AD9874ABST Analog Devices Inc, AD9874ABST Datasheet - Page 33

• 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

Figure 23b shows that omitting the LO frequencies given by
Equation 12 for m = 1, 3, and 5 and by Equation 13 accounts
for most of the spurs. Some of the remaining low level spurs can
be attributed to coupling from the SSI digital output. As a
result, users are also advised to optimize the output bit rate
(f
strength to achieve the lowest spurious and noise figure perfor-
mance for a particular LO frequency and f
especially the case for particularly narrow-band channels in
which low level spurs can degrade the AD9874’s sensitivity
performance.
Despite the many spurs, sweet spots in the LO frequency are
generally wide enough to accommodate the maximum signal
bandwidth of the AD9874. As evidence of this property, Fig-
ure 24 shows that the in-band noise is quite constant for LO
frequencies ranging from 70 MHz to 71 MHz.
REV. A
CLKOUT
Figure 24. Expanded View from 70 MHz to 71 MHz
–50
–60
–70
–80
–90
via the SSIORD register) and the digital output driver
70.0
Figure 23b. Same as Figure 23a Excluding LO Frequencies Known to Produce Large In-Band Spurs
–50
–60
–70
–80
–90
0
LO FREQUENCY – MHz
70.5
50
CLK
setting. This is
100
71.0
LO FREQUENCY – MHz
–33–
150
Spurious Responses
The spectral purity of the LO (including its phase noise) is an
important consideration since LO spurs can mix with undesired
signals present at the AD9874’s IFIN input to produce an in-band
response. To demonstrate the low LO spur level introduced within
the AD9874, Figure 25 plots the demodulated output power as a
function of the input IF frequency for an LO frequency of
71.1 MHz and a clock frequency of 18 MHz.
The two large –10 dBFS spikes near the center of the plot are
the desired responses at f
i.e., at 68.85 MHz and 73.35 MHz. LO spurs at f
would result in spurious responses at offsets of
desired responses. Close-in spurs of this kind are not visible on
the plot, but small spurious responses at f
at 50.85 MHz, 55.35 MHz, 86.85 MHz, and 91.35 MHz, are
visible at the –90 dBFS level. This data indicates that the AD9874
does an excellent job of preserving the purity of the LO signal.
Figure 25 can also be used to gauge how well the AD9874
rejects undesired signals. For example, the half-IF response (at
69.975 MHz and 72.225 MHz) is approximately –100 dBFS,
giving a selectivity of 90 dB for this spurious response. The
largest spurious response at approximately –70 dBFS occurs
with input frequencies of 70.35 MHz and 71.85 MHz. These
spurs result from third order nonlinearity in the signal path
(i.e., abs [3
Figure 25. Response of AD9874 to a –20 dBm IF
Input when f
–100
–120
–20
–40
–60
–80
0
200
50
RESPONSES
f
DESIRED
LO
– 3
60
LO
= 71.1 MHz
f
LO
IF_Input
250
IF FREQUENCY – MHz
,
70
f
] = f
IF2_ADC
D =
CLK
80
f
, where f
CLK
/8).
LO
/4 = 4.5MHz
300
f
IF2_ADC
f
90
IF2_ADC
AD9874
SPUR
LO
around the
= f
f
f
SPUR
100
CLK
CLK
, i.e.,
/8,