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| 型號(hào)： | AD9873-EB |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Analog Front End Converter for Set-Top Box, Cable Modem |
| 中文描述： | 模擬前端轉(zhuǎn)換器，用于機(jī)頂盒，電纜調(diào)制解調(diào)器 |
| 文件頁(yè)數(shù)： | 25/39頁(yè) |
| 文件大小： | 935K |
| 代理商： | AD9873-EB |
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REV. 0
AD9873
–25–
TxI[11:6]
t
HD
t
SU
MCLK
Tx SYNC
Tx IQ
TxI[5:0]
TxQ[5:0]
TxQ[11:6]
TxI[11:6]'
TxI[5:0]'
TxQ[5:0]'
TxQ[11:6]'
TxI[5:0]"
TxI[11:6]"
Figure 9. Transmit Timing Diagram
The I/Q sample rate f
IQCLK
puts a bandwidth limit on the maxi-
mum transmit spectrum. This is the familiar Nyquist limit and is
equal to one-half f
IQCLK
which hereafter will be referred to as f
NYQ
.
Half-Band Filters (HBFs)
HBF 1 is a 15-tap filter that provides a factor-of-two increase
in sampling rate. HBF 2 is an 11-tap filter offering an additional
factor-of-two increase in sampling rate. Together, HBF 1 and 2
provide a factor-of-four increase in the sampling rate (4 f
IQCLK
or 8 f
NYQ
).
In relation to phase response, both HBFs are linear phase filters.
As such, virtually no phase distortion is introduced within the
passband of the filters. This is an important feature as phase
distortion is generally intolerable in a data transmission system.
Cascaded Integrator—COMB (CIC) Filter
A CIC filter is unlike a typical FIR filter in that it offers the
flexibility to handle differing input and output sample rates
(only in integer ratios, however). In the purest sense, a CIC
filter can provide either an increase or a decrease in sample
rate at the output relative to the input, depending on the
architecture. If the integration stage precedes the comb stage,
the CIC filter provides sample rate reduction (decimation).
When the comb stage precedes the integrator stage, the CIC
filter provides an increase in sample rate (interpolation). In the
AD9873, the CIC filter is configured as a programmable inter-
polator and provides a sample rate increase by a factor of R = 3
or R = 4. In addition to the ability to provide a change in sample
rate between input and output, a CIC filter also has an intrinsic
low-pass frequency response characteristic. The frequency
response of a CIC filter is dependent on three factors:
1. The rate change ratio, R.
2. The order of the filter, n.
3. The number of unit delays per stage, m.
It can be shown that the system function H(z), of a CIC filter is
given by:
H( )
=
R
z
z
R
z
Rm
n
k
k
n
=
∑
=
1 1
1
1
1
0
1
The form on the far right has the advantage of providing a result
for z = 1 (corresponding to zero frequency or dc). The alternate
form yields an indeterminate form (0/0) for z = 1, but is other-
wise identical. The only variable parameter for the AD9873
’
s
CIC filter is R; m and n are fixed at 1 and 3, respectively. Thus,
the CIC system function for the AD9873 simplifies to:
H( )
=
R
z
z
R
z
R
k
k
=
∑
=
1 1
1
1
1
3
0
1
3
The transfer function is given by:
H( )
R
e
e
R
fR
f
π
j
fR
j
f
sin(
sin(
)
)
(
)
=
1
=
1 1
1
2
2
3
3
π
π
π
The frequency response in this form is such that
“
f
”
is scaled to
the output sample rate of the CIC filter. That is, f = 1 corresponds
to the frequency of the output sample rate of the CIC filter. H(f/R)
will yield the frequency response with respect to the input sample
of the CIC filter.
Combined Filter Response
The combined frequency response of HBF 1, HBF 2 and CIC is
shown in Figure 10a to 10c and Figure 11a to 11c.
The usable bandwidth of the filter chain puts a limit on the maxi-
mum data rate that can be propagated through the AD9873.
A look at the passband detail of the combined filter response
(Figure 10d and Figure 11d) indicates that in order to maintain
an amplitude error of no more than 1 dB, we are restricted to
signals having a bandwidth of no more than about 60% of f
NYQ
.
Thus, in order to keep the bandwidth of the data in the flat portion
of the filter passband, the user must oversample the baseband data
by at least a factor of two prior to presenting it to the AD9873.
Note that without oversampling, the Nyquist bandwidth of the
baseband data corresponds to the f
NYQ
. As such, the upper end
of the data bandwidth will suffer 6 dB or more of attenuation
due to the frequency response of the digital filters. Furthermore, if
the baseband data applied to the AD9873 has been pulse-shaped,
there is an additional concern. Typically, pulse-shaping is applied
to the baseband data via a filter having a raised cosine response.
In such cases, an
α
value is used to modify the bandwidth of the
data where the value of
α
is such that 0
≤
α
≤
1. A value of 0 causes
the data bandwidth to correspond to the Nyquist bandwidth. A
value of 1 causes the data bandwidth to be extended to twice the
Nyquist bandwidth. Thus, with 2
×
oversampling of the baseband
data and
α
= 1, the Nyquist bandwidth of the data will correspond
with the I/Q Nyquist bandwidth. As stated earlier, this results in
problems near the upper edge of the data bandwidth due to the
frequency response of the filters. The maximum value of
α
that
can be implemented is 0.45. This is because the data bandwidth
becomes:
1/2(1+
α
)
f
NYQ
= 0.725
f
NYQ
,
which puts the data bandwidth at the extreme edge of the flat
portion of the filter response.
相關(guān)PDF資料 |
PDF描述 |
|---|---|
| AD9873JS | Analog Front End Converter for Set-Top Box, Cable Modem |
| AD9874 | IF Digitizing Subsystem |
| AD9874BST | IF Digitizing Subsystem |
| AD9874EB | IF Digitizing Subsystem |
| AD9875BSTRL | Broadband Modem Mixed-Signal Front End |
相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| AD9873JS | 制造商:Analog Devices 功能描述: |
| AD9874 | 制造商:AD 制造商全稱:Analog Devices 功能描述:IF Digitizing Subsystem |
| AD9874ABST | 功能描述:IC IF DIGIT SUBSYSTEM 48-LQFP RoHS:否 類別:RF/IF 和 RFID >> RF 其它 IC 和模塊 系列:- 標(biāo)準(zhǔn)包裝:100 系列:* |
| AD9874ABST | 制造商:Analog Devices 功能描述:IF DIGITIZING SYBSYSTEM ((NW)) 制造商:Analog Devices 功能描述:IC, IF DIGITIZING SUBSYSTEM, LQFP-48 |
| AD9874ABST | 制造商:Analog Devices 功能描述:IC IF DIG SUBSYSTEM |
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