Specifications

CA10 Course Notes
Sharp Electronics (UK) Limited
CE Technical Support Group Page 35
RF AGC operation
As signals received from off air transmissions can vary tremendously from area to area, it is necessary
to provide some form of automatic gain control on the incoming RF signal. The circuit shown below
performs this action.
Figure 34 : Automatic Gain Control Circuit
It can be seen the at the heart of the system is the main system microprocessor, IC101. Via the clock and
data lines, IC101 will instruct the tuner to tune to the desired signal ( the data for this operation would have
been set by the user at the initial installation and the control values held within the non volatile memory,
IC104 ). The signal from the tuner will enter IC201 ( video IF ) in which there is a detection circuit which
will give out a DC voltage on pin 54 ( AGC OUT ) that is dependant upon signal strength. This is fed
straight back into the tuner AGC pin which will first control the gain of the RF stages within and then the
IF stages, i.e. maximum gain is always given to the RF amplifier to reduce noise.
Automatic tuning operation
During the automatic tuning function it is necessary to detect the presence of a signal. While this is done
by detecting the presence of a valid carrier signal, it is also necessary to monitor the amplitude of the RF
signal to ensure that the RF signal can be correctly decoded. This is one of the functions of the circuit
based around one half of IC501.
IC501 is used as a Schmitt trigger feeds back either a high or low signal to indicate the presence of a RF
signal to the system microprocessor, IC101. The threshold of the Schmitt trigger is controlled by the
components fitted to pin 2 of IC501, which gives a level of 2.39V on pin 2 ( inverting input ). When the
level on pin 3 ( non-inverting input ) reaches this level, pin 1 will change state. This change of state is
detected by IC101 on pin 48 ( AGC ).