et correction SO that I/Q imbalances in the alternately with a traditional mixer or modulator to the transmit signal chain can be minimized. Other desired desired IF. This traditional technique is being replaced by features include data rate interpolation and I/Q phase digital means because of the added flexibility offered correction. Driver Nyquist Filter Antenna Filter Input Input Input Tuning Control Loops Input Figure 2.3-8 Multi-channel transmit with single up-convert super-heterodyne. CHAPTER 2.3 Software defined radio Modulator Baseband Band Filter Filter Antenna Input Tuning Control Loops Figure 2.3-9 Single-carrier direct-conversion transmit. Finally, power gain is achieved through a pre-amp and in the digital domain, the possibilities are limitless if the PA. Aside from the fact that these devices must operate functions are configurable through software. Even where across a wide range of frequencies, it is desirable to adjust complex functions are implemented in the analog the RF output power. There could be regulatory issues domain, various errors such as quadrature errors can that require some frequencies to be transmitted at lower cause performance issues. power than others. While the PA gain is usually fixed, the In the digital domain, quadrature and other functions pre-amp may be in the form of a VGA. can be exactly generated. Once in the digital domain, the accuracy of the function is limited only by the bit pre- cision of the math used to implement the function. For Implementation issues example, it is always much easier to add a few more bits to a digital multiplier than it is to improve the linearity of The reality is that without improvements in semi- an analog mixer. conductor technology through the late 1990s, SDR as Finally, because most of the signal chain is digital, outlined here would still be a concept and not a reality. performance would be more consistent for each system Although the evolution of DSP technology has certainly manufactured, eliminating much of the product variation bee