L64704 LSI Logic Corporation, L64704 Datasheet - Page 127

• Manufacturer Part Number: L64704
• Description: Satellite Decoder Technical Manual 5/97
• Manufacturer: LSI Logic Corporation

5.7.2
Power Control
Loop
Table 5.5
PWR_BW as a
Function of Symbol
Rate
Figure 5.12
AGC Loop Control
and a unit signal level, a recommended value for R is 1.7, which corre-
sponds to PWR_REF = 84. For L = 4, a recommended value for R is 3.4,
which also corresponds to PWR_REF = 84.
The ratio of the signal level to the ADC range is constant (S:R = 1:1.7)
in
level to keep the S:R ratio constant.
The L64704 measures the signal power at the output of the matched
filter and compares the measured value to an expected value that the
microcontroller has written into the PWR_REF register.
The microcontroller can adjust the power loop bandwidth using the
PWR_BW[1:0] bits (Group 4, APR 20). The loop bandwidth is propor-
tional to PWR_BW and proportional to the symbol rate. Set the
PWR_BW bits according to
PWR_BW
0
1
2
3
The power control signal drives the
this output to drive an external passive RC filter that feeds the gain
control stage
PWR
The R and C values for the passive low-pass filter should be such that
R
Automatic Gain Control (AGC)
AGC
Figure
C
AGC
5.11. If the ADC range changes, the AGC scales the signal
64 s
(Figure
Symbol Rate (MHz)
20 - 45
10 - 20
5 - 10
2 - 5
N
P
5 V
.
5.12).
R ~ 470 ohm
Table
R
AGC
5.5.
~ 30K
modulated output PWRP. You use
C
AGC
~ 2.2 nF
V
AGC
5-23