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Alignment
Tuning of a single coil is all that is required for complete
alignment of the IF amplifier. This is most easily
accomplished with the test set–up shown in Figure 26. The
tuner is set to a given channel and a CW signal that is
precisely set to the picture carrier frequency of that channel,
is connected to the tuner RF input. The dc power supply is
adjusted until the tuner output, measured by the frequency
counter, is equal to the required IF picture carrier (45.75 MHz
in the USA). The VCO coil is then adjusted so that the voltage
across the 8.2 k resistor approaches zero. A voltage level of
less than 5.0 mV should be easy to attain. The RF signal and
the dc supply are removed and alignment is completed.
The tuning system should be designed so that the required
varactor bias is approximately 2.5 V when phase–locked to
the nominal IF signal. This centers the AFT amplifier's
current source/sink output, Pin 11, yielding the maximum
compliance voltage for optimum hold–in and pull–in
characteristics. When interfacing Pin 11 with the tuning
system's control bias, the output current must not exceed
4.0 mA. This current can be limited with the addition of a
series output resistor if the AFT amplifier is required to drive
a low resistance load.
Differential Phase and Sound Buzz
Even with all the care taken in this design, some residual
differential phase still remains. Although small, refer to
Figure 8, it results in an output on the phase detector that
modulates the VCO and the sound intercarrier. This in turn
has the potential of degrading the stereo sound performance.
In addition, there is a quadrature differential phase shift that
is produced by the shape of the IF bandpass filter. Both
produce currents in the output of the phase detector which in
turn phase modulates the VCO. This phase modulation is
imposed on the sound intercarrier resulting in a video related
sound buzz. These currents can be canceled by injecting the
correct amplitude and phase of demodulated video into the
MOTOROLA ANALOG IC DEVICE DATA
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Figure 26. Alignment Configuration
CW Picture Carrier
RF
Local
Mixer
Amp
OSC
Tuner
V CC
8
Bandpass
Filter
9
Frequency
Counter
MC44302A
V
8.2 k
2.5 V
V CC
AFT
Switch
25
12
11
AFT
AFT
Amp
Clamp
IF
Phase
Limiter
Amp
Detector
Limiter
PLL filter. This can be accomplished with the addition of the
differential phase correction circuit shown in Figure 27. The
phase detector current that is due to the in–phase differential
gain is canceled by the resistor current, and the quadrature
component that is induced by the IF filter is canceled by the
capacitor current. With proper adjustment, the differential
phase distortion can be reduced to less than 0.5 degrees as
well as eliminating any perceptible sound buzz. The source
for the demodulated video to be injected into the PLL filter
can be obtained from Pins 5 or 6. This must be determined
experimentally for a given printed circuit board layout in order
to obtain the best results. With the use of the correction
circuit, this system achieves a similar level of performance to
that of a parallel sound IF system.
Electrostatic Protection
Most pins on the IC have electrostatic protection diodes to
V CC and ground. It is therefore imperative that no pin is taken
below ground or above V CC by more than one diode drop,
approximately 0.6 V, without current limiting.
Figure 27. Differential Phase Correction Circuit
V CC
VCO
Coil
19
20
21
VCO
VCO
Freq
Sweep
Doubler
From Negative Video Output Pin 5
or Positive Video Output Pin 6
500 k
5.0–25 pF
82 k
0.1
To PLL Filter
(Main VCO Loop)
Pin 19
17
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