AD9874ABSTZ datasheet(37/40 Pages) AD

37 / 40 page

REV. A

AD9874

–37–

Split Path Rx Architecture

A split path Rx architecture may be attractive for those applica-

tions whose instantaneous dynamic range requirements exceed

the capability of a single AD9874 device. To cope with these

higher dynamic range requirements, two AD9874s can be oper-

ated in parallel with their respective clip points offset by a fixed

amount. Adding a fixed amount of attenuation in front of the

AD9874 and/or programming the attenuation setting of its

internal VGA can adjust the input-referred clip point. To save

power and simplify hardware, the LO and CLK circuits of the

device can also be shared. Connecting the SYNCB pins of the two

devices and pulsing this line low synchronizes the two devices.

An example of this concept for possible use in a GSM base station

is shown in Figure 29. The signal chain consists of a high linearity

RF front end and IF stage followed by two AD9874s operating in

parallel. The RF front end consists of a duplexer and preselect

filter to pass the GSM RF band of interest. A high performance

LNA isolates the duplexer from the preselect filter while providing

sufficient gain to minimize system NF. An RF mixer is used to

downconvert the entire GSM band to a suitable IF, where much of

the channel selectivity is accomplished. The 170.6 MHz IF is

chosen to avoid any self-induced spurs from the AD9874. The IF

stage consists of two SAW filters isolated by a 15 dB gain stage.

The cascaded SAW filter response must provide sufficient

blocker rejection in order for the receiver to meet its sensitivity

requirements under worst-case blocker conditions. A composite

response having 27 dB, 60 dB, and 100 dB rejection at frequency

offsets of

0.8 MHz,

1.6 MHz, and

6.5 MHz, respectively,

provides enough blocker suppression to ensure that the AD9874

with the lower clip point will not be overdriven by any blocker.

This configuration results in the best possible receiver sensitivity

under all blocking conditions.

The output of the last SAW filters drives the two AD9874s via a

direct signal path and an attenuated signal path. The direct path

corresponds to the AD9874 having the lowest clip point and

provides the highest receiver sensitivity with a system noise

figure of 4.7 dB. The VGA of this device is set for maximum

attenuation, so its clip point is approximately –17 dBm. Since

conversion gain from the antenna to the AD9874 is 19 dB, the

digital output of this path will nominally be selected unless the

target signal’s power exceeds –36 dBm at the antenna. The

attenuated path corresponds to the AD9874 having the highest

input-referred clip point, and its digital output point of this path

is set to 7 dBm by inserting a 30 dB attenuator and setting the

AD9874’s VGA to the middle of its 12 dB range. This setting

VCO

LNA

MIXER

IF SAW 1

IF SAW 2

DUPLEXER PRESELECT

GAIN = –2dB

NF = 2dB

GAIN = 22dB

NF = 1dB

GAIN = –3dB

NF = 3dB

GAIN = 5dB

NF = 12dB

AMP

GAIN = –9dB

NF = –9dB

GAIN = 15dB

NF = 2dB

13MHz

DSP

ASIC

36dB

PAD

SYNCB

DOUTA

CLKOUT

CLKP

CLKN

LOP

LON

AD9874

MASTER

LOOP

FILTER

IOUTC

LOP

LON

IFIN

AD9874

SLAVE

DOUTA

CLKOUT

SYNCB

CLKP

CLKN

IOUTL

LOOP

FILTER

CVAR

0.1 F

LOSC

RBIAS

COSC

VDDC

IFIN

IOUTC

ATTENUATED PATH WITH

CLIP POINT = 7.0dBm

DIRECT PATH WITH

CLIP POINT = –17dBm

fREF

Figure 29. Example of Split Path Rx Architecture to Increase Receiver Dynamic Range Capabilities

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