Applications Information

SOUND FM MODULATOR

Frequency deviation is determined by the Q of the tank cir-

cuit at pin 13 and the current entering the audio input pin 1

impedance (15 kX) and any impedance network connect-

ed externally A signal of 60 mVrms at pin 1 will yield about

g

25 kHz deviation when configured as shown in

Figure 2

VIDEO CLAMP

When video is not available at DC levels within the RF mod-

ulator common-mode range or if the DC level of the video is

not temperature stable then it should be AC-coupled as

shown in the typical applications circuit (

Figure 2 ) The

The clamp coupling capacitor is charged during every sync

pulse and discharged when video information is present

The discharge current is approximately 20 mA This current

and the amount of acceptable tilt over a line of video deter-

mines the value of the coupling capacitor C1 For most ap-

plications 1 mF is sufficient

RF MODULATION

Two RF channels are available with carrier frequencies up

to 100 MHz being determined by L-C tank circuits at pins

4 5 and 67 The signal inputs (pins 10 and 11) are com-

mon to both modulators but removing the power supply

from an RF oscillator will also disable that modulator

The offset between the two signal pins determines the level

of the RF carrier output To preserve the DC content of the

video signal amplitude modulation of the RF carrier is done

in one direction only with increasing video (toward peak

white) decreasing the carrier level This means the active

composite video signal at pin 11 must be offset with respect

to pin 10 and the sync pulse should produce the largest

offset

The largest video signal (peak white) should not be able to

suppress the carrier completely particularly if sound trans-

mission is needed This requires that pin 10 be biased

above the largest expected video signal Because peak

white level is often difficult to define a good rule to follow is

to bias pin 10 at a level which is four times the sync ampli-

tude above the sync tip level at pin 11 For example the DC

bias at pin 10 with 05V sync clamped to 52V on pin 11

Typical Application

TLH5079 – 7

FIGURE 2 Two Channel Video Modulator with FM Sound

7