MC1374

3

MOTOROLA ANALOG IC DEVICE DATA

Figure 2. TV Modulator

Bias

Section

FM Oscillator/Modulator

Audio In

Sound Carrier

OSC B+

Sound Carrier

Oscillator

VCC

AM Modulator

RF Out

AM Oscillator

RF Tank

14

4

3

2

8

9

7

6

R10

R11

R12

6.0k

R16

Q1

Q2

R13

325

Q25

Q24

Q23

D1

R15

R14

Q3

Q4

Q5

Q6

C1

R17

Q7

Q12 Q13

Q14 Q15

Q21

Q22

Q10

Q11

Q26

Q27

Q8

Q9

Q16

Q17

Q18

R1

R2

R3

R4

R5

R6

R7

R8

R9

5

113

12

11

Gnd

Sound Carrier

In

Gain

Video In

Q19 Q20

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

VCC.

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

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.