L64780 LSI Logic Corporation, L64780 Datasheet - Page 25

• Manufacturer Part Number: L64780
• Description: Dvb-t Cofdm Demodulator Technical Manual 2/00
• Manufacturer: LSI Logic Corporation

2.4 Real-to-Complex Conversion
Table 2.1
ADC sampling frequency
Low-IF center frequency
Approximate and Exact Frequencies for 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
to the sampling frequency, f
shows that f
L64780 ensures that this is the case. The output of the block is complex
numbers at the rate of f
The frequencies in
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.
Real-to-Complex Conversion
CLK18
Symbol
f
CLK18
f
LIF
= 4 f
Table 2.1
Approx. Frequency, MHz Exact Frequency, MHz
LIF
CLK18
almost exactly). The AFC circuitry of the
CLK18
/2.
apply for the normal version of the DVB-T
, used by the ADC (see
18.28
4.57
LIF
) of the low IF is related
Table
128/7
32/7
2.1, which
2-7