SL5067
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.
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.
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.
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.
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.
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
resistor connected to ground or V
CC
depending on whether
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.
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.
FM/AM select
The voltage on the FM/AM select pin should be defined by
two external resistors between v
cc
and ground, see Figs 6 and
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
value (5V or less) is used between V
CC
and GND since this will
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
GEC [ GENERAL ELECTRIC COMPANY ]
