Specifications

CA10 Course Notes
Sharp Electronics (UK) Limited
CE Technical Support GroupPage 32
this is provided from the +150 volt line via the dropper resistors R211, R213 and R214 and IC202. IC202
is effectively a 32 volt high stability zener diode.
Other supplies need to be present to make the tuner work correctly. Although there is only one other
supply source ( +5V ) this is used internally for the digital to analogue converter, RF amplifier, tuning
stages and IF processing circuits. The low pass filtering circuit on the supply line comprising of L201 and
C204 ensure that no high frequency noise enters the tuner, this could lead to the tuner oscillating
erratically resulting in picture corruption.
Once the tuner has tuned into a channel, the IF signal output passes out of pins 10 and 11 via the SAWF
filter, SF201, and into the main IF amplifier contained within IC201. IC201 then further amplifies the signal
and converts it into a composite baseband signal.
Automatic frequency control
To keep the station on tune at all times, an automatic frequency control system is utilised. Once again
this circuit is based around the system microprocessor. Unlike previous chassis, there is no external AFC
circuitry, as all the processing is done within IC101 and IC201. If it is seen the signal is going off tune,
then the microprocessor will change the RF tuning point within the tuner via the I C bus line. The
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reference for the AFC circuit is maintained by the detector coil T201, which has to be correctly set to
ensure that the set does not drift off tune on off air broadcasts.
Tuning features
As this receiver uses a digitally controlled tuner, it is possible to add some advanced user features to the
receiver. Direct channel entry is possible using the remote control and auto sort tuning is provided.
As the receiver leaves the factory, the auto sort tuning feature is enabled, this is so that the end user does
not have to manually tune the unit in when getting it home. When the receiver is first turned on, the end
user is presented with a screen that instructs them to connect the aerial, VCR ( while playing a tape ) and
satellite receiver ( switched on and tuned to a station ) to the rear of the set via the RF socket. They are
then requested to press any button on the remote control, the auto tuning/sorting sequence will now be
initiated.
First the signal carrier is detected, then the sync detector detects a line sync signal and the tuning
operation slows down and the signal is fine tuned, both of these operations are contained within IC201
and control is maintained by IC101 via the I C bus 2. Next the teletext header is decoded by IC106 and
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the time taken to decode it is stored within the NVM, IC104, via the I C bus 1. The data value relating to
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the VT line voltage is also stored. This process continues for all channels.
When this process is completed the text header ( if transmitted ), time taken to obtain the text header and
tuning point will all be stored in the NVM. The set will now sort this information depending on the text
header and time taken to obtain the text header. Contained within the EPROM, IC105 is the instructions
to store BBC1 on PR1, BBC2 on PR2, ITV on PR3, Channel 4 or SC4 on PR4 and Channel 5 ( if being
transmitted ) on PR5. As the VCR was playing a tape ( no text header ) this will be put on CH6 and any
other equipment connected at RF will be put on the next available program numbers.
As the receiver may pick up signals from other transmitters, it is necessary for the system processor to
decide which ones are more desirable to put in the first positions. This is done by examining the time to
decode up the text header. The faster the text header is decoded, the stronger the signal and so the
strongest broadcast is stored in the primary position for that station. As it is possible that the incorrect
station may be stored in the primary position, i.e. Carlton ITV instead of Meridian ITV, a tuning editing
menu is provided so that the end user can change this.
Note that any stations that are too weak to be stored in the primary positions are stored in the remaining
free locations after the VCR and satellite channels ( if connected ). For a circuit description of this
process, please refer to the Automatic tuning circuit operation section of these notes.
IF signals
From pin 6 of IC201, the base band signal ( a frequency spectrum of the base band signal is shown on
the next page ) passes into the emitter follower, Q202. This ensures that the signal is matched to the
following stages and allows buffering from the output of IC201. From the emitter of Q202, the signal splits