SL5067

8

APPLICATION NOTES

Overview

The key to good modulator performance is to ensure good

and compact circuit layout with adequate grounding of all

supplies. Earth loops must be avoided or kept as small as

possible since RF coupling either through the air, or through

the ground plane itself is the single most important factor in

degrading modulator performance. Double sided board with a

groundplane should be used, and all sensitive pins must be

properly decoupled as close to the device as practicable.

Oscillator design and layout

The oscillator should be kept as small as possible to

minimise parasitics. It is recommended that the circuit diagram

shown in these application notes is used if the entire UHF band

is to be covered. For lower frequencies or for applications

requiring less tuning range, component values can be

adjusted. Surface mount components should be used

throughout the circuit and particular care must be taken with

placement as the two coils should be as close to the oscillator

pins as possible.( See Figs. 16 and 17)

For applications at low VHF frequencies, it is suggested

that the values of the coupling capacitors on pins 9, 10, 11 and

12 are increased, 2.2pF capacitors (or greater) may be used

for frequencies up to 500MHz but it must be remembered that

the larger the coupling capacitor used, the smaller the tuning

range will be, as the varactor diode capacitance will form a

lower percentage of the total tuning capacitance of the loop.

For fixed frequencies (or small tuning ranges) up to

100MHz, 15pF or 18pF capacitors may be used.

Varactor tuning of the SL5067 should not be attempted

unless the application either uses a synthesiser, or a

temperature compensating network is used. The capacitance

of most varactor diodes changes greatly with temperature,

and this must be compensated for if the modulator is to remain

on tune to the correct channel.

For applications requiring tuning over only a few channels,

an air variable capacitor plus appropriate temperature

compensation may be used.

Modulated outputs

Care must be taken with the routing of the modulated

outputs and also with the mod index pin, pin3. It is suggested

that pin 1 is used, and that the unused modulated output on pin

2 is terminated in a way which looks as physically and

electrically similar to the used output on pin 1.

Experiments have shown that a RF coupling problem can

exist between pins 2 and 3. This manifests itself at frequencies

over 600MHz in applications where pin 3 is not taken directly

to ground. Good decoupling of pin 3 (with 10pF and 10nF) will

help to reduce these effects.

The modulated outputs must be routed away from the

oscillator tank as there is danger of the local oscillator signal

coupling directly into the modulated outputs. This will produce

distortions in the modulated signal giving bad performance in

such characteristics as differential phase and gain. For VHF

and other applications below 500MHz RF coupling is not such

a problem, however similar care should still be taken with

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 only way to achieve acceptable performance at

higher frequencies where common mode noise has made it

impossible to achieve a low enough minimum power signal to

give the necessary 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

Care must also be taken with the layout of the sound tank,

in order to minimise harmonics,and reduce coupling between

the audio and video parts of the circuit. The sound tank must

be situated as close to the device pins as possible. If this is not

done, RF may couple into the sound tank, via the tracks

connecting the sound oscillator to the inductor and capacitor.

In practice, it is easiest to mount the sound tank capacitor

close to, or directly on pin 15 and 16, with the inductor slightly

further away. This appears to give the best linearity.

In some cases where some coupling and/or distortion

problems are occurring, the addition of small 2p2 capacitors

from either side of the tank circuit to ground may improve both

FM deviation and linearity.

For optimum performance (in the FM case) the sound tank

should be selected to give a Q of around 10. The circuits

shown in the datasheet give a value of approximately 9, and

are the suggested normalised values to be used.

Lower values of Q will give greater FM deviation per volt

input (kHz/Volt), but also increase the level of the 3rd harmonic

of the sound subcarrier. This is shown in Fig. 10.

The Q of the inductor chosen should be at least 2.5 times

the Q of the tank circuit itself.

It is not recommended that a Q of over 16 is used, as the

amplitude of the sound subcarrier fundamental will start to

decrease once a Q of approx 12 has been reached. Thus if a

Q of 20 were used in order to give good harmonic

performance, there would be an unacceptable trade off in

terms of picture carrier to sound subcarrier ratio, which would

be approx 20dB.

MISCELLANEOUS POINTS

Board layout and decoupling

Good decoupling techniques must be used throughout with

the use of surface mount components wherever possible. For

best performance, all supplies and sensitive pins should be

decoupled as close to the device as possible, with a

combination of capacitors, say 100pF and 10nF to ground.

The use of double sided board with a groundplane is strongly

advised. This should be of particular help in the reduction of

oscillator coupling.

Mod index pin

As already stated, great care must be taken with the mod

index pin, pin 3. This should be decoupled with chip

components as close to the pin as possible. Ideally the mod

index should be defined with a DC voltage, thus requiring the

use of two external resistors, see Figs 4 and 5. It is also

possible to define mod index through the use of a single

negative or positive modulation is required.

Synthesiser drive

It is suggested that any synthesiser (if used) is driven

differentially. This is done by taking both of the prescaler

outputs (pin 7 and 8) to the synthesiser via 1nF or 10nF

capacitors.

FM/AM select

The voltage on the FM/AM select pin should be defined by

7. The application diagram Fig. 14 shows a potentiometer,

RV2 which is used to define the voltage on this pin in the demo

board in practice it is suggested that in low total resistance

ensure a constant voltage on pin 18 irrespective of any small

internal resistance variations between devices, thus ensuring

a constant PC/SC ratio. It should be noted that the sound

subcarrier level is referenced to the AGC sidebands rather