Pin 11Video Input

Pin 11 when configured as shown is

the RF modulator video input In this application video is

coupled directly from the video clamp Alternatively video

could be DC-coupled directly to pin 11 if it is already within

the DC common-mode input range of the RF modulator (see

curves) In any case the video sync tip at pin 11 must have

a constant DC level independent of video content Because

of circuit symmetry pins 10 and 11 may be interchanged

Pin 12RF Supply

Pin 12 is the RF supply with C12 and

C7 serving as RF decouple capacitors Increasing C12 or

C7 may result in less effective RF decoupling while de-

creasing them may cause supply interaction It is important

that C7 be grounded at the RF output ground

Pin 13Sound Tank

Pin 13 is the collector output of the

sound oscillator L3 and C13 determine the oscillating fre-

L3 or C13 will lower the operating frequency while decreas-

ing them will raise the frequency L3 and C13 also help

define the Q of the tank on which FM modulator de-

viation level depends As C13 increases Q increases and

frequency deviation decreases Likewise decreasing C13

increases deviation The other factor concerning Q is the

external resistance across the tank The series combination

resistive network will decrease Q and increase deviation It

should be noted that because the level of phase modulation

of the 45 MHz signal remains constant variation in Q will

not effect distortion of the frequency modulation process if

the audio at pin 1 is left constant The amplitude of the

sound subcarrier is directly proportional to Q so increasing

the unloaded Q or either of the resistors mentioned above

will increase the sound subcarrier amplitude For proper op-

eration of the frequency modulator the sound subcarrier

amplitude should be greater than 2 Vp-p

Pin 14Sound Supply

Pin 14 is the sound supply and C14

is an RF decouple capacitor Decreasing C14 may result in

increased supply interaction

Printed Circuit Layout

Printed circuit board layout is critical in preventing RF feed-

through The location of RF bypass capacitors on supply is

very important

Figure 4 shows an example of a properly

layed out circuit board It is recommended that this layout be

used

TLH5079 – 10

FIGURE 4 Printed Circuit Board and Component Diagram

(Component Side 1X)

9