ADC and DAC Tradeoffs This chapter introduces the relationship between ADCs, DACs, and software radios. Uniform sampling is the process of estimating signal amplitude once each T seconds, sampling at a consistent frequency of fs = 1=T Hz. Although s s there are other types of sampling, SDRs employ uniform-sampling ADCs. | Software Radio Architecture Object-Oriented Approaches to Wireless Systems Engineering Joseph Mitola III Copyright @2000 John Wiley Sons Inc. ISBNs 0-471-38492-5 Hardback 0-471-21664-X Electronic 9 ADC and DAC Tradeoffs This chapter introduces the relationship between ADCs DACs and software radios. Uniform sampling is the process of estimating signal amplitude once each Ts seconds sampling at a consistent frequency of fs 1 Ts Hz. Although there are other types of sampling SDRs employ uniform-sampling ADCs. I. REVIEW OF ADC FUNDAMENTALS Since the wideband ADC is one of the fundamental components of the software radio this chapter begins with a review of relevant results from sampling theory. The analog signal to be converted must be compatible with the capabilities of the ADC or DAC. In particular the bandwidths and linear dynamic range of the two must be compatible. Figure 9-1 shows a mismatch between an analog signal and the ADC. For uniform sampling rate fs the maximum frequency for which the analog signal can be unambiguously reconstructed is the Nyquist rate fs 2. The wideband analog signal extends beyond the Nyquist frequency in the figure. Because of the periodicity of the sampled spectrum those components that extend beyond the Nyquist frequency fold back into the sampled spectrum as shown in the shaded parts of the figure thus the term folding frequency . This is well known as aliasing 274 275 . Although some aliasing is unavoidable an ADC designed for software-radios must keep the total power in the aliased components below the minimum level that will not unacceptably distort the weakest subscriber signal. Figure 9-1 Aliasing distorts signals in the Nyquist passband. 289 290 ADC AND DAC TRADEOFFS A. Dynamic Range DNR Budget If acceptable distortion is defined in terms of the BER then dynamic range DNR may be set by the following procedure 1. Set BERthreshold from QoS considerations 2. BER f MODULATION CIR FEC 3. BER BERTHRESHOLD CIR cirthreshold from f 4.
