High-frequency tuner, also known as tuner, has the functions of selecting channels, amplifying signals and changing frequencies. The function of antenna and input circuit is to select the weak TV signal of the channel to be received, which is selectively amplified by the high-frequency amplifier and then mixed with the sine wave with higher frequency output by the local oscillator to obtain the intermediate frequency signal. The high-frequency tuner has good selectivity, and can suppress interference signals such as image (if higher than twice the signal frequency) interference and if interference. The coupling between the isolation mixer and the antenna can prevent the local oscillator signal from radiating through the antenna and interfering with other receivers.

The mixer converts the received RF TV signals of different channels into IF signals with fixed frequency. China stipulates that the image intermediate frequency is 38 MHz, and the first audio intermediate frequency is 3 1.5 MHz. Subsequent IF amplifiers can obtain good selectivity and high gain due to fixed frequency. The total gain of a general high-frequency tuner is about 20 dB. ?

2. Intermediate amplifier and detection?

The IF amplifier amplifies the image IF signal and the first audio IF signal sent by the high frequency tuner. Its main task is to amplify the image intermediate frequency signal, but the amplification factor of audio intermediate frequency signal is very small, so it is often called the image intermediate frequency amplifier. Intermediate amplifier is the main amplification unit of the whole TV receiver, and the gain is required to be above 60 dB. ? In order to adapt to the transmission of residual sideband and suppress interference, the IF characteristic curve has a special shape, which is formed by SAWF once. ?

The first task of video detector is to detect video image signal from intermediate frequency image signal. Generally, large signal detection is envelope detection. The second task of the video detector is to generate the second audio intermediate frequency signal of 6.5 MHz from the beat frequency of the image intermediate frequency and the audio intermediate frequency by using the nonlinearity of the diode. ?

The output signal of the detector should be provided to PAL encoder, synchronous separation circuit, automatic gain control (AGC) circuit and audio intermediate amplifier circuit, so the video is pre-amplified to enhance its load capacity. The amplification from the antenna to the video preset is called the common channel. ?

The function of automatic noise suppression (ANC) circuit is to automatically suppress the interference pulse, so as not to affect the normal operation of synchronous separation circuit. The common method is to separate the interference pulse and superimpose it on the original signal after inversion, thus canceling the interference pulse. ? The function of automatic gain control (AGC) circuit is to detect a DC voltage that varies with the input signal level, so as to control the gains of intermediate frequency amplifier and high frequency amplifier, and keep the output amplitude of video detection basically unchanged. ?

3. Sound track?

The 6.5 MHz second audio intermediate frequency signal taken from the video preamplifier is sent to the audio intermediate frequency amplifier, amplified and limited, and then sent to the frequency discriminator for frequency discrimination, audio signal detection and low amplification, and finally TV audio is obtained at the speaker.

4.PAL decoder?

For details about PAL decoder, please refer to Section 3.3.6 PAL decoder and the block diagram of PAL decoder shown in Figure 3-2 1.

5. Synchronization and scanning circuits?

The video image signal is sent to the synchronous separation circuit after the interference pulse is eliminated by ANC circuit, and the composite synchronous signal is separated. After the composite synchronization signal is amplified, it is separated from the field synchronization signal by the integrating circuit. Then, the field synchronization signal is used to control the sawtooth signal generated by the field oscillator to synchronize with the transmitting end. The field sawtooth signal is amplified by the field driving stage and the field output stage, and a field scanning current is generated in the field deflection coil. ?

In order to improve the anti-interference ability of line scanning circuit, modern TV receiver adopts automatic frequency and phase control (AFPC) circuit. The composite synchronization signal is directly applied to the phase detector of AFPC circuit and compared with the line oscillation signal. If there is a frequency and phase difference between them, a voltage proportional to the error is output to control the frequency and phase of the line oscillator, so as to synchronize with the transmitter. Because of the low-pass filter in AFPC circuit, the anti-interference ability of line synchronization is enhanced.

The line oscillation signal synchronized with the transmitting end is amplified by the line pushing stage and the line output stage, and a line deflection current is generated in the line deflection coil. Boost and rectify the retrace pulse of line scanning to obtain the high voltage and medium voltage required by CRT and the voltage required by video amplification circuit (combined with PAL decoder primary color matrix). ? The auxiliary circuits of color kinescope include convergence, geometric distortion correction, white balance adjustment, color purity adjustment and degaussing. Circuit. ?

6. Remote control system?

The remote control system consists of a local keyboard, a program memory, an infrared remote control transmitter, an infrared receiver, a microcontroller and an interface circuit. ?

The local keyboard is located on the TV panel, and the user can complete the channel selection, preset or various function control of the TV through the operation of the local keyboard. ?

The keyboard function on the infrared remote control transmitter is basically similar to the local keyboard, but the difference is that it can be far away from the TV and control the TV through infrared command signals. When a key of the infrared remote controller is pressed, the encoder in the remote controller outputs a set of corresponding binary codes, which are modulated on a carrier wave of 38 kHz, and then the infrared light emitting diode is modulated into an infrared remote control command signal and sent out. After receiving the infrared remote control command signal, the infrared photodiode installed in the infrared receiver behind the TV panel gets the binary code of the command through amplification, detection and shaping, and sends it to the microcontroller for decoding, thus identifying the control type and content, and accordingly sending corresponding signals through the interface circuit to adjust the TV.

The program memory adopts EEPROM, which is used to store the tuning voltage data of multiple channels and various functional control parameters. And also stores the information of the last watched TV program, including channel number, TV /AV status, volume, brightness, contrast, color saturation, etc. ?

Microcontroller is the core of the remote control system, which consists of 8-bit arithmetic and logic operators, various registers, voltage or frequency synthesizers, RAM (data memory), ROM (solidified all channel selection, preset and various function control programs), I/O ports, instruction decoders, buses, master clocks, etc. Perform user's remote control instructions together with peripheral circuits, such as channel selection, preset, volume, brightness and other functional controls. ?

The interface circuit converts various functional control instruction codes sent by the microcontroller into 64-level analog control voltage after decoding and D/A conversion, and then controls the volume, brightness, color saturation and power supply.