L64780 LSI Logic Corporation, L64780 Datasheet - Page 30

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

2.7.4 AFC_INIT_FREQ Register Definition
2.7.5 AFC External Loop Filter
2-12
In AFC digital loop mode, the AFC_INIT_FREQ register is defined as a
two’s complement 24-bit register with the following usage:
Equation 2.13
where W
In AFC analog loop mode, the AFC_INIT_FREQ register is defined as a
two’s complement 24-bit register and in the L64780 has the following
usage:
Equation 2.14
where W
The control signal of the frequency loop drives the Sigma-Delta
modulated output AFCOUT. Use the AFCOUT signal to drive an external
passive RC filter that feeds the timing clock control stage, as shown in
Figure
Figure 2.5
The 3 dB cutoff frequency of this circuit is f
estimate of the Frequency Loop Bandwidth and resulting in a low-pass
filter time constant:
Equation 2.15
Architectural Overview
AFCOUT
2.5.
0
0
is the central frequency of the VCXO.
is the frequency of the VCXO.
5V
AFC Loop Pass Filter
W
T
R= 10 k
W
1
i
i
=
=
RC
W
W
0
0
+
=
R
5K
------------------
300 s
143 10
-------------------------
AFC
2
vcxo
20
2
= 100 k
23
3
AFC_INIT_FREQ
AFC_INIT_FREQ
C
AFC
0
= 3.3 nF
= 482 Hz, giving a rough