When the signal inputs are exactly balanced ideally there is

no RF carrier at the output Circuit board layout is critical to

this measurement For optimum performance the output

and supply decoupling circuitry should be configured as

shown in

Figure 3

TLH5079 – 8

RF decouple supply directly to output ground

FIGURE 3 Correct RF Supply Decoupling

The video clamp level is derived from a resistive divider con-

pin 10 which is biased externally should also be referenced

to supply (see

Figure 2 )

Pin 1Audio Input

Pin 1 is the audio input to the sound

FM generator Frequency deviation is proportional to the

signal at this pin A pre-emphasis network comprised of R1

C2 and the device input impedance yields the following re-

sponse with an 80 mVrms audio input

Pre-Emphasis Network

Response

TLH5079 – 9

Increasing R1 lowers the boost frequency and decreases

deviation below the boost frequency Increasing C2 only

lowers the boost frequency C1 is a coupling capacitor and

must be a low impedance compared to the sum of R1 and

the device input impedance (15 kX)

Pin 2Video Clamp

The video clamp restores the DC

component to AC-coupled video The video is AC-coupled

to the clamp via C3 Decreasing C3 will cause a larger tilt

between vertical sync pulses in the clamped video wave-

form

Pin 3Ground

Although separate on the chip level all

ground terminate at pin 3

Pins 45Channel 4 Oscillator

Pins 4 and 5 are the col-

lector outputs of the channel 4 oscillator L1 and C5 set the

ing L1 will decrease the oscillator frequency while decreas-

ing L1 will increase the oscillator frequency Decreasing C5

will increase the oscillator frequency and lower the tank Q

causing possible drift problems R2 and R3 are the oscillator

loads which determine the oscillator amplitude and the tank

Q Increasing these resistors increases the Q and the oscil-

lator amplitude possibly overdriving the RF modulator

which will increase output RF harmonics Decreasing R2

and R3 reduces the tank Q and may cause increased drift

C4 is an RF decoupling capacitor Increasing C4 may result

in less effective decoupling at RF Decreasing C4 may intro-

duce RF to supply coupling

Pins 67Channel 3 Oscillator

Pins 6 and 7 are the chan-

nel 3 oscillator outputs Every component at these pins has

the same purpose and effect as those at pins 4 and 5

Pin 8Channel 4 RF Output

Pin 8 is the channel 4 RF

output and R13 is the load resistor The RF signal is AC

coupled via C15 to the output filter which is a two channel

VSB filter L5 is parallel resonant with the filter input capaci-

tance minimizing loss in the output network R14 terminated

the filter output

Pin 9Channel 3 RF Output

Pin 9 is the channel 3 RF

output with all components performing the same functions

as those in the pin 8 description

Pin 10RF Modulator Sound Subcarrier Input

Pin 10 is

one of the RF modulator inputs and may be used for video

or sound It is used as a sound subcarrier input in

Figure 2

R8 R9 and R10 set the DC bias on this pin which deter-

mines the modulation depth of the RF output (see Applica-

tion Notes) R12 and C11 AC-couple the sound subcarrier

from the sound modulator to the RF modulator R12 and

R11 form a resistive divider that determines the level of

sound at pin 10 which in turn sets the picture carrier to

sound subcarrier ratio Increasing the ratio of R11R12 will

increase the sound subcarrier at the output C10 forms an

AC ground preventing R8 R9 and R10 from having any

effects on the circuit other than setting the DC potential at

pin 10 R11 and R12 also effect the FM sound modulator

(see pin 13 description)

8