L64704 LSI Logic Corporation, L64704 Datasheet - Page 109

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

5.5.1
Input
Decimation
5.5.2
Clock
Acquisition and
Tracking Modes
The CLK_RATIO[2:0] bits (Group 4, APR 18) set the ratio of the VCO
clock to the required sample clock in the L64704. For example, if the
input signal is 5-Mbaud and the oversampling ratio is N = 4, then the
input sample frequency is 20 MHz. Under the same conditions, it is pos-
sible to use a VCO running at 40 MHz by specifying CLK_RATIO = 1.
This corresponds to an input decimation of 2, and in this case every
second sample coming from the ADC is fed to the L64704.
This feature allows the satellite decoder control program to choose a
reduced VCO range when the L64704 must operate over a wide range
of baud rates. Note that setting the CLK_RATIO bits does not switch any
decimation filters in the data path; therefore the external IF SAW filtering
must fit the baud rate and the oversampling ratio selected via the
CLK_DR bits (Group 4, APR 14).
The Clock Synchronizer operates in two modes:
5.5.2.1 Clock Acquisition Mode
In the acquisition mode (indicated by CLK_LCF = 0; Group 3, APR 9),
the Automatic Frequency Controller (AFC) is active but the Timing Error
Detector (TED) is not. Software can disable the AFC after the Clock
Synchronizer completes acquisition of the timing loop. This prevents the
system from switching back to acquisition mode if a short interrupt
occurs during transmission. Setting the CLK_LCF_SUPPRESS bit
(Group 4, APR 14) disables the AFC.
The AFC control function determines whether the VCO clock is within the
pull-in range of the L64704. If the VCO clock is within the pull-in range,
the CLK_LCF bit is set to 1. If the VCO clock is too slow, then the AFC
drives a positive voltage on the loop output in order to generate a positive
frequency sweep. If the VCO clock is too fast, then the AFC drives a low
voltage on the loop output in order to generate a negative frequency
sweep.
Channel Clock Recovery
Clock Acquisition
Tracking
5-5