MC1374 View Datasheet(PDF) - Motorola => Freescale

Part Name

Description

Manufacturer

MC1374

TV MODULATOR CIRCUIT

Motorola => Freescale 

MC1374

FM Section

The oscillator center is approximately the resonance of the

inductor L2 from Pin 2 to Pin 3 and the effective capacitance

C3 from Pin 3 to ground. For overall oscillator stability, it is

best to keep XL in the range of 300 Ω to 1.0 kΩ.

The modulator transfer characteristic at 4.5 MHz is shown

in Figure 15. Transfer curves at other frequencies have a very

similar shape, but differ in deviation per input volt, as shown in

Figures 13 and 17.

Most applications will not require DC connection to the

audio input, Pin 14. However, some improvements can be

achieved by the addition of biasing circuitry. The unaided

device will establish its own Pin 14 bias at 4 θ, or about 3.0 V.

This bias is a little too high for optimum modulation linearity.

Figure 14 shows better than 2 to 1 improvement in distortion

between the unaided device and pulling Pin 14 down to 2.6 V

to 2.7 V. This can be accomplished by a simple divider, if the

supply voltage is relatively constant.

The impedance of the divider has a bearing on the

frequency versus temperature stability of the FM system. A

divider of 180 kΩ and 30 kΩ (for VCC = 12 V) will give good

temperature stabilization results. However, as Figure 18

shows, a divider is not a good method if the supply voltage

varies. The designer must make the decisions here, based

on considerations of economy, distortion and temperature

requirements and power supply capability. If the distortion

requirements are not stringent, then no bias components are

needed. If, in this case, the temperature compensation needs

to be improved in the high ambient area, the tuning capacitor

from Pin 3 to ground can be selected from N75 or N150

temperature compensation types.

Another reason for DC input to Pin 14 is the possibility of

automatic frequency control. Where high accuracy of

inter–carrier frequency is required, it may be desirable to feed

back the DC output of an AFC or phase detector for nominal

carrier frequency control. Only limited control range could be

used without adversely affecting the distortion performance,

but very little frequency compensation will be needed.

One added convenience in the FM section is the separate

Pin “oscillator B+” which permits disabling of the sound

system during alignment of the AM section. Usually it can be

hard wired to the VCC source without decoupling.

Standard practice in television is to provide pre–emphasis

of higher audio frequencies at the transmitter and a matching

de–emphasis in the TV receiver audio amplifier. The purpose

of this is to counteract the fact that less energy is usually

present in the higher frequencies, and also that fewer

modulation sidebands are within the deviation window. Both

factors degrade signal to noise ration. Pre–emphasis of 75 µs

is standard practice. For cases where it has not been

provided, a suitable pre–emphasis network is covered in

Figure 20.

It would seem natural to take the FM system output from

Pin 2, the emitter follower output, but this output is high in

harmonic content. Taking the output from Pin 3 sacrifices

somewhat in source impedance but results in a clean output

fundamental, with all harmonics more than 40 dB down. This

choice removes the need for additional filtering components.

The source impedance of Pin 3 is approximately 2.0 kΩ, and

the open circuit amplitude is about 900 mV pp for the test

circuit shown in Figure 11.

The application circuit of Figure 1 shows the

recommended approach to coupling the FM output from Pin 3

to the AM modulator input, Pin 1. The input impedance at Pin

1 is very high, so the intercarrier level is determined by the

source impedance of Pin 3 driving through C4 into the video

bias circuit impedance of R4 and R5, about 2.2 k. This

provides an intercarrier level of 500 mV pp, which is correct

for the 1.0 V peak video level chosen in this design. Resistor

R6 and the input capacitance of Pin 1 provide some

decoupling of stray pickup of RF oscillator or AM output which

may be coupled to the sound circuitry.

Figure 11. FM Test Circuit

fo C3 L2

(MHz) (pF) (µH)

10.7 12 10

4.5 120 10

1.76 200 40

C14

0.01µF

VCC

7

8

6

9

5

10

4

11

Intercarrier

L2

Sound Output

(Use FET Probe)

10µH

3

C5

12

C3

120pF

2

0.001

µF

1

13

14

Optional Bias R

(See Text)

R12 C6

+ 1µF

Audio

R13 Input

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

1.4

Figure 12. Modulator Sensitivity

TA = 25°C

2.0

3.0 4.0 5.0 6.0 7.0 8.0 9.0 10 14

fosc, OSCILLATOR FREQUENCY [MHz]

6

MOTOROLA ANALOG IC DEVICE DATA

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