12-Bit/ 65 MSPS IF Sampling A/D Converter
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DIGITAL WIDEBAND RECEIVERS
Several key technologies are now being introduced that may
forever alter the vision of radio. Figure 43 shows the typical
dual conversion superheterodyne receiver. The 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. This 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.
SHARED ONE RECEIVER PER CHANNEL
Figure 43. Narrowband Digital Receiver Architecture
If demodulation takes place in the analog domain then tradi-
tional discriminators, envelop detectors, phase locked loops or
other synchronous detectors are generally employed to strip the
modulation from the selected carrier.
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 43. As
shown in the figure, prior to ADC conversion, the signal must
be mixed down, filtered, and the I and Q components separated.
These functions are realizable through DSP techniques, how-
ever several key technology breakthroughs are required: high
dynamic range ADCs such as the AD6640, new DSPs (highly
programmable with onboard memory, fast), digital tuners and
filters such as the AD6620, wide band mixers and amplifiers.
SHARED CHANNEL SELECTION
Figure 44. Wideband Digital Receiver Architecture
Figure 44 shows such a wideband system. This design shows
that the front end variable local oscillator has been replaced with
a fixed oscillator and the back end has been replaced with a
wide dynamic range ADC, digital tuner and DSP. This tech-
nique offers many benefits.
First, many passive discrete components have been eliminated
that formed the tuning and filtering functions. These 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
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.
In the wideband digital radio (Figure 44), the first down conver-
sion functions in much the same way as a block converter does.
An entire band is shifted in frequency to the desired interme-
diate frequency. In the case of cellular base station receivers,
5 MHz to 30 MHz of bandwidth are down-converted simulta-
neously to an IF frequency suitable for digitizing with a wide-
band analog-to-digital converter. Once digitized the broadband
digital data stream contains all of the in-band signals. The
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.
In the narrowband receiver (Figure 43), the signal to be received
must be tuned. This is accomplished by using a variable local
oscillator at the first mix down stage. The first IF then uses a
narrow band filter to reject out of band signals and condition
the selected carrier for signal demodulation.
In the digital wideband receiver (Figure 44), the variable local
oscillator has been replaced with a fixed oscillator, so tuning
must be accomplished in another manner. Tuning is performed
digitally using a digital down conversion and filter chip fre-
quently called a channelizer. The term channelizer is used
because the purpose of these chips is to select one channel out
of many within the broadband spectrum present in the digital
data stream of the ADC.
Figure 45. AD6620 Digital Channelizer
Figure 45 shows the block diagram of a typical channelizer, such
as the AD6620. Channelizers consist of a complex NCO (Nu-
merically Controlled Oscillator), dual multiplier (mixer), and
matched digital filters. These are the same functions that would
be required in an analog receiver, however implemented in
digital form. The 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 re-
ceived, although only one wideband RF receiver is needed for
the entire band. Data from the channelizer may then be pro-
cessed using a digital signal processor such as the ADSP-2181
or the SHARC® processor, the ADSP-21062. This data may
then be processed through software to demodulate the informa-
tion from the carrier.
SHARC is a registered trademark of Analog Devices, Inc.