MC1374
AM Section
In television, one of the most serious concerns is the
prevention of the intermodulation of color (3.58 MHz) and
sound (4.5 MHz) frequencies, which causes a 920 kHz signal
to appear in the spectrum. Very little (3rd order) nonlinearity is
needed to cause this problem. The results in Figure 6 are
unsatisfactory, and demonstrate that too much of the
available dynamic range of the MC1374 has been used.
Figures 8 and 10 show that by either reducing standard
signal level, or reducing gain, acceptable results may be
obtained.
At VHF frequencies, small imbalances within the device
introduce substantial amounts of 2nd harmonic in the RF
output. At 67 MHz, the 2nd harmonic is only 6 to 8 dB below
the maximum fundamental. For this reason, a double pi low
pass filter is shown in the test circuit of Figure 3 and works
well for Channel 3 and 4 lab work. For a fully commercial
application, a vestigial sideband filter will be required. The
general form and approximate values are shown in Figure 19.
It must be exactly aligned to the particular channel.
The AM modulator transfer function in Figure 3 shows that
the video input can be of either polarity (and can be applied at
either input). When the voltages on Pin 1 and Pin 11 are
equal, the RF output is theoretically zero. As the difference
between V
and V increases, the RF output
Pin 11
Pin 1
increases linearly until all of the current from both I current
1
sources (Q8 and Q9) is flowing in one side of the modulator.
This occurs when ±(V
typically 1.15 mA. The peak–to–peak RF output is the 2I R .
Usually the value of R is chosen to be 75 Ω to ease the
design of the output filter and match into TV distribution
– V
) = I R , where I is
Pin11
Pin1
1
G
1
1
L
L
systems. The theoretical range of input voltage and R is
G
quite wide, but noise and available sound level limit the useful
video (sync tip) amplitude to between 0.25 Vpk and 1.0 Vpk.
It is recommended that the value of R be chosen so that
G
only about half of the dynamic range will be used at sync tip
level.
The operating window of Figure 5 shows a cross–hatched
area where Pin 1 and Pin 11 voltages must always be in order
to avoid saturation in any part of the modulator. The letter φ
represents one diode drop, or about 0.75 V. The oscillator
Figure 3. AM Modulator Transfer Function
Pins 6 and 7 must be biased to a level of V
2I R (or
1 L
CC – φ –
lower) and the input Pins 1 and 11 must always be at least 2φ
below that. It is permissible to operate down to 1.6 V,
saturating the current sources, but whenever possible, the
minimum should be 3φ above ground.
The oscillator will operate dependably up to about
105 MHz with a broad range of tank circuit component
values. It is desirable to use a small L and a large C to
minimize the dependence on IC internal capacitance. An
operating Q between 10 and 20 is recommended. The values
2I R
1
L
of R , R and R are chosen to produce the desired Q and to
1
2
3
set the Pin 6 and 7 dc voltage as discussed above.
Unbalanced operation, i.e., Pin 6 or 7 bypassed to ground, is
not recommended. Although the oscillator will still run, and
the modulator will produce a useable signal, this mode
causes substantial base–band video feedthrough.
Bandswitching, as Figure 1 shows, can still be accomplished
economically without using the unbalanced method.
0
–I R
+I R
1
1
G
G
Differential Input, V –V (V)
11
1
Figure 4. AM Test Circuit
R2
The oscillator frequency with respect to temperature in the
test circuit shows less than ±20 kHz total shift from 0° to 50°C
as shown in Figure 7. At higher temperatures the slope
approaches 2.0 kHz/°C. Improvement in this region would
require a temperature compensating tuning capacitor of the
N75 family.
470
0.1µH
L1
0.001
470
R3
C2 56
R1
Crystal control is feasible using the circuit shown in Figure
21. The crystal is a 3rd overtone series type, used in series
resonance. The L1, C2 resonance is adjusted well below the
crystal frequency and is sufficiently tolerant to permit fixed
values. A frequency shift versus temperature of less than
1.0 Hz/°C can be expected from this approach. The resistors
Ra and Rb are to suppress parasitic resonances.
Coupling of output RF to wiring and components on Pins 1
and 11 can cause as much as 300 kHz shift in carrier (at
67 MHz) over the video input range. A careful layout can
keep this shift below 10 kHz. Oscillator may also be
inadvertently coupled to the RF output, with the undesired
470
6
7
V
1
8
9
V
CC
1
RL
75
10µF
RF
+
11
22
µH
22
µH
Video
Input
22
47
22
1.0k
12
13
5
effect of preventing a good null when V = V . Reasonable
care will yield carrier rejection ratios of 36 to 40 dB below sync
tip level carrier.
11
1
R
V
G
11
4
MOTOROLA ANALOG IC DEVICE DATA
MOTOROLA [ MOTOROLA ]
