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MC44355DW Ver la hoja de datos (PDF) - Motorola => Freescale

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MC44355DW Datasheet PDF : 20 Pages
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MC44353 MC44354 MC44355
Test Pattern Generator
A simple test pattern is generated on the IC which can be
switched in under bus control to permit a TV receiver to easily
tune in to the modulator output. The pattern consists of two
white vertical bars on a black background and a 976 Hz audio
test signal.
Figure 14. TPSG Typical Video Waveform
TE2
7/10
maximum permissible modulation depth (50 kHz FM
deviation, 85% AM depth), may be adjusted under bus
control.
UHF Oscillator
The UHF oscillator is designed to operate over a range of
450 to 860 MHz. The oscillator drives an external LC tank
circuit differentially, and is tuned by a varicap diode. The
varicap tuning voltage, as described in an earlier section, is
provided from an on chip operational amplifier and external
filter arrangement which is controlled by the PLL Section of
the chip. The UHF frequency thus generated is used by the
modulator as the TV channel carrier frequency.
TE1
0
10
24 28
20
30
3/10
44
40
50
60 64
Video Modulator and Sound Mixer
This section of the circuit accepts as inputs:
1. composite video;
2. the selected sound subcarrier I2C frequency;
3. the UHF carrier frequency at the selected channel;
4. the test pattern generator waveform.
Time in µS
MODULATOR SECTION
Video Input–Clamp and Peak White Clip
The modulator requires a composite video input with
negative going sync pulses and a nominal level of 1.0 Vpp.
This signal is ac coupled to the video input where the sync tip
level is clamped.
The video signal is then passed to a peak white clip circuit
whose function is to soft clip the top of the video waveform if
the amplitude from the sync tip to peak white goes too high.
In this way over-modulation of the carrier by the video is
avoided.
Sound Subcarrier Modulator
The sound modulator system consists of an FM modulator
incorporating the sound subcarrier oscillator, and an AM
modulator. The audio input signal is ac coupled into the
amplifier which then drives the two types of modulator. In
order to provide the accuracy needed for sound subcarrier
frequencies, the sound oscillator consists of a
phase/frequency locked loop. An external LC tank circuit is
required, and the oscillator frequency is controlled by varicap
tuning diodes. The resulting oscillator frequency is divided
down by a divider whose ratio can be controlled via the bus.
A phase/frequency comparator is then used to compare this
frequency with a reference frequency (Fref 2), obtained from
the main PLL Section. The resulting error voltage is used to
control the varicap. To allow all tuning voltage to be derived
from VCC, a hyper–abrupt type of tuning diode is required to
cover the necessary capacitance range. If only a single
sound subcarrier frequency is being used, for example for
PAL only or NTSC, then a less abrupt varicap diode may be
used. The sound phase frequency comparator also requires
an external loop filter.
The oscillator provides subcarrier frequencies of 4.5, 5.5,
6.0 and 6.5 MHz, selectable via the bus. For all applications
except system L, the subcarrier is frequency modulated with
the audio signal. For system L, amplitude modulation is
employed. The level of audio at the input needed to give the
Selection is made via the control bus between the
composite video input and the on chip test pattern generator.
The video and sound inter-carrier are used to amplitude
modulate the UHF carrier. Negative modulation is used,
except in the case of System L where positive modulation is
used.In this part of the circuit, the video modulation depth and
the sound to picture carrier ratio may be programmed under
bus control. In system L mode the video modulation depth
has the same range, but may extend to higher percentage
values.
RF O/P Buffer
The TV signal generated in the video modulator and mixer
section is fed to an emitter follower output stage, capable of
driving a terminated line. This output is provided with a
separate VCC pin in order to avoid large circulating currents
on the IC. It can provide at the output typically 84 dBµV of
signal across a 75 load.
Transient Output Inhibit
To minimize the risk of interference to other channels
while the UHF PLL is acquiring lock on the desired frequency
at Power-on, the UHF output stages are turned off for each
power-on from zero and from standby mode. There is a
timeout of 263 ms until the output is enabled. This allows the
PLL to settle on its programmed frequency. Care must be
taken therefore in determining the loop filter components so
that the loop transient does not exceed this delay.
Data Retention
The circuit contains 4 bytes of memory holding the last
frequency and control bits information. The circuit can retain
this information at power down if a suitable VCC is supplied.
The Standby VCC of nominal 5.0 V must be applied to pin
VCC Mod. The 5.0 V current in data retention is approximately
500 µA. Note that the voltage source on this pin must be able
to supply a much higher current in normal operation (typically
12 mA).
The circuit will enter into Data Retention Mode when the
VCCA pin voltage drops below approximately 3.0 V.
10
MOTOROLA ANALOG IC DEVICE DATA
 

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