AD9042

REV. A

–19–

DIGITAL WIDEBAND RECEIVERS

Introduction

Several key technologies are now being introduced that may

forever alter the vision of radio. Figure 49 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.

ADCs

VARIABLE

FIXED

NARROWBAND

FILTER

NARROWBAND

FILTER

I

Q

LNA

RF

e.g. 900MHz

SHARED

ONE RECEIVER PER CHANNEL

Figure 49. Narrowband Digital Receiver Architecture

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.

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.

These 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).

WIDEBAND

ADC

FIXED

WIDEBAND

MIXER

WIDEBAND

FILTER

LNA

RF

e.g. 900MHz

SHARED

"n" CHANNELS

TO DSP

12.5MHz

(416 CHANNELS)

CHANNEL SELECTION

Figure 50. Wideband Digital Receiver Architecture

Figure 50 shows such a wideband system. This 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. This technique 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.

System Description

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. 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 49), 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 50), 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 the many within the broadband of spectrum actually present

in the digital data stream of the ADC.

DECIMATION

FILTER

LOW-PASS

FILTER

DIGITAL

TUNER

COS

SIN

DECIMATION

FILTER

LOW-PASS

FILTER

DATA

I

Q

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. 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

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. This data

may then be processed through software to demodulate the

information from the carrier.