AD9874BST AD [Analog Devices], AD9874BST Datasheet - Page 36

• Manufacturer Part Number: AD9874BST
• Description: IF Digitizing Subsystem
• Manufacturer: AD [Analog Devices]

AD9874
“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 AD9874 device. To cope with these higher
dynamic range requirements, two AD9874s 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 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
together 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
GAIN = –2dB
NF = 2dB
DUPLEXER PRESELECT
Figure 29. Example of Split Path Rx Architecture to Increase Receiver Dynamic Range Capabilities
LNA
GAIN = 22dB
NF = 1dB
GAIN = –3dB
NF = 3dB
MIXER
X
GAIN = 5dB
NF = 12dB
IF SAW 1
ATTENUATED PATH WITH
“CLIP POINT” = 7.0dBm
GAIN = 15dB
NF = 2dB
IF
AMP
“CLIP POINT” = –17dBm
DIRECT PATH WITH
IF SAW 2
GAIN = –9dB
NF = –9dB
–36–
0.1 F
36dB
PAD
stage consists of two SAW filters isolated by a 15 dB gain stage. The
cascaded SAW filter response must provide sufficient blocker rejec-
tion in order for the receiver to meets its sensitivity requirements
under worst-case blocker conditions. A composite response having
27, 60, and 100 dB rejection at frequency offsets of ±0.8 MHz, ±1.6
MHz, and ±6.5 MHz, respectively, provides enough blocker sup-
pression 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 AD9874 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
VDDC
R
L
BIAS
OSC
C
OSC
VCO
C
R D
VAR
19
47
43
42
47
43
42
19
20
20
IFIN
LOP
LON
CLKP
CLKN
FILTER
CLKP
CLKN
IFIN
LOP
LON
LOOP
C
C
P
Z
AD9874
“SLAVE”
R
“MASTER”
AD9874
F
IOUTC
IOUTL
IOUTC
15
38
15
FILTER
LOOP
CLKOUT
CLKOUT
DOUTA
DOUTA
SYNCB
SYNCB
f
f
REF
REF
FS
PC
PD
PC
FS
PE
PD
PE
25
24
23
33
31
29
28
35
35
29
28
25
24
23
33
31
13MHz
ASIC
DSP
OR
REV. 0