away from the desired input, this problem can be averted. However, with DR = PADC. Fullscale G - 0.001 a multi-carrier receiver, the harmonics can cover a very wide band of frequencies. Generally, even-order har- - NF cascaded required (2.3.5) monics are not much of a problem. At very low signal Software defined radio CHAPTER 2.3 levels, the key harmonics are the third and fifth must be achieved before the spectrum is folded upon harmonics of the input spectrum. Since the input may be itself. a wide band, the third and fifth harmonics are 3 and 5 times as wide, respectively. Given this, it becomes Data conversion difficult to try and place these signals in a part of the spectrum where they will cause no problems. In cases For both transmit and receive, the data converters are where careful signal planning is not possible, dithering usually key components in the signal plan. The key ele- techniques provide relief to many of these problems ments for both are the dynamic range, which is bound on [12]. Figure 2.3-10 shows how it is possible to place one end by the noise floor of the converter and the max- some harmonics out of band if sufficiently oversampled. imum input or output range on the other. Other equally In this example, the second and third harmonics are important issues include distortion, both harmonic and placed in such a manner that the aliased components fall intermodulation. Although related, they have somewhat away from the desired fundamentals. different effects in the limitation of performance. There are many different converter topologies and each has their benefits and limitations [13, 14]. While Transition band requirements there is no set architecture that provides better perfor- Since all of the channel selection in an SDR is done in the mance for SDR, the selection is best made based on the digital domain, the analog filter requirements are differ- performance requirements for the application and then ent. Their purpose is primarily to prevent the overlap of a study of the available data she