AD6640STSLASHPCB datasheet(19/25 Pages) AD

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AD6640

–18–

REV. A

ADCs

VARIABLE

IF1

IF2

FIXED

NARROWBAND

FILTER

NARROWBAND

FILTER

LNA

e.g. 900MHz

SHARED

ONE RECEIVER PER CHANNEL

Figure 25. 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 25. As shown

in the figure, prior to ADC conversion, the signal must be mixed

down and 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 AD6640, new DSPs (highly programmable

with onboard memory, fast), digital tuners and filters such as the

AD6620, wide band mixers, and amplifiers.

WIDEBAND

ADC

FIXED

WIDEBAND

MIXER

WIDEBAND

FILTER

LNA

e.g. 900MHz

SHARED

"n" CHANNELS

TO DSP

12.5MHz

(416 CHANNELS)

CHANNEL SELECTION

DIGITAL TUNER/FILTER

DSP

DIGITAL TUNER/FILTER

DSP

Figure 26. Wideband Digital Receiver Architecture

Figure 26 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 technique

offers many benefits. First, many passive discrete components

that formed the tuning and filtering functions have been elimi-

nated. These passive components often require tweaking and

special handling during assembly and final system alignment.

Digital components 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 26), 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 intermediate

frequency. In the case of cellular base station receivers, 5 MHz

to 30 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 25), 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 26), 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.

DECIMATION

FILTER

LOW-PASS

FILTER

DIGITAL

TUNER

COS

SIN

DECIMATION

FILTER

LOW-PASS

FILTER

DATA

Figure 27. AD6620 Digital Channelizer

Figure 27 shows the block diagram of a typical channelizer, such

as the AD6620. 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, but implemented in digital

form. The digital output from the channelizer is the desired car-

rier, frequently in I & Q format; all other signals have been fil-

tered 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

® proces-

sor, the ADSP-21062. This data may then be processed through

software to demodulate the information from the carrier.

System Requirements

Figure 28 shows a typical wideband receiver subsystem based

around the AD6640. This strip consists of a wideband IF filter,

amplifier, ADC, latches, channelizer, and interface to a digital

signal processor. This design shows a typical clocking scheme

used in many receiver designs. All timing within the system is

referenced back to a single clock. While this is not necessary, it

does facilitate PLL design, ease of manufacturing, system test,

and calibration. Keeping in mind that the overall performance

goal is to maintain the best possible dynamic range, many con-

siderations must be made.

One of the biggest challenges is selecting the amplifier used to

drive the AD6640. Since this is a communications application,

it is common to directly sample an intermediate frequency (IF)

signal. As such, IF gain blocks can be implemented instead of

baseband op amps. For these gain block amplifiers, the critical

specifications are third order intercept point and noise figure. A

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