AD9042
D IGITAL WID EBAND RECEIVERS
Intr oduction
is used for demodulation, different routines may be used to
demodulate different standards such as AM, FM, GMSK or any
other desired standard. In addition, as new standards arise or
new software revisions are generated, they may be field installed
with standard software update channels. A radio that performs
demodulation in software as opposed to hardware is often
referred to as a soft radio because it may be changed or modified
simply through code revision.
Several key technologies are now being introduced that may
forever alter the vision of radio. Figure 49 shows the typical
dual conversion superheterodyne receiver. T he signal picked up
by the antenna is mixed down to an intermediate frequency (IF)
using a mixer with a variable local oscillator (LO); the variable
LO is used to “tune-in” the desired signal. T his first IF is
mixed down to a second IF using another mixer stage and a
fixed LO. Demodulation takes place at the second or third IF
using either analog or digital techniques.
System D escr iption
In the wideband digital radio (Figure 50), the first down
conversion functions in much the same way as a block converter
does. An entire band is shifted in frequency to the desired
intermediate frequency. In the case of cellular base station
receivers, 5 MHz to 20 MHz of bandwidth are down-converted
simultaneously to an IF frequency suitable for digitizing with a
wideband analog-to-digital converter. Once digitized the
broadband digital data stream contains all of the in-band
signals. T he remainder of the radio is constructed digitally using
special purpose and general purpose programmable DSP to
perform filtering, demodulation and signal conditioning not
unlike the analog counter parts.
ADCs
NARROWBAND
FILTER
NARROWBAND
FILTER
LNA
I
Q
IF
IF
2
RF
1
e.g.
900MHz
FIXED
VARIABLE
ONE RECEIVER PER CHANNEL
SHARED
Figure 49. Narrowband Digital Receiver Architecture
In the narrowband receiver (Figure 49), the signal to be received
must be tuned. T his is accomplished by using a variable local
oscillator at the first mix down stage. T he first IF then uses a
narrow band filter to reject out of band signals and condition
the selected carrier for signal demodulation.
If demodulation takes place in the analog domain then
traditional discriminators, envelop detectors, phase locked loops
or other synchronous detectors are generally employed to strip
the modulation from the selected carrier.
In the digital wideband receiver (Figure 50), the variable local
oscillator has been replaced with a fixed oscillator, so tuning
must be accomplished in another manner. T uning is performed
digitally using a digital down conversion and filter chip fre-
quently called a channelizer. T he term channelizer is used
because the purpose of these chips is to select one channel out
of the many within the broadband of spectrum actually present
in the digital data stream of the ADC.
However, as general purpose DSP chips such as the ADSP-2181
become more popular, they will be used in many baseband-
sampled applications like the one shown in Figure 49. As
shown in the figure, prior to ADC conversion, the signal must
be mixed down, filtered, and the I and Q components separated.
T hese functions are realizable through DSP techniques,
however several key technology breakthroughs are required:
high dynamic range ADCs such as the AD9042, new DSPs
(highly programmable with onboard memory, fast), digital tuner
& filter (with programmable frequency and BW) and wide band
mixers (high dynamic range with >12.5 MHz BW).
DECIMATION
FILTER
LOW-PASS
FILTER
I
COS
DIGITAL
TUNER
DATA
WIDEBAND
ADC
WIDEBAND
SIN
WIDEBAND
MIXER
FILTER
DECIMATION
FILTER
LOW-PASS
FILTER
LNA
Q
"n" CHANNELS
TO DSP
RF
e.g.
900MHz
12.5MHz
(416 CHANNELS)
Figure 51. Digital Channelizer
Figure 51 shows the block diagram of a typical channelizer.
Channelizers consist of a complex NCO (Numerically
Controlled Oscillator), dual multiplier (mixer), and matched
digital filters. T hese are the same functions that would be
required in an analog receiver, however implemented in digital
form. T he digital output from the channelizer is the desired
carrier, frequently in I & Q format; all other signals have been
filtered and removed based on the filtering characteristics
desired. Since the channelizer output consists of one selected
RF channel, one tuner chip is required for each frequency
received, although only one wideband RF receiver is needed for
the entire band. Data from the channelizer may then be
processed using a digital signal processor such as the ADSP-
2181 or the SHARC processor, the ADSP-21062. T his data
may then be processed through software to demodulate the
information from the carrier.
FIXED
CHANNEL SELECTION
SHARED
Figure 50. Wideband Digital Receiver Architecture
Figure 50 shows such a wideband system. T his design shows
that the front end variable local oscillator has been replaced with
a fixed oscillator (for single band radios) and the back end has
been replaced with a wide dynamic range ADC, digital tuner
and DSP. T his technique offers many benefits.
First, many passive discrete components have been eliminated
that formed the tuning and filtering functions. T hese passive
components often require “tweaking” and special handling
during assembly and final system alignment. Digital compo-
nents require no such adjustments; tuner and filter characteristics
are always exactly the same. Moreover, the tuning and filtering
characteristics can be changed through software. Since software
REV. A
–19–
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