ta converters are divided into two main categories, namely, Nyquist rate and oversam- pling. In the Nyquist rate type, the input occupies a large fraction of the available band- width (Nyquist zone), whereas in the oversampling type, the input occupies only a small fraction of the Nyquist zone, simplifying the antialiasing filter (AAF) design and lowering the quantization noise at the expense of higher sampling rate (see Fig. 5.22). Ideal sine wave (time domain) Ideal sine wave (frequency domain) Frequency Ideal DAC output (time domain) Ideal DAC output (frequency domain) fout Images from first Nyquist zone (amplitude determined by sinc function) Zone 1 Zone 2 Zone 3 Zone 4 Nyquist zones Sine waveform sampled at DAC clock rate Frequency Figure 5.21 Example illustration of Nyquist zones [15]. 712 Chapter 5 Sampling at Nyquist rate Oversampling Vinput Anti-aliasing transition band Vinput Anti-aliasing transition band f B f 12 42 / (12/B) 42 / 12 42 / (12/3') A² 2 /12 B = fs S / Oversampling factor Oversampling factor f B f 12 f 12 S A : ADC step size (the quantization error is confined between +//2 and is zero at mid-step) Figure 5.22 Illustration of Nyquist rate and oversampling data conversion [15]. 5.5.2 Direct RF Sampling Transmitter Design Direct digital domain to RF domain conversion with a high-speed RF DAC is a technology disruption for BS transmitters. The RF DAC partition uses direct digital synthesis to move the quadrature modulator, local oscillator (LO), and signal filtering analog functions into the digital domain (see Fig. 5.23). The RF DAC with direct digital synthesis partition capi- talizes on the fact that digital processors scale better than their analog counterparts in terms of lower power consumption, faster speed, smaller die area, and lower cost. The RF DAC is the enabling technology that makes this possible because it bridges the digital-to-analog domains. An RF DAC is generally characterized as a mixed signal device that operates in multiple Nyquist zones with conversion rates above 1.5 GS