Advertisement
Video and Sound Intercarrier Demodulation
To ensure that the above performance improvements
were not lost elsewhere, great care was taken with the
design of the video demodulator and video amplifiers. One
example is in the architectural placement of the phase shift
amplifier (Figure 22) that is required for video demodulation.
This amplifier was placed in series with the IF signal side of
the demodulator, instead of the oscillator side as is common
practice. The 90 phase shift is obtained by a capacitively
coupling each of the differential amplifier driver emitters to
the video demodulator inputs. This results in an output
current that is at 90 with respect to the input voltage over a
wide range of frequencies. Small phase errors that are
caused by the transistor dynamic small–signal emitter
resistance are corrected with the use of cross–coupled
emitter resistors. This arrangement leads to a simpler design
with the ability to tailor the demodulation angle for the lowest
possible distortion at the IF/demodulator interface. The
dynamic emitter resistances, which can give rise to distortion,
are now in quadrature with the capacitive reactance and
therefore contribute very little to the resultant output.
After the PLL attains phase–lock, video and sound
demodulation is obtained by the use of two separate double
balanced multipliers. Video demodulation is accomplished by
multiplying the non–limited 90 phase shifted carrier signal,
with the regenerated vision carrier that is obtained from the
Frequency Doubler output. Both positive and negative video
outputs are produced. The phase relationship between the
video demodulator inputs is 0 since the carrier signal is
phase shifted 90 . This is done in order to cancel out the 90
phase shift that is present at the inputs of the Phase Detector
when it is locked. The sound intercarrier signal is also
recovered by a multiplier in a similar manner to that of the
video. In this case the carrier signal is not phase shifted, and
the phase relationship between the sound demodulator
inputs is 90 . A consequence of this phase relationship is that
only the higher frequency video components are
demodulated while the lower frequency components, those
that fall within the vestigial sideband, are suppressed. With
negative polarity modulation systems, a significant reduction
in the level of white character sound buzz and hum is
achieved. This is most noticeable when demodulating video
signals that contain a high luma level which can cause the
modulation index to exceed 100 percent.
Figure 22. 90 Phase Shift Amplifier
+V in
Video Outputs
Each of the video outputs are part of a wide bandwidth
operational amplifier with internal dc feedback and frequency
compensation. The AGC reference provides the same
composite video output level of approximately 2.2 Vpp for
MOTOROLA ANALOG IC DEVICE DATA
Downloaded from
Elcodis.com
electronic components distributor
I out
I out
+
+
V ref
–V in
MC44302A
both positive and negative polarities of video modulation. The
positive video output appears at Pin 6 and is intended to drive
the luma and chroma channels. This output contains a White
Spot Inverter that is used to invert and clamp any
demodulated noise that is significantly above the white level.
This effectively removes the whiter than white noise
produced by the true synchronous demodulator and prevents
the CRT from being overdriven and defocused. The white
spot inversion threshold and clamp levels are set to
approximately 4.0 V and 2.5 V respectively. The negative
video output appears at Pin 5 and is intended to be used as
a sync separator source. With a simple preseparator low
pass noise filter, this output will provide optimum sync
performance. The video outputs are designed to drive a
resistive load that is in the range of 2.0 kΩ. Lower resistance
values could increase differential phase and gain distortion.
Figure 23. Positive Video Output with
4.0 V
3.7 V
2.5 V
1.2 V
AM & FM Sound IF and Detection
The intercarrier sound that is present at Pin 28 normally
connects through a ceramic bandpass filter to either the FM
IF and Detector input at Pin 2, or the AM IF and Detector
input at Pin 23. With the FM IF, intercarrier sound is limited by
a five stage ac coupled amplifier yielding high sensitivity and
a high level of AM rejection. The typical limiting threshold is
80 µV, and the AM rejection ratio is in excess of 50 dB. FM
detection is accomplished by a self tuning quadrature
demodulator. An internal reactance stage with phase
compensation is controlled to automatically adjust the tuning
of an external tank circuit eliminating the need for manual
alignment. The tank is a parallel circuit consisting of a fixed
value inductor, capacitor, and resistor. The tuning range is
controlled by the ratio of the internal capacitance change to
that of the fixed external tank capacitance. The internal
capacitance is controlled by the voltage present on the
Sound AFT Filter, Pin 7. The capacitance ranges from
0.25 pF to 19 pF, refer to Figure 9. Figure 10 shows the self
tuning frequency range for three inductor values. In general,
for fixed frequency applications, the external tank
capacitance should be in the range of 56 pF to 82 pF. This
should allow sufficient tuning range to account for the
component tolerances. The L–C values should be selected
so that the AFT filter operates below 2.4 V when properly
tuned to the sound intercarrier. This yields the best low signal
lock–in performance, since the AFT filter voltage approaches
1.0 V under no signal conditions. Multi–standard applications
that require a wide intercarrier tuning range can be
accomplished by using a small external capacitance with a
White Spot Inversion
White Spot Inversion Threshold
Normal
0% and
100%
Carrier
Levels
White Spot Clamp Level
11
Advertisement
