Briefly introduce the working principle and function of color TV AFT circuit

The function of the AFT circuit is to automatically and accurately control the local oscillation frequency of the high frequency tuner to the specified nominal frequency to ensure the quality of the color tone image and sound tone.

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When the ambient frequency changes due to changes in ambient temperature, voltage fluctuations, etc., the overall adjustment of the color TV image channel will be affected, and the color TV receiving effect nest will deteriorate.

The AFT voltage outputted by the intermediate amplifier circuit, the DC control voltage proportional to the oscillation rate offset, is applied to the varactor diode of the local oscillation in the high frequency tuner, and automatically pulls the oscillation frequency back to the original specified The nominal frequency is up to ensure that the color TV is not affected by environmental changes, so that the receiving effect is always good.

The automatic frequency trimming circuit (AFT) is set to stabilize the high frequency tuner local oscillator frequency. It is provided by the phase-detection voltage of the IF phase-locked loop. When the local oscillator frequency drifts and the intermediate frequency changes, AFT≠0, AFT voltage is read back (A/D) by the MCU and processed as the trimming voltage of VT. VT is applied to the high-frequency tuner local oscillator circuit to correct the local oscillator frequency. Drift and fine tune it to the correct value.

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