Automatic tuning method and apparatus of double conversion tuner

A method for automatically tuning a double conversion tuner, said method comprising the steps of:

• a) tuning a voltage controlled oscillator (VCO) circuit by a phase locked loop (PLL) circuit and tuning a radio frequency (RF) signal by a tuning voltage generated by a program contained within a memory element of a microprocessor which compares the automatic gain control (AGC) voltage of a picture carrier wave with a maximum AGC voltage;
• b) if the maximum AGC voltage is less than or equal to the picture carrier wave AGC voltage, then an intermediate frequency (IF) AGC voltage is used as the tuning voltage and stored in memory;
• however, if the maximum AGC voltage is greater than the picture carrier wave AGC voltage, then the tuning voltage is adjusted to the maximum AGC voltage and stored in memory, and then a 4.5 MHz signal is added to the oscillation frequency of the VCO circuit;
• c) detecting a sound carrier wave and adjusting its tuning voltage such that the AGC voltage of the sound carrier wave has a maximum gain value, then storing said value in the memory and then adjusting the sound carrier wave by the oscillation frequency of the VCO circuit;
• d) adjusting the RF signal by an RF tuning voltage having a value equal to the difference between the picture carrier wave voltage and the sound carrier wave voltage, and storing said RF tuning voltage in said memory;
• e) subtracting the AGC voltage of the picture carrier wave from the AGC voltage of the sound carrier wave and generating a second picture carrier wave AGC and equating the AGC voltage of the second sound carrier wave to the AGC voltage of the picture carrier wave;
• f) comparing the absolute value of the AGC voltage difference between the second picture carrier wave and second sound carrier wave generated in step (e) with a fixed numeric value corresponding to the gain value difference at the time when an AGC voltage of a third picture/sound carrier wave has become equal to a predetermined degree;
• g) using the AGC voltage of said third picture/sound carrier wave as a final tuning voltage when the absolute value and the fixed numeric value are equal at step (f), and, when the values are different, if the AGC voltage of the third picture carrier wave is greater than the AGC voltage of the third sound carrier wave, then changing the tuning voltage according to this compared value and storing it in memory;
• and
• h) changing the AGC voltage of the third picture/sound carrier wave in step (g) in accordance with changes in the tuning voltage of the third picture carrier wave, and then repeating steps (f) and (g) above.