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參數(shù)資料
| 型號： | L64780 |
| 廠商： | LSI Corporation |
| 英文描述： | DVB-T COFDM Demodulator(對地數(shù)字視頻廣播 正交編碼頻率分割多路處理解調(diào)器) |
| 中文描述： | 的DVB - T COFDM解調(diào)器（對地數(shù)字視頻廣播正交編碼頻率分割多路處理解調(diào)器） |
| 文件頁數(shù)： | 25/126頁 |
| 文件大?。?/td> | 953K |
| 代理商： | L64780 |
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Real-to-Complex Conversion
2-7
2.4 Real-to-Complex Conversion
The DVB-T signal occupies a bandwidth of approximately 7.61 MHz; the
signal is symmetrically disposed in extent about a notional center carrier.
As radiated, this normally lies in the UHF range; however, its spectrum
is not symmetrical, because its upper and lower sidebands are different
(a double-sideband modulated signal does have a symmetrical
spectrum). Thus, a complex representation is needed to describe it at
baseband. This spectrum can be derived using the in-phase I and
quadrature-phase Q demodulators, but each would have to be sampled
separately, using two ADCs and many well-matched components.
The L64780 chip avoids the complexity and expensive matching of
analog components. It uses a single ADC, and derives the complex-
baseband representation by internal digital processing. The ADC
samples the DVB-T signal at a low IF, then the real-to-complex block
digitally shifts the samples to complex baseband form, centered on zero
frequency. It is fundamental to the operation of this conversion
processing that the nominal center frequency (f
LIF
) of the low IF is related
to the sampling frequency, f
CLK18
, used by the ADC (see
Table 2.1
, which
shows that f
CLK18
= 4 f
LIF
almost exactly). The AFC circuitry of the
L64780 ensures that this is the case. The output of the block is complex
numbers at the rate of f
CLK18
/2.
The frequencies in
Table 2.1
apply for the normal version of the DVB-T
specification for use with UHF channels spaced by 8 MHz. To adopt the
DVB-T specification for use with 7 MHz or 6 MHz channel spacing, scale
the clock frequency and all other frequencies within the system (including
the bit-rate capacity and low IF) by factors of 7/8 and 6/8, respectively.
Table 2.1
Approximate and Exact Frequencies for Real to Complex Conversion
Symbol
Approx. Frequency, MHz Exact Frequency, MHz
ADC sampling frequency
f
CLK18
18.28
128/7
Low-IF center frequency
f
LIF
4.57
32/7
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相關代理商/技術參數(shù) |
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|---|---|
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