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參數資料
| 型號： | AD9870 |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 通信及網絡 |
| 英文描述： | IF Digitizing Subsystem |
| 中文描述： | SPECIALTY TELECOM CIRCUIT, PQFP48 |
| 封裝： | 1.40 MM HEIGHT, PLASTIC, TQFP-48 |
| 文件頁數： | 16/20頁 |
| 文件大小： | 233K |
| 代理商： | AD9870 |
REV. 0
AD9870
–16–
Table IX. SPI Registers Associated with AGC
A
d
dress
(Hex)
Bit
Breakdown
Default
Value
Width
Name
0x03
(7)
(6:0)
(7:0)
(7:4)
(3:0)
(7:4)
(3:0)
(2:0)
1
7
8
4
4
4
4
3
0
0x3F
0xFF
0
0
0
0
0
ATTEN
AGCG(14:8)
AGCG(7:0)
AGCA
AGCD
AGCO
AGCD
AGCR
0x04
0x05
0x06
System Noise Figure (NF) vs. VGA (or AGC) Control
The AD9870’s system NF is a strong function of the gain set-
ting of the VGA. The noise present at the output of the VGA
and input of the ADC is relatively large and independent of the
VGA setting. Under small signal conditions in which the VGA is
set to its maximum gain, this noise referred back to the input of
the LNA’s input has less of an effect on raising the AD9870’s
system NF. However, under large signal conditions in which the
gain of the VGA must be reduced to prevent ADC clipping, this
noise quickly becomes a significant contributor in determining
the AD9870’s overall NF. Figure 11 shows how the NF of the
AD9870 in AGC mode remains relatively constant as an inter-
ferer signal input power is increased until its power reaches a
programmed reference level (i.e., –3 dB) at which point the NF
degrades almost 1 dB per dB as the interferer signal is increased
beyond this point, forcing the VGA gain to decrease. As a result, it
is recommended that the AGC referenced level be set to 3 (i.e.,
AGCR = 1) to maintain the best possible NF over the widest
input signal range.
INTERFERER AMPLITUDE
–
dBm
45
24
–
85
–
20
–
80
N
–
–
75
–
70
–
65
–
60
–
55
–
50
–
45
–
40
–
35
–
30
–
25
42
27
21
18
36
30
39
33
15
12
9
NOISE FIGURE
MEAN AGC VALUE
ADC CLIPS
AT
–
24 dBm
M
150
100
50
0
–
50
–
100
–
150
Figure 11. Noise Figure vs. Interferer Signal Level with an
IF = 73.35 MHz and CLK = 18 MSPS and AGCR = 1
DECIMATION FILTER
The decimation filter consists of a complex mix by f
CLK
/8 and a
cascade of three linear phase FIR filters: DEC1, DEC2, and DEC3
as shown in Figure 12. DEC1 downsamples by a factor of 20
using a fourth-order comb filter. DEC2 also uses a fourth-
order comb filter, but its decimation factor is set by the M
control register. DEC3 is a decimate-by-3 FIR filter.
M
DEC2
SINC
4
FILTER
M + 1
DEC3
FIR
FILTER
3
I
Q
COMPLEX
DATA TO
SSI PORT
COS
SIN
DATA FROM
MODULATOR
DEC1
SINC
4
FILTER
20
Figure 12. Decimation Filter Architecture
Figure 13a shows the response of the complete decimation filter
on a linear frequency axis for frequencies up to the third alias.
As this figure shows, the alias with the least attenuation is
located at the lower end of the third alias band and has an
attenuation of 83 dB.
FREQUENCY
–
kHz
0
–
40
–
100
0
30
10
20
–
20
–
60
–
80
70
40
60
50
d
FOLDING
POINT
5kHz PASSBAND
88dB ATTENUATION
(MIN)
83dB ATTENUATION
(MIN)
>100dB ATTENUATION
Figure 13a. Frequency Response for f
CLKOUT
= 20 kHz,
Showing the First Three Alias Bands
Figure 13b shows the full response of the decimation filter with
the decimation factor set to 60 on a logarithmic frequency
scale, while Figure 13c shows the folded frequency response
on a linear frequency scale and Figure 13d shows a blowup of
the passband. The location of the cutoff frequency shown in Figure
13b is inversely proportional to the decimation factor. However,
since both DEC1 and DEC2 are fourth-order comb filters, their
combination is also a fourth-order comb filter and thus the
shapes of the frequency responses shown in Figures 13c and
13d are independent of the decimation factor.
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PDF描述 |
|---|---|
| AD9870EB | IF Digitizing Subsystem |
| AD9873 | Analog Front End Converter for Set-Top Box, Cable Modem |
| AD9873-EB | Analog Front End Converter for Set-Top Box, Cable Modem |
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相關代理商/技術參數 |
參數描述 |
|---|---|
| AD9870AST | 制造商:Rochester Electronics LLC 功能描述:GENERAL PURPOSE IF SUBSYSTEM - Tape and Reel 制造商:Analog Devices 功能描述: |
| AD9870EB | 制造商:AD 制造商全稱:Analog Devices 功能描述:IF Digitizing Subsystem |
| AD9873 | 制造商:AD 制造商全稱:Analog Devices 功能描述:Analog Front End Converter for Set-Top Box, Cable Modem |
| AD9873-EB | 制造商:Analog Devices 功能描述: |
| AD9873JS | 制造商:Analog Devices 功能描述: |
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