AD9874BST AD [Analog Devices], AD9874BST Datasheet - Page 25

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

Once the AD9874 has been tuned, the noise figure degradation
attributed solely to the temperature drift of the LC and RC resona-
tors is minimal. Since the drift of the RC resonator is actually
negligible compared to that of the LC resonator, the external
L and C components’ temperature drift characteristics tend to
dominate. Figure 13d shows the degradation in noise figure as the
product of the LC value is allowed to vary from –12.5% to +12.5%.
Note, the noise figure remains relatively constant over a ± 3.5%
range (i.e., ± 35,000 ppm) suggesting that most applications will
not be required to retune over the operating temperature range.
Figure 13d. Typical Noise Figure Degradation from L and
C Component Drift (f
DECIMATION FILTER
The decimation filter shown in Figure 14 consists of an f
complex mixer and a cascade of three linear phase FIR filters:
DEC1, DEC2, and DEC3. DEC1 downsamples by a factor of 12
using a fourth order comb filter. DEC2 also uses a fourth order
comb filter, but its decimation factor is set by the M field of
Register 0x07. DEC3 is either a decimate-by-5 FIR filter or a
decimate-by-4 FIR filter depending on the value of the K Bit within
Register 0x07. Thus, the composite decimation factor can be set
to either 60 × M or 48 × M for K equal to 0 or 1, respectively.
The output data rate (f
frequency (f
Due to the transition region associated with the decimation
filter’s frequency response, the decimation factor must be
selected such that f
bandwidth. This ensures low amplitude ripple in the pass band
along with the ability to provide further application-specific
digital filtering prior to demodulation.
REV. 0
MODULATOR
FROM -
DATA
12
11
10
9
8
Figure 14. Decimation Filter Architecture
–15
CLK
COS
SIN
) divided by the digital filter’s decimation factor.
BW = 10kHz
–10
OUT
SINC
FILTER
BW = 75kHz
DEC1
CLK
is equal to or greater than twice the signal
OUT
4
–5
= 18 MSPS, f
) is equal to the modulator clock
12
LC ERROR – %
SINC
FILTER
BW = 30kHz
0
DEC2
4
M
M + 1
IF
5
= 73.3501 MHz)
FIR
FILTER
DEC3
10
K
OR
4
5
Q
COMPLEX
DATA TO
SSI PORT
I
15
CLK
/8
–25–
Figure 15a shows the response of the decimation filter at a decimation
factor of 900 (K = 0, M = 14) and a sampling clock frequency of
18 MHz. In this example, the output data rate (f
with a usable complex signal bandwidth of 10 kHz centered around
dc. As this figure shows, the first and second alias bands (occurring
at even integer multiples of f
provide at least 88 dB of attenuation. Note, signals falling around
frequency offsets that are odd integer multiples of f
10 kHz, 30 kHz, and 50 kHz) will fall back into the transition
band of the digital filter.
Figure 15b shows the response of the decimation filter with a
decimation factor of 48 and a sampling clock rate of 26 MHz. The
alias attenuation is now at least 94 dB, and this attenuation occurs
for frequencies at the edges of the fourth alias band. The differ-
ence between the alias attenuation characteristics of Figure 15b
and those of Figure 15a is due to the fact that the third decimation
stage decimates by a factor of 5 for Figure 15a versus a factor of 4
for Figure 15b.
Figure 15a. Decimation Filter Frequency Response
for f
Figure 15b. Decimation Filter Frequency Response
for f
–100
–120
OUT
–100
–120
OUT
–20
–40
–60
–80
–20
–40
–60
–80
0
0
0
0
FOLD-
POINT
= 20 kSPS (f
= 541.666 kSPS (f
ING
10
–98dB
0.5
20
5.0kHz PASS BAND
–88dB
135.466kHz PASS BAND
30
CLK
FREQUENCY – kHz
OUT
FREQUENCY – MHz
1.0
–115dB
= 18 MHz, OSR = 900)
40
–88dB
CLK
/2) have the least attenuation but
50
= 26 MHz, OSR = 48)
1.5
60
–101dB
70
2.0
–94dB
OUT
80
AD9874
) is 20 kSPS,
OUT
–103dB
90
2.5
/2 (i.e.,
100