Voltage References

clamp is released, the veo offset is slowly removed, the
sweep generator is inhibited and the phase detector remains
permanently enabled. With the AFT clamp removed, a large
error voltage appears at the AFT output, driving the system
back to the correct frequency. Since the La loop is slow and
the veo loop is fast, the IF changes slowly and the veo tracks
it, maintaining phase lock until the final static conditions are
reached. For large frequency errors during this period, the
slew rate of the La loop is increased but not to the extent
where it would cause any veo tracking problems. This
technique allows the acquisition time of the circuit to be
considerably shortened while still using a larger than normal
Adjacent
t t
AP S
t
Desired
t t
AP S
MC44301
Figure 11. The AFT System In Action
time constant in the La loop. In this way, any possibility of
phase modulating the La with video is removed. Figure 11
illustrates the AFT system in action.
To accommodate all types of tuners the Las, positive or
negative La drive can be selected externally by operation of
the AFT switch. The AFT switch also has a third position which
disconnects the drive to the tuner. Under this condition,
the TV set can be tuned in the normal manner and so appears
to have a conventional type of AFT. Other PLL systems cannot
be tuned manually in this way, having an abrupt
capture characteristic, and because of this, have not gained
general acceptance.
Typical IF Input
Filter Response
IF Bandpass
Properly Tuned Channel
8J
Adjacent Nominal Channel with Initial 2.0 MHz Offset
p AS Initial nominal offset of VCO and LO (AF1). When a beat note is
I detected, AFT bias is held and VCO is swept another 2.0 MHz low with
I phase detector inhibHed, then the VCO is swept high wnh the phase
I I detector active. Upon phase lock, the AFT clamp is removed and the
initial veo offset is slowly removed. capture of desired picture carrier is
I I I assured even H mistuned ±2.0 MHz.
I I I
I I I
ttl I t -2.0 MHz Mistuning with Initial 2.0 MHz Offset
P AS
Desired t t +2.0 MHz Mistuning with Initial 2.0 MHz Offset
P AS
MOTOROLA LINEAR/INTERFACE ICs DEVICE DATA
9-245