Unfolding and Folding of Architectures - Digital Design of Signal Processing Systems: A Practical Approach - page 346

Antenna

Processing

ADC

Sample
Rate
Conversion

SW
Algorithms ported
on DSP, GPP, or
FPGA as run-time
reconfigurable
modules

HW
DSPs,
FPGAs,
ASICs,
GPPs

Flexible RF
Hardware

Duplexer

DAC

Control

Figure 8.1 Typical software-defined radio architecture

X

f N

f

f

0

-f N f N f L f 0 f H

0

cos( 2 π f

0 t

)

Figure 8.2 Baseband signal mixed with a carrier for signal translation in the frequency band

at very high frequency is a difficult engineering problem, and a relaxation in the band results in cost-
effective designs. Although this relaxation results in adjacent channels in the filtered signals and a
wider bandwidth of the signal to be digitized, the adjacent channels are then easily filtered out in the
digital domain as shown in Figure 8.3(a).
In many designs the analog signal is first translated to a common intermediate frequency f IF . The
signal is also filtered to remove images and harmonics. The highest frequency content of this signal is
f IF þ B 0 /2, where B 0 is the bandwidth of the filtered signal with components of the adjacent channels
still part of the signal. Applying Nyquist, the signal can be digitized at twice this frequency. The
sampling frequency in this case needs to be greater than 2( f IF þ B 0 /2). The digital signal is then
digitally mixed using a quadrature mixer, and low-pass filters then remove adjacent channels from
the quadrature I and Q signals. The signal is then decimated to only keep the required number of
samples at base band. A typical receiver that digitizes a signal centered at f IF for further processing is
shown in Figure 8.3(b).
As the sampling frequency is so critical, further reduction in this frequency should be explored. In
this context, for cases where the rest of the spectrum of the band-pass received signal or the IF signal
is cleaned of noise, the signal can be directly sampled at a lower sampling frequency. This technique
sub-samples an analog signal for intentional aliasing and down-conversion to an unused lower
spectrum. The minimum sampling rate for band-pass sampling that avoides any possible aliasing of
a band-pass signal is [1]:

2 f L

K f s

f H
K

2

ð 8 : 1 Þ

1