AD9874ABST Analog Devices Inc, AD9874ABST Datasheet - Page 35

• Manufacturer Part Number: AD9874ABST
• Description: IC IF DIGIT SUBSYSTEM 48-LQFP
• Manufacturer: Analog Devices Inc

Specifications of AD9874ABST

• Frequency: 10MHz ~ 300MHz
• Rohs Status: RoHS non-compliant
• Function: IF Digitizing Subsystem
• Rf Type: UHF, Cellular, TETRA, GSM, EDGE, APCO25
• Secondary Attributes: 16dB Front End Attenuator
• Package / Case: 48-LQFP
• Ic Function: A/D Converter (A-D)
• Supply Voltage Range: 2.7V To 3.6V
• Operating Temperature Range: -40°C To +85°C
• Digital Ic Case Style: LQFP
• No. Of Pins: 48
• Msl: MSL 3 - 168 Hours
• For Use With: AD9874-EB - BOARD EVAL FOR AD9874
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

This second IF signal is then digitized by the - ADC, demodu-
lated into its quadrature I and Q components, filtered via matching
decimation filters, and reformatted to enable a synchronous serial
interface to a DSP. In this example, the AD9874’s LO and CLK
synthesizers are both enabled, requiring some additional passive
components (for the synthesizer’s loop filters and CLK oscillator)
and a VCO for the LO synthesizer. Note that not all of the
required decoupling capacitors are shown. Refer to the previous
section and Figure 26 for more information on required external
passive components.
The selection of the first IF frequency is often based on the
availability of low cost standard crystal or SAW filters as well as
system frequency planning considerations. In general, crystal
filters are often used for narrow-band radios having channel
bandwidths below 50 kHz with IFs below 120 MHz, while SAW
filters are more suited for channel bandwidths greater than
50 kHz with IFs greater than 70 MHz. The ultimate stop-band
rejection required by the IF filter will depend on how much
suppression is required at the AD9874’s image band resulting
from downconversion to the second IF. This image band is
offset from the first IF by twice the second IF frequency (i.e.,
The selectivity and bandwidth of the IF filter will depend on
both the magnitude and frequency offset(s) of the adjacent
REV. A
f
CLK
/4, depending on high or low side injection).
INPUT
RF
PRESELECT
FILTER
Figure 27. Typical Dual Conversion Superheterodyne Application Using the AD9874
LNA
TUNER
ADF42xx
PLL SYN
REFIN
VCO
OSCILLATOR
CRYSTAL
IF CRYSTAL OR
SAW FILTER
FILTER
LOOP
IF2 =
IFIN
SYNTH.
–16dB
LO
f
LNA
CLK
/8
VCO
–35–
VDDA
FILTER
LOOP
channel blocker(s) that could overdrive the AD9874’s input
or generate in-band intermodulation components. Further
suppression is performed within the AD9874 by its inherent
band-pass response and digital decimation filters. Note that
some applications will require additional application-specific
filtering performed in the DSP that follows the AD9874 to
remove the adjacent channel and/or implement a matched
filter for optimum signal detection.
The output data rate of the AD9874, f
to be at least twice the bandwidth or symbol rate of the desired
signal to ensure that the decimation filters provide a flat pass-
band response as well as to allow for postprocessing by a DSP.
Once f
filters should be set such that the input clock rate, f
between the AD9874’s rated operating range of 13 MHz to
26 MHz and no significant spurious products related to f
within the desired pass band, resulting in a reduction in sensitiv-
ity performance. If a spurious component is found to limit the
sensitivity performance, the decimation factor can often be
modified slightly to find a spurious free pass band. Selecting a
higher f
the first IF’s filtering requirements often depend on the tran-
sition region between the IF frequency and the image band
(i.e.,
SAMPLE CLOCK
SYNTHESIZER
OUT
CLK
f
CLK
- ADC
is determined, the decimation factor of the digital
is typically more desirable given a choice, since
/4 ). Lastly, the output SSI clock rate, f
DAC AGC
REFERENCE
DECIMATION
VOLTAGE
FILTER
VDDC
CONTROL LOGIC
FROM DSP
FORMATTING/SSI
AD9874
SPI
OUT
CLKOUT
DOUTB
DOUTA
, should be chosen
FS
AD9874
CLK
TO
DSP
CLKOUT
, falls
CLK
,
fall