[FM radio] What is the principle of FM radio? FM radio production process
What is the principle of FM radio and what is the production process of FM radio
1. Objectives of the experiment
Cognitive goals:
(1) To understand the simple radio Simple principle.
(2) Understand the general steps of the whole machine installation.
(3) Learn the operating procedures for soldering batches of components.
Skill target:
(1) Learn to use a multimeter to detect printed circuit boards and resistors.
(2) Master the six-step operation of soldering a batch of resistors on the printed board.
(3) Complete the soldering of all resistors on the printed board.
2. Experiment content
(1) Learn to use an electric soldering iron and be able to use its skilled soldering;
(2) ) Proficient in the basic methods of soldering various electronic components with an electric iron;
(3) Learn to identify simple electronic components and electronic circuits;
(4) Learn and master the working principle of the radio ;
(5) Weld the components according to the drawings, assemble a radio, and master its debugging methods.
III. Experimental equipment
(1) Simple radio lecture notes and teaching materials.
(2) Multimeter, electric soldering iron, soldering iron stand, solder, rosin, diagonal pliers, needle nose pliers, tweezers, scissors, broken saw blades, etc.
(3) Printed board, resistor (R1=100KΩ, R2=4148Ω, R3=1.5KΩ, R4=4.7KΩ, R5=2.2KΩ, R6=330Ω).
(4) Capacitors and other electrical appliances
C1=30P, C2=30P, C3=30P, C4=10Mf, C5=403, C9=150P, C10=1P, C11 =4.7μF, C12=15P, C13=103, C14=103, C15=500P, C16=4.7μF, C17=10μF, C18=203, C19=403, C20=10μF, C21=220μF, C22=220μF, C23 =403, C24=403. Inductance: L1=3.5T, L2 antenna coil, L3=14.5T, L4=10.5T, L5=100Mh. Filters: CF1 455B, CF2 10.7M. Speakers, main and auxiliary boards, jumpers, potentiometers, etc.
Four. Experimental principle
The principle of the radio is to restore the high-frequency signal received from the antenna to an audio signal through detection (demodulation), and send it to the radio. Headphones become sound waves. Due to the development of broadcasting, there are many radio waves of different frequencies in the sky. If all these many radio waves are received, the audio signal will be like in a busy city, and many sounds will be mixed together, and as a result, nothing can be heard. In order to try to select the desired program, after the receiving antenna, there is a selective circuit, its function is to select the desired signal (station) and “filter out” the unwanted signal to avoid interference, which is When we listen to the radio, we use the “Select Channel” button. The output of the selective circuit is to select the high-frequency AM signal of a certain radio station. It is impossible to use it to directly push the earphone (electroacoustic device), and it must be restored to the original audio signal. This restoration circuit is called demodulation. , send the demodulated audio signal to the earphone, you can receive the broadcast.
The above is the simplest radio called direct detector, but the high-frequency antenna electrical signal obtained from the receiving antenna is generally very weak, and it is not suitable to send it directly to the detector. A high-frequency amplifier is inserted between the selection circuit and the detector to amplify the high-frequency signal. Even if a high-frequency amplifier has been added, the power of the detection output is usually only a few milliwatts, which can be heard with headphones, but it is too small to use a speaker, so an audio amplifier is added after the detection output to push the speaker.
High-amplifier radios are more sensitive and powerful than direct-detection radios, but have poor selectivity and complex tuning. Amplify the high-frequency signal received from the antenna by hundreds or even tens of thousands of times. Generally, several stages of high-frequency amplification are required. Each stage of the circuit has a resonant circuit. When the received frequency changes, the resonant circuit has to be regenerated. In order to overcome these shortcomings, almost all modern radios use super-heterodyne circuits.
The characteristics of superheterodyne are: the carrier frequency of the selected high-frequency signal becomes a lower fixed intermediate frequency (465KHz), and then the intermediate frequency amplifier is used to amplify to meet the detection requirements, and then detection is performed. In the superheterodyne receiver, in order to generate frequency conversion, there is also an external sinusoidal signal, which is usually called a heterodyne signal, and the circuit that generates the heterodyne signal is used to be called local oscillation. In the radio, the frequency of the local oscillator and the frequency of the received signal are different by an intermediate frequency, so the selection circuit before the mixer, and the local oscillator use a unified tuning line, such as a coaxial double capacitor (PVC) for tuning, so that the difference is Keep a fixed IF value. Since the intermediate frequency is fixed and the frequency is lower than that of the high-frequency modulated signal, the gain of the intermediate amplifier can be made larger, the work is relatively stable, and the passband characteristics can also be ideal, so that the detector can obtain a large enough signal. So that the whole machine can output the audio signal with better sound quality.
The radio converts the received high-frequency signal of the radio station into an intermediate frequency signal with a fixed frequency by a frequency conversion stage circuit, and then performs multi-stage amplification on the intermediate frequency signal, and then detects and low-amplifies it. In this way, the sensitivity and selectivity can be greatly improved, and the acceptance sensitivity of the whole band can be uniform. Since the intermediate frequency is low and fixed, the intermediate frequency tuning amplifier circuit can achieve good selectivity, high gain and not easy to self-excite. The basic composition of the superheterodyne radio is shown in the figure:
After the signal received by the antenna is selected by the input loop, and the sine wave oscillating signal (the frequency of which is always higher than that selected by the input loop) The frequency of the signal is higher than 465kHz), which is jointly sent to the frequency conversion stage for mixing to generate a fixed difference frequency signal, that is, 465kHz. The 465kHz intermediate frequency signal is amplified by several stages of intermediate frequency selective amplifying circuits, and then added to the detection stage for detection. The demodulated audio signal is amplified by the pre-low amplifier stage and the power amplifier stage to push the speaker to make sound. The fluctuating DC component signal output by the detector stage can reflect the strength of the input high-frequency signal, and it is passed through the automatic gain control circuit to control the gain of the mid-amp stage. In this way, the gain of the mid-amp stage can be reduced when the high-frequency signal is strong, so that the output volume does not change with the strength of the high-frequency signal, and the work of the mid-stage stage is stable and its performance is improved.
How it works
1, AM Working principle
The medium wave broadcast signal 520-1620KHZ, after being selected by the input loop composed of L3 and CO-3, sent to the CXA1691BM integrated circuit (IC) pin 10, mixed with the local oscillator signal frequency. The local oscillator signal is generated by the internal circuit of IC circuit 5 externally connected to B1, C8, CO-4 to form a local oscillator circuit. After mixing, IC14 pin outputs various combined signals. B2 and CF1 form a 455KHZ intermediate frequency frequency selection circuit, which changes the high frequency carrier into a unified intermediate frequency carrier (455KHZ), and then outputs from IC23 pin, which is controlled by an external volume potentiometer RV through IC4 pin. , into the IC24 pin for audio amplification and power amplification, and then output from the IC27 pin, C23 is coupled to the speaker. The other output from IC23 and the external C16 are sent to the AGC circuit in the IC22 pin for automatic gain control.
2, FMHow it works
FM signal 64 -108KHZ is input from the ANT rod antenna, sent to Q1 for pre-selection and amplification through L1 and C1, and coupled to the input loop composed of L2 and C3 through C2 to obtain the selection of the range of 64-108KHZ, from C4 to IC12. The input high-frequency wave gets high-frequency amplification, and there are L4 and CO-1 to form a high-amp circuit, and choose to accept FM radio programs. The FM local oscillator circuit is composed of L5 and CO-2. CO-1 and C0-2 have coaxial variable capacitors, the purpose is that the frequency of the local oscillator signal changes with the frequency of the FM signal, and the difference is always 10.7MHZ. The difference frequency combined ceramic filter CF2 of the local oscillator signal and the radio signal is selected, so that the FM high frequency carrier becomes a unified intermediate frequency carrier. The intermediate frequency is amplified at the input IC17 pin, and the audio signal is demodulated through the frequency discrimination circuit and the additional circuit B3, and output from the IC23 pin. After the internal volume potentiometer RV is controlled by the IC4 pin, the output is output to the IC24 pin and coupled to the speaker via C23. The 10.7MHZ offset of the frequency discrimination output is realized through the AFC loop inside the IC, to the IC21 pin output, and to the IC6 pin through C15, R13.
Five, experimental steps
Basic welding steps (five-step welding method):
( 1) Preparation for welding: The preparation before welding includes cleaning of the welding part, component installation and soldering.
Flux and tool preparation. Solder the wire in the left hand and hold the soldering iron in the right hand (the tip of the soldering iron should be kept clean, and the soldering tip should be kept in the soldering state at all times).
(2) Heating the weldment: Pay attention to heating the whole weldment and heat it evenly.
(3) Feeding the welding wire: After heating the welding piece to a certain temperature, the welding wire and the iron contact the welding piece from the opposite side.
(4) Remove the welding wire: When the welding wire melts a certain amount, immediately remove the welding wire.
(5) Remove the soldering iron: Remove the soldering iron after the solder has penetrated the soldering pad or the welding part of the weldment.
The specific operations are as follows:
Assembly steps of the radio
(1) Detection of components . Before installing the machine, strictly check the components used one by one to see if there are any missing or damaged components.
(2) The installation of Install the device at the designated position according to the teacher’s instructions, and then solder the remaining resistors, capacitors, inductors and other components to the designated position according to the circuit diagram. When inserting components, pay attention to the polarity of the electrolytic capacitor and the pins of the triode to prevent wrong insertion.
(3) Install the potentiometer and the headphone jack at the designated position and solder them with a soldering iron. Be careful to keep the potentiometer in a plane.
(4) Finally, solder the speaker and power cord.
(5) Check whether the welding is correct.
(6) After installation and soldering, carefully check the position and lead polarity of each component against the circuit diagram and circuit board diagram, and also pay attention to whether there is any tinning.
Commissioning steps for the radio
(1) AM ( >AM) part
①Mid-frequency: Pull the medium-wave antenna coil down the edge of the magnetic rod, and seal it with wax or toothpick, you can listen to the medium-wave Electric, adjust the white mid-cycle to the loudest and clearest.
②Coverage: Aim at the local radio station, adjust the low-end coverage (red mid-vibration), and the high-end coverage (tune C0-4 for fine-tuning) twice.
③Tune in: Find the local radio station near 600kHz, adjust the antenna coil to the best position, find the local radio station near 1500kHz, adjust C0-3 to the best position, repeat twice.
(2) FM (FM) section
①Intermediate frequency: Pull the L4 frequency selection coil and L5 oscillation coil apart appropriately. After receiving the FM radio station, adjust the B3 frequency discriminator mid-cycle (black) to the ideal position.
②Override: Adjust L5 to the local low-end radio station (to the scale), and adjust C0-2 to fine-tune the local high-end radio station, repeat twice.
③Tune in: First find a local radio station near 90MHz to tune L4, then find a local high-end radio station near 105MHz to prepare for welding, heat the weldment, send in the welding wire, remove the welding wire, remove the welding iron C0-1 for fine-tuning, repeat two times Second, finally, L4 and L5 must be firmly sealed with high-frequency wax, otherwise it is easy to generate mechanical vibration.
