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Phase errors, resulting in quadrature video distortion, can
also be caused by dc errors in the phase detector and AFT
amplifier. Most of the dc offsets are caused by mismatches in
the current mirrors of the push–pull output stage, refer to
Figure 20. Switches SW1, SW2, and SW3 are driven by a
1.0 MHz square wave with an accurate 1:1 mark/space ratio.
Switches SW1 and SW2 maintain the same sense of error
signal, while SW2 ensures errors due to the top PNP current
mirrors average to zero on the external loop filter capacitor. In
a similar way, SW3 by interchanging Q3 and Q4, cancels
errors due to the bottom NPN mirror. With phase errors
reduced to a minimum, there is no need for any external
phase adjustments. The phase detector output is filtered and
it is used to control the VCO in a corrective manner. When the
PLL establishes a locked condition, there will be a 90 phase
shift between the two phase detector inputs.
The Voltage Controlled Oscillator and Frequency Doubler
circuits are shown in Figure 21. The oscillator operates at
one half of the picture carrier frequency and is tuned by a
control bias that is applied to the reactance stage input.
Reactance tuning allows a higher Q to be maintained in the
10
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SW1
1.0 MHz
Square
Wave
I – ∆I
I + ∆I
IF Carrier
Signal
(Limited)
Regenerated
Carrier
(Limited)
Figure 21. VCO and Frequency Doubler
Control
Bias
Reactance
Tuning Stage
MC44302A
Figure 20. Phase Detector
V CC
Q1
Q2
SW3
Q3
Q4
tank circuit as opposed to a phase shift type of oscillator with
the same tuning range. The oscillator frequency is internally
doubled to picture carrier frequency by a balanced multiplier.
Note that the multiplier input signals are at 90 to each other
for frequency doubling.
Since the oscillator operates at one half of the picture
carrier frequency, radiation from the external tuned circuit
components will not desensitize the system, even if picked
up by the amplifier input leads. This significantly reduces the
possibility of a PLL push–off condition. Running the oscillator
at twice the picture carrier and dividing it down is another way
of solving the IF input radiation problem, but there are two
significant disadvantages. First and foremost, radiation into
the antenna now becomes a problem. In the U.S.A. twice the
picture carrier falls directly into the passband of channel 6,
producing a very noticeable beat. Any second order
harmonics, four times picture carrier, will fall into the
passband of channel 8. Second, it is more difficult to produce
a stable oscillator that operates at twice the IF frequency than
one that operates at one half of the IF frequency.
V CC
20
21
4.7 k
Bias
Oscillator
Frequency Doubler
(f OSC = 0.5 Pix Carrier)
Balanced Multiplier
SW2
VCO
PLL
I – ∆I
19
Filter
2∆I
I + ∆I
1.0 MHz
Square
Wave
PLL Limiter for
Video/Sound
Demodulations
(f = Pix Carrier)
MOTOROLA ANALOG IC DEVICE DATA
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