2 ANALOG-TO-DIGITAL CONVERTERS
The ADC is a key component in any radio that uses direct digitization of the RF input signal or
that uses digitization after an initial downconversion to an IF. The other key component is the
digital signal processor (discussed in Section 3).
2.1 Sampling Methods and Analog Filtering
The sampling process is critical for radio receivers using digitization at the RF or IF. The content
of the resulting sampled signal waveform is highly dependent on the relationship between the
sampling rate employed and the minimum and maximum frequency components of the analog
input signal. Some common sampling techniques that utilize a uniform spacing between the
samples include sampling at twice the maximum frequency, oversampling, quadrature sampling,
and bandpass sampling (also called downsampling or direct downconversion). Sampling
techniques with nonuniform spacing between the samples do exist but they are not widely used
and therefore are not considered in this report.
When a continuous-time analog signal is sampled uniformly, the spectrum of the original signal
F(f) is repeated at integer multiples of the sampling frequency (i.e., F(f) becomes periodic). This
is an inherent effect of sampling and cannot be avoided. This phenomenon is shown graphically
in Figure 1. Figure 1(a) shows the spectrum of the original analog signal F(f). Figure 1(b) shows
the spectrum of the sampled signal F
s
(f) using a sampling rate of f
s
=2f
max
.
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