Automatic tuning control circuit

Digital Tuning System
    Figure 2502 is a block diagram of a digital tuning system using a PLL (Phase Locked Loop) synthesizer. The most important feature of the digital tuning system
is that it can completely eliminate the discreteness in the tuning operation and the reception frequency deviation caused by time and temperature, and overcome the problems of inaccurate channel selection and platform hopping
. Using a PLL frequency synthesizer can synchronize the local oscillator frequency with the reference frequency of the quartz oscillator to maintain correct reception.
    Another important feature is that it has various tuning functions and rich channel selection functions. The tuner using the PLL synthesizer can save the need
to turn the dial and press a button to skip a channel. The tuner can also automatically Tune.
    The PLL frequency synthesizer is a digital circuit and does not require a dial, because it can accurately display the frequency numbers received, and
can also memorize the desired station and directly preset the frequency to be received through 10 keys.
    The digital tuning system shown in Figure 25-32 has a CMOS single-chip microprocessor (42-pin dual in-line) as its control core, and
an ECL type integrated circuit (9-pin single in-line) used as a preset scaler. ), used for PLL is a CMOS integrated circuit (dual row 16 pins).
    The main performance of the control microprocessor is as follows:
    (1) The RAM in the CPU is 96B×4 and the ROM is 2KB.
    (2) 52 instructions.
    (3) The clock cycle is 20 degrees.
(4) 1 level interrupt.
(5) 34 output channels.
    2. The application of processors in frequency modulation (FM) tuners
    FM radio technology has developed rapidly. The performance (distortion and signal-to-noise ratio) of radios using FM technology is greatly improved compared to AM radios.
The most significant thing is that the application of microprocessors and the development of software have greatly expanded the functions of radios, which has also led to a
great improvement in the quantity and quality of digital tuners.
    Due to the use of memory and preset keys, the FM receiver has a memory function. It can not only memorize channels, but also memorize intermediate frequency
band switches, high frequency band switches, and time control and other complex functions.
    The controller is a new technology under research that can automatically search and find the desired
radio station with the help of the computing and control functions of the microprocessor. At the same time, with the help of microprocessor control, the high amplifier circuit and other circuits of the radio part are in the best condition.
    The preset channel function can realize automatic recording or unattended recording, and users can also record two or even multiple radio stations.
    Some tuners currently on the market use microprocessors to allow the memory program to be extended beyond a week, while controlling the audio system
as a whole.
    A typical 4-bit single-chip microprocessor that performs this function is an NMOS-type IC with 1 KB ROM and 256 bit RAM. Combined
with the microprocessor are clocks and other functions. In the clock function, there is a touch-adjustable time preset and display function.
The hours and minutes of a certain day of the week can be preset to be displayed 24 hours a day. In addition, the frequencies of 8 FM stations can be memorized.
From the power supply to the tuner , preamplifier, main amplifier (power), tape recorder and other actions, all can be realized by the timing function program (32 steps).
    The frequency code is 8 bits; two 8X8=64 bit RAMs are needed to memorize 8 radio frequencies. The audio equipment in the home
needs to maintain the memorized content when the 220 V AC power supply is cut off, so CMOS memory RAM with low power consumption is used. This eliminates the need for
a large auxiliary power supply.
    In the timing controller, the necessary number of bits for each step is 8X4-32 bits, and for 32 steps. The memory capacity of 256 bits is obviously not
enough, so a 256X4=1024-bit RAM is used as internal memory.