MC1374
Figure 2. TV Modulator
Bias
Section
FM Oscillator/Modulator
Sound Carrier
AM Modulator
V RF Out
CC
AM Oscillator
RF Tank
Sound Carrier
Oscillator
Audio In
OSC B+
14
4
3
2
8
9
7
6
R12
6.0k
R10
R11
R16
Q21
Q7
Q22
Q1
Q2
C1
R13
325
Q19 Q20
Q12 Q13
Q14 Q15
R17
Q25
Q24
R14
Q3
Q6
Q4
Q5
Q10
Q8
Q11
Q9
R15
Q26
Q27
Q16
Q17
Q18
Q23
D1
R1
R2
R3
R4
R5
R6
R7
R8
R9
5
Gnd
1
13 12
Gain
11
Video In
Sound Carrier
In
GENERAL INFORMATION
The MC1374 contains an RF oscillator, RF modulator, and
a phase shift type FM modulator, arranged to permit good
printed circuit layout of a complete TV modulation system.
The RF oscillator is similar to the one used in MC1373, and is
coupled internally in the same way. Its frequency is controlled
by an external tank on Pins 6 and 7, or by a crystal circuit, and
will operate to approximately 105 MHz. The video modulator
is a balanced type as used in the well known MC1496.
Modulated sound carrier and composite video information
can be put in separately on Pins 1 and 11 to minimize
unwanted crosstalk. A single resistor on Pins 12 and 13 is
selected to set the modulator gain. The RF output at Pin 9 is
a current source which drives a load connected from Pin 9 to
base station (1.76 MHz), and high enough to be used as an
FM IF test signal source (10.7 MHz). At 4.5 MHz, a deviation
of ±25 kHz can be achieved with 0.6% distortion (typical).
In the circuit above, devices Q1 through Q7 are active in
the oscillator function. Differential amplifier Q3, Q4, Q5, and
Q6 acts as a gain stage, sinking current from input section
Q1, Q2 and the phase shift network R17, C1. Input amplifier
Q1, Q2 can vary the amount of “in phase” Q4 current to be
combined with phase shifter current in load resistor R16. The
R16 voltage is applied to emitter follower Q7 which drives an
external L–C circuit. Feedback from the center of the L–C
circuit back to the base of Q6 closes the loop. As audio input
is applied which would offset the stable oscillatory phase, the
frequency changes to counteract. The input to Pin 14 can
include a dc feedback current for AFC over a limited range.
The modulated FM signal from Pin 3 is coupled to Pin 1 of
the RF modulator and is then modulated onto the AM carrier.
V
CC.
The FM system was designed specifically for the TV
intercarrier function. For circuit economy, one phase shift
circuit was built into the ship. Still, it will operate from 1.4 MHz
to 14 MHz, low enough to be used in a cordless telephone
3
MOTOROLA ANALOG IC DEVICE DATA
MOTOROLA [ MOTOROLA ]
