SL5162
Standard
NTSC (M)
PAL (G)
PAL (I)
SECAM (L)
Negative
Negative
Negative
Positive
Video
Modulation
Mod. index (%) SC freq. (MHz)
83
83
83
94
4·5
5·5
6·0
6·5
Sound
Modulation
FM
FM
FM
AM
Pre-emphasis (
µ
s)
75
50
50
Through
Table 2 Video modulation/standard selection
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.
64µs
WHITE
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.
4
µs
20
µs
4
µs
12
µs
4
µs
20
µs
BLACK
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.
SYNC
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.
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.
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
the VCO. Any of the of the Mitel Media Products I
2
C and 3-wire
bus synthesisers such as SP5611, SP5502, SP5026 should
be suitable for use with the SL5162
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
the entire V
CC
range it is recommended that the sound tank
circuit is started via an effective pulse from the I
2
C bus
synthesiser used in the system. This is shown in the demon-
stration board circuit, Figure 4. To do this, one of the
4
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