considerations 3.1.1  Superheterodyne Transceivers A block diagram of a typical superheterodyne radio transceiver using air as a medium is shown in Figure 3.1. Modulated signals are transmitted and received at some frequency by an antenna. If the radio is receiving information, then the signals are passed from the antenna to the receiver (Rx) part of the radio. If the radio is transmitting, then signals are passed to the antenna from the transmitter (Tx) part of the radio. Radios either transmit and receive at the same time (called a full duplex transceiver) or alternate between transmitting and receiving (called a half duplex transceiver). In a half duplex transceiver it is possible to put a switch between the antenna and the Rx and Tx paths to provide improved isolation, while in a full duplex transceiver the switch must be omitted and the two input filters have the sole responsibility of isolating the Tx and Rx paths without the aid of a switch. In the Rx path, the signals are first passed through a filter to remove interfer-ence in frequency bands other than the one of interest. The signal is then ampli-fied by a low noise amplifier (LNA) to increase the power in weak signals, while adding as little noise (unwanted random signals) as possible. This amplifier may or may not include some form of adjustable gain or gain steps. The spectrum is then further filtered by an image filter and then downconverted by a mixer to an intermediate frequency (IF). The IF frequency must be chosen with great care, taking into account many factors including interaction of spurs and mixing of LO harmonics. The mixer (also sometimes called a multiplier) mixes the incoming spectrum of radio-frequency (RF) signals with the output from the RF frequency synthesizer which is an accurate frequency reference generator also called a local oscillator (LO). The LO is tuned so that the frequency of the desired IF signal is always at the same frequency. The LO can be either low-side injected (the LO is at a frequency less than the RF frequency) or high-side injected (the LO is at a fre-quency greater than the RF frequency). For low-side injection, the IF frequency is given by: IFRFLOfff=- (3.1)