L64780 LSI Logic Corporation, L64780 Datasheet - Page 29

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

2.7.3 AFC Loop Gain
The block takes the sequence of complex numbers from the output of the
FFT block and produces a numerical control signal. It also takes the
sequence of complex numbers from the real-to-complex conversion block
and applies the frequency shift to the numbers according to the
numerical control signal. Then, it passes the data on to the FFT block.
The digital frequency synchronization block comprises two parts: one to
shift the frequency of the complex baseband signal, and one to measure
the error in the center frequency of the complex baseband signal as
presented to the FFT block.
Multiplying the signal by a rotating vector shifts the frequency of the
complex baseband signal. A number-controlled oscillator (NCO) with sine
and cosine outputs generates the rotating vector. The frequency range
of the NCO is
The error in the frequency of the local oscillator translates directly into
an error in the center frequency of the complex baseband signal. The
range of measurement is
the 2 k mode. The numerical control signal is derived from the measured
frequency error.
Use the microprocessor interface to read and initialize the value of the
control signal. This allows a previously stored value to be used when a
channel is selected, thus making acquisition faster.
In digital AFC mode, the optimal sensitivity (KSENS) is when the
AFC_SENSITIVITY field in the Parameter Registers (see
set to 0b110:
Equation 2.11
In analog AFC mode, the optimal AFC_SENSITIVITY is defined as:
Equation 2.12
Automatic Frequency Control (AFC)
K
K
143 kHz.
SENS
SENS
=
1.56 10
-----------------
K
74.4
VCXO
51.7 kHz in the 8 k mode, and
–
2
page
143 kHz in
4-18) is
2-11