AD9864 Analog Devices, AD9864 Datasheet - Page 40

• Manufacturer Part Number: AD9864
• Description: IF Digitizing Subsystem
• Manufacturer: Analog Devices

Specifications of AD9864

• Resolution (bits): 24bit
• # Chan: 1
• Sample Rate: 18MSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni,SE-Uni
• Adc Architecture: Sigma-Delta
• Pkg Type: CSP

AD9864
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 AD9864 device. To cope with these
higher dynamic range requirements, two AD9864s 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
AD9864 and/or programming the attenuation setting of its in-
ternal 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 sta-
tion is shown in Figure 73. The signal chain consists of a high
linearity RF front end and IF stage followed by two AD9864s
0.1µF
TO OTHER
AD9864s
R
VDDC
L
BIAS
OSC
Figure 72. Example of Synchronizing Multiple AD9864s
C
VCO
OSC
C
R
VAR
D
47
43
42
19
20
19
20
47
43
42
FILTER
IFIN
LOP
LON
CLKP
CLKN
CLKP
CLKN
IFIN
LOP
LON
LOOP
C
C
P
Z
AD9864
R
AD9864
MASTER
F
SLAVE
IOUTL
IOUTC
IOUTC
15
38
15
LOOP
FILTER
CLKOUT
CLKOUT
DOUTA
SYNCB
SYNCB
DOUTA
FREF
FREF
PD
PC
PE
PD
PC
FS
PE
FS
25
24
23
33
31
29
28
35
35
31
29
28
25
24
23
33
TO OTHER
AD9864s
FROM
CRYSTAL
OSCILLATION
TO
DSP
TO DSP
FROM
DSP
Rev. 0 | Page 40 of 44
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 in order for the receiver to meet its sensitivity
requirements under worst-case blocker conditions. A compos-
ite response having 27 dB, 60 dB, and 100 dB rejection at
frequency offsets of ±0.8 MHz, ±1.6 MHz, and ±6.5 MHz, r
spectively, provides enough blocker suppression to ensure tha
the AD9864 with the lower clip point will not be overd
any blocker. This configuration results in the best possible re
ceiver sensitivity under all blocking conditions.
The output of the last SAW filters drives the two AD9864s 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 will nominally be 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 AD9864’s VGA 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. Since 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.
Since GSM is based on a TDMA scheme, digital data (or path)
selection can occur on a slot-by-slot basis. The AD9864 would
be configured 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 should be used for the current time slot.
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