AD9864 datasheet(44/48 Pages) AD

44 / 48 page

Data Sheet

AD9864

Rev. A | Page 43 of 47

Split Path Rx Architecture

A split path Rx architecture may be attractive for those

applications whose instantaneous dynamic range requirements

exceed the capability of a single AD9864 device. To cope with

these higher dynamic range requirements, two AD9864 devices

can be operated in parallel with their respective clip points

offset by a fixed amount. Adding a fixed amount of attenuation

in front of the AD9864 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 75. The signal chain consists of a high

linearity RF front end and IF stage followed by two AD9864

devices 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 AD9864. 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 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 AD9864 with the

lower clip point is not 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 AD9864

devices via a direct signal path and an attenuated signal path.

The direct path corresponds to the AD9864 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

AD9864 is 19 dB, the digital output of this path is nominally

selected unless the target signal’s power exceeds −36 dBm at the

antenna. The attenuated path corresponds to the AD9864

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 VGA of the AD9864 to the middle of

its 12 dB range. This setting results in a ±6 dB adjustment of the

clip point, allowing the clip point difference to be calibrated to

exactly 24 dB, so that a simple 5-bit shift would make up the

gain difference. The attenuated path can handle signal levels up

to −12 dB at the antenna before being overdriven. Because the

SAW filters provide sufficient blocker suppression, the digital

data from this path need only be selected when the target signal

exceeds −36 dBm. Although the sensitivity of the receiver with

the attenuated path is 20 dB lower than the direct path, the

strong target signal ensures a sufficiently high carrier-to-noise

ratio.

Because GSM is based on a TDMA scheme, digital data (or

path) selection can occur on a slot-by-slot basis. Configure the

AD9864 to provide serial I and Q data at a frame rate of

541.67 kSPS, as well as additional information including a 2-bit

reset field and a 6-bit RSSI field. These two fields contain the

information needed to decide whether the direct or attenuated

path is to be used for the current time slot.

AD9864 Datasheet(PDF) 44 Page - Analog Devices

AD9864 Datasheet(HTML) 44 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Datasheet Download

Link URL