4

SL5162

Modulation

Video

Standard

NTSC (M)

PAL (G)

PAL (I)

SECAM (L)

Negative

Negative

Negative

Positive

Mod. index (%)

SC freq. (MHz)

Sound

Modulation

83

83

83

94

4·5

5·5

6·0

6·5

FM

FM

FM

AM

Pre-emphasis (

75

50

50

Through

Table 2 Video modulation/standard selection

Figure 3 TSG waveform

APPLICATION NOTES

The key to achieving acceptable modulator performance is

to ensure a well planned circuit layout with good RF decoupling

of all supplies and sensitive pins. Ground loops should be

avoided or kept as small as possible since RF coupling is the

single most important characteristic in degrading modulator

performance. Where possible, double sided board with a

ground plane should be used and care taken to decouple all

sensitive pins as close to the device as possible.

Oscillator Design and Layout

The oscillator layout should be kept as small as possible

to minimise parasitics. It is also recommended that the ground

plane is kept as far away from the oscillator itself since this will

minimise the unwanted capacitance from the tank components

to the ground plane. This has two advantages:

1. It increases the oscillator tuning range

2. It minimises the amount of RF coupled into the ground

plane by the oscillator.

The circuit diagram shown in Figure 4 can be used for UHF

or VHF applications. Surface mount components should be

used wherever possible since these will minimise parasitics

and also allow a more compact circuit design. For applications

at VHF the values of the tuning components must be modified:

the coupling capacitors should be increased to a value of

2·2pF or greater. For fixed frequency applications up to

100MHz (or limited tuning range applications) 15pF coupling

capacitors can be used. Varactor tuning of the SL5162 should

not be attempted unless the oscillator is synthesised, due to

the wide temperature variation of varactor diodes. The

application shown in Figure 4 uses a Mitel 1.3GHz TV

synthesiser. This provides the required stability and tuning for

bus synthesisers such as SP5611, SP5502, SP5026 should

be suitable for use with the SL5162

64

µs

4

µs

20

µs

4

µs

4

µs

12

µs

20

µs

WHITE

BLACK

SYNC

Modulated Outputs

The routing of the modulated outputs requires special

attention since these are particularly vulnerable to coupling

from the VCO: unwanted coupling of the LO frequency to the

RF output will modify the modulation depth. Typically, in

instances where RF coupling is present, the amount of

coupling (and the phase) will vary as the oscillator is tuned

across the band, causing the modulation index to be either

higher or lower than the situation where no RF coupling is

present. Thus the modulation index will vary as the device is

tuned from one channel to another. For VHF and other

applications below 500MHz, oscillator coupling is not such an

issue, however similar care should still be taken with the

layout in order to maximise device performance.

Use of a Balun

It is possible to further improve device performance with

the use of a balun to remove the effects of common mode

coupling. Although using a balun will add to component cost,

it may be the best way to achieve maximum performance at

higher frequencies where common mode noise has made it

impossible to achieve the required SNR or dynamic range in

the output signal. A low cost balun wound on a ferrite bead

former should be sufficient to provide adequate performance

in the majority of applications.

Sound Tank Circuit

It is recommended that the sound tank shown in Figure 4

is used. For PAL G, NTSC or SECAM applications, the value

of the inductor may be modified to achieve the required

subcarrier frequency. The tank circuit can be tuned between

4MHz and 7MHz. The sound subcarrier is automatically gain

controlled to a value which gives a 16dB picture-to-sound

carrier ratio when the SL5162 is set to negative modulation.

The audio input signal is AC coupled through pin 11. The FM

sensitivity of the of the sound section is 0·7kHz/mV, therefore

a 140mVp-p input signal should give a 650kHz deviation of

the sound subcarrier.

Stereo Sound

For stereo applications, a previously modulated second

subcarrier should be input via the SUB 2 input (pin 13). For

example, with the German Panda system, this would normally

be at 5·74MHz. An input level of 200mV p-p should provide

the required picture carrier/sound carrier ratio of 22dB.

Video Modulation/Standard Selection

The SL5162 may be switched between negative and

positive modulation standards via MSS, pin 14. This configures

both the sense of video modulation (negative or positive), and

switches the modulation on the sound subcarrier to AM when

positive video modulation is selected. These standards are

switched as shown in Table 3. To ensure reliable startup over

synthesiser used in the system. This is shown in the demon-

stration board circuit, Figure 4. To do this, one of the

TEST SIGNAL GENERATOR (TSG)

The internal test signal generator is driven from a 500kHz

ceramic resonator. The TSG waveform is shown in Figure 3

and has an effective input video amplitude of 1V sync tip to

white. Note that when TSG is enabled, the sound subcarrier

modulation is disabled and so the audio is muted.