AD6620SSLASHPCB datasheet(42/43 Pages) AD

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AD6620

–42–

REV. 0

For most narrow-band uses of the AD6600/AD6620 combina-

tion, a high oversampling ratio is desired. This spreads the

quantization noise of the A/D over a wider spectrum and allows

the digital filtering of the AD6620 to remove much of this noise.

This effectively increases the SNR of the AD6600. This process

of oversampling and digital filtering is called “process gain” and

its contribution to SNR can be calculated from the equation

below.

Sample Rate of Channel

Signal Bandwidth









10 log

The process of oversampling can also provide the benefit of

lowering the noise floor of the A/D. This can increase the effec-

tive dynamic range of a receiver if the sampling rate is chosen

such that the signal harmonics and/or intermodular distortion

(IMD) products fall out of the band of interest. In this case

these spurs could be filtered by the AD6620 and the quantiza-

tion noise would be the dominant dynamic range limitation of

the AD6600/AD6620 receiver solution.

A DSP is then used to perform the demodulation of the digital

channel. This has the advantage of allowing for in-system con-

figuration options and can even allow for improved modulation

techniques to be applied in the future. This assumes that the

AD6600 and the circuitry on its front end are compatible with

the modulation standard to be used.

For more information on using the AD6600 and AD6620 in a

Single Carrier application, refer to Analog Devices’ Application

note AN-502.

Using the AD6620 in a Wideband System

The AD6620 is fully capable of being utilized in a wide-band

architecture system where A/Ds such as the AD6640 or the

AD9042 usually run at higher sample rates than those typically

found in a narrow-band system. A correspondingly wider band

can then be digitized. The digitization of this wide bandwidth

allows many more channels to be digitized using the same A/D

and IF circuitry. The core configuration of such a system is

shown in Figure 62.

INPUT

12 DATA BITS

ENCODE

SCLK

SDI

SDO

SDFS

IN[15...5]

AD6620

AD6640

SCLK

SDO

SDI

SDFS

DSP

CLK

Figure 62. Implementation of a Multicarrier Receivers

The AD6640 and the AD6620 are both designed to run as fast

as 65 MHz. In these applications the AD6620 will be used to

process only one channel and will process the data at the A/D

sample rate. Additional channels can be processed by taking the

AD6640 high speed data stream to additional AD6620s. Each

AD6620 can then be tuned to a different channel.

The AD6620 provides a great deal of selectivity by mixing down

a channel of interest as in the narrow-band case and filtering the

out-of-band noise and adjacent channels. Unlike the narrow-

band solutions it is much more difficult to place the spurious

content out of the band of interest because more of this band-

width is used due to the larger number of carrier channels. The

aliased spurs of one channel are likely to fold back on another

channel. This places a greater requirement on the Spurious

Free Dynamic Range (SFDR) of the A/D than in the narrow-

band case. The SFDR is then usually the limiting factor of the

wide-band system.

Provided that the A/D has sufficient SFDR for the air interface

requirements, the AD6620 can use process gain, as in the narrow-

band case, by filtering the out-of-band noise and adjacent chan-

nel power. This increases the SNR of the digital data stream.

As in the Narrow-band System a DSP is then used to demodu-

late the digital data. The same advantages of flexibility exist in

the wide-band case as they did in the narrow-band case. Future

improvements in demodulation algorithms can be implemented

in the receiver, provided that the front end hardware is compat-

ible with the desired modulation standard.

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