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參數(shù)資料
| 型號： | AD9877ABSZ |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 18/36頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC PROCESSOR FRONT END 100MQFP |
| 標(biāo)準(zhǔn)包裝： | 1 |
| 位數(shù)： | 12 |
| 通道數(shù)： | 3 |
| 功率（瓦特）： | 1.17W |
| 電壓 - 電源，模擬： | 3.3V |
| 電壓 - 電源，數(shù)字： | 3.3V |
| 封裝/外殼： | 100-BQFP |
| 供應(yīng)商設(shè)備封裝： | 100-MQFP（14x20） |
| 包裝： | 托盤 |
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AD9877
Rev. B | Page 25 of 36
COMBINED FILTER RESPONSE
The combined frequency response of HBF 1, HBF 2, and CIC is
and Figure 33.
The usable bandwidth of the filter chain puts a limit on the
maximum data rate that can be propagated through the
AD9877. A look at the pass-band detail of the combined filter
response (Figure 30 and Figure 34) indicates that to maintain an
amplitude error of no more than 1 dB, use signals having a
bandwidth of no more than about 60% of fNYQ.
To keep the bandwidth of the data in the flat portion of the filter
pass band, the user must oversample the baseband data by at
least a factor of 2 prior to representing it to the AD9877. With-
out oversampling, the Nyquist bandwidth of the baseband data
corresponds to the fNYQ. Consequently, the upper end of the data
bandwidth suffers 6 dB or more of attenuation due to the
frequency response of the digital filters.
There is an additional concern if the baseband data applied to
the AD9877 has been pulse shaped. 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 corresponds 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
(
)
NYQ
f
725
.
0
1
2
1
=
α
+
which puts the data bandwidth at the extreme edge of the flat
portion of the filter response.
If a particular application requires an α value between 0.45
and 1, the user must oversample the baseband data by at least a
factor of 4. The combined HBF1, HBF2, and CIC filter
introduces a worst-case droop of less than 0.2 dB over the
frequency range of the data to be transmitted.
FREQUENCY (fS/2)
0
1.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
10
0
MAGNITUDE
(dB)
–10
–20
–30
–40
–50
–60
–70
–80
02716-027
Figure 27. Cascaded Filter 12× Interpolator (N = 3)
FREQUENCY (fS/2)
0
1.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
10
0
MAGNITUDE
(dB)
–10
–20
–30
–40
–50
–60
–70
–80
02716-028
Figure 28. Input Signal Spectrum (N = 3), α = 0.25
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