Quite often the IF amplifier is in the form of an AGC. izers, receive signal processors (RSPs), or DDCs. The goal of the AGC is to use the maximum gain possible The final element in the SDR is the DSP. Since this without overdriving the remainder of the signal chain. is general purpose DSP, it can be programmed for any Software defined radio CHAPTER 2.3 required processing task. Typical tasks include equaliza- through digital logic and the availability of good, tion, detection, rake receiver functions, and even net- cost-effective digital-to-analog converters. As with the work interfacing, to name a few. Because they are fully related receive function, the purpose of this device is to programmable, they can be used for just about any signal shape the bandwidth of the desired channel and then processing task as well as controlling all of the features in up-convert by digital means to the desired IF frequency. the other elements of the block diagram. As DSP If multiple channels are required, they can be synthe- processing capabilities increase, DSPs may well take over sized on one chip. After translation, each of the channels many of the functions within the digital preprocessors. can be summed together and interpolated to the desired data rate and then sent to a DAC. If desired, digital Transmit pre-distortion can be added in conjunction with the DSP to correct for distortion later in the signal chain. Transmit functions are also based on some form of Either a mixer or a modulator is used for frequency super-heterodyne or direct conversion. Figures 2.3-8 and translation to the final RF frequency. If direct RF modu- 2.3-9 illustrate these two options. The multi-carrier lation employed, an RF modulator will be used. If an IF is option is best suited to single- and multi-carrier applica- used (either directly from a DAC or a traditional IF tions, while the direct conversion offers an excellent, up-conversion), a mixer will be used to translate to the final low-cost solution for single-carrier applications. As RF freq