Advertisement
Upper Channel
Snd
Adjacent
Upper Channel
Snd
It must be noted that in the operating description of this
device, any reference made to the amount of VCO offset or
sweep is the actual effect on the IF passband. The true VCO
frequency change is only one half of that stated due to the
Frequency Doubler circuit.
The AFT system is designed to control all types of varactor
tuned local oscillators via the AFT Mode Switch input at
Pin 12. This input is used to activate the output of the AFT
control amplifier that appears at Pin 11, and to select the
control voltage polarity versus IF frequency. With the AFT
Mode Switch input connected to V CC , Pin 11 is placed in a
sourcing mode when the IF carrier frequency is below
MOTOROLA ANALOG IC DEVICE DATA
Downloaded from
Elcodis.com
electronic components distributor
Figure 24. Acquisition Circuit Operation
39.75
41.25
Adjacent
41.75
Pix Trap
Adjacent
Upper Channel
Pix
Snd
Snd
Adjacent
Desired
Channel
Pix
Snd
Phase Detector Inhibited
Desired
Channel
Pix
Pix
Snd
Adjacent
Upper Channel
Snd
Pix
Snd
MC44302A
43.75
45.75
47.25
Adjacent
Snd Trap
Desired
Adjacent
Channel
Lower Channel
Pix
Snd
Adjacent
Lower Channel
Pix
Snd
Carried
Detected
Phase Detector Active
Adjacent
Lower Channel
Snd
Pix
Desired
Channel
Lower Channel
Pix
Snd
nominal. With the AFT Mode Switch input grounded, Pin 11 is
placed in a sinking mode when the IF carrier frequency is
below nominal. With the AFT Mode Switch input
disconnected, Pin 11 is internally clamped to one half of V CC ,
refer to Figures 6 and 25. Under this condition the TV set can
be tuned manually and appear to have a conventional type of
AFT with a smooth capture characteristic. Most other PLL
AFT systems cannot be manually tuned in this manner as
they tend to exhibit an undesirable abrupt capture
characteristic. Digital phase–locked loop tuning systems can
also be controlled with the addition of a varactor diode used
to shift the PLL reference oscillator.
Typical IF Bandpass Filter Response
Properly Tuned Desired Channel
Pix
Initial 2.0 MHz VCO with the AFT clamped. Note that
if the Desired Channel picture carrier appears, it will
be centered in the IF passband.
Pix
When a beat note is detected, the VCO is swept another 2.0 MHz
low with the phase detector is inhibited. The VCO is then swept high
with the phase detector enabled. Upon phase lock, the AFT clamp is
removed, the initial VCO offset is slowly released, and the VCO
Sweep is inhibited. Capture of the desired picture carrier is assured
even if mistuned 2.0 MHz.
–2.0 MHz mistuning of the Desired
Channel with an initial 2.0 MHz offset.
Adjacent
2.0 MHz mistuning of the Desired
Channel with an initial 2.0 MHz offset.
Pix
15
Advertisement
