Aft Circuit - Sanyo AVM-2550S Training Manual

The Automatic Fine Tuning (AFT) program incorporated in the CPU functions
to fine tune the tuner local oscillator to the center of the actual broadcast
frequency. This is necessary because the transmitted TV signal may not be
exactly on its assigned channel frequency.
channels are purposely slightly offset to reduce interference from broadcast
frequencies. The operating range of the AFT is ± 2.25 MHz from FCC center.
Specifically, the AFT searches the frequency band ± 2.25 MHz from FCC
center in 62.5 KHz steps by changing the PLL division ratios while evaluating
the binary signals from IC101, the Signal Processor.
The station center is determined to have been found when the Time Base
signal at pin 26 is High and the AFT S-Curve signal at pin 29 is between 3.3
VDC and 1.67 VDC. However, the slope (right down) is also checked to
distinguish the station center from a pseudo-tuning point.
S-CURVE
In addition, certain CATV
IC801
CPU
R167
AFT
29
R168
R372
R371
Q372
R377
TIME
26
BASE
R376

AFT CIRCUIT

The Time Base signal is the AND signal of the horizontal sync signal from the
flyback transformer and the horizontal sync signal from the video (Y) signal.
The AFT S-Curve signal is an indication of the video IF carrier frequency
45.75 MHz.
The Time Base and AFT S-Curve signals are checked after each stepping
action. If station center cannot be confirmed after a complete search of the
upper and lower limits of the AFT range, the frequency is returned to FCC
center and the AFT action ceased.
The AFT only operates when first entering the channel. When entering the
channel C05 or C06, the operating range of the AFT is ± 2.25MHz from FCC
center +125 KHz (2 steps).
7.6 V
R161
C161
R162
C258
Q371
R373
AFT Circuit
– 12 –
IC101
SIGNAL
PROCESSOR
13
18
C371