The basic working principle of the radio can be simply summarized as follows: the first step is to receive the radio waves of the corresponding frequency, the second step is to extract the sound information modulated on the radio waves, and the third step is to restore the sound information to the adult ear energy. The sound that I heard. As shown in Figure 1, the simple radio system consists of five modules. After the tuning signal is selected by the tuning loop, the received radio signal is very weak. After passing through the tuning circuit, it needs to be amplified by a high-frequency amplifier circuit to a certain amplitude before it can be sent to the diode and the filter capacitor. The voltage doubler detection circuit performs detection, demodulates the amplitude modulation signal envelope, obtains the audio signal in front of the modulation, and amplifies the audio signal at a low frequency, and the sound information is still an electrical signal with a very low amplitude, and our human ear is listening. If not, after being amplified by the transistor current, it is sent to the speaker and restored to an audible sound wave signal.
The simple radio is very simple to make, by changing the size of the inductor and capacitor to receive a simple radio in the middle section from 550kHz to 1600kHz. And the radio audio output is basically up to the extent that the human ear can hear it.
According to the basic working principle of the above radio, the working circuit diagram of the simple radio shown in Fig. 2 is designed by using the chip CD4011.
2. Analysis of circuit working principle
The circuit diagram is shown in Figure 2. Under normal circumstances, such NAND gate circuits are in both on and off states, that is, output high level and low level. In fact, during the transition between high and low levels, there is a transition zone in which the middle portion of the transition zone is substantially linear. Therefore, the feedback circuit can be used to select an appropriate operating state so that each NAND gate is in an amplified state. The circuit and working principle are shown in Figure 2. The input terminals of each NAND gate are connected together as inverters to convert high and low levels. L1, C1 is the receiving resonant circuit, R1 is the DC feedback resistor, C2 is the AC bypass function, and the received signal is amplified by U1A, and then coupled to the double voltage detection composed of D1, D2, R2 and C4 through C3 coupling. The circuit is detected, and then further amplified by the gates U1B, U1C, U1D and sent to the headphone output. The C1 on the way can be selected as a single-string variable capacitor. The coil L can be wound around 80~100T on a 50mm magnetic bar. The earphone should be a high-impedance earphone. It can also add a first-level integrated audio amplifier circuit such as LM386, or use a triode for impedance transformation. By increasing and decreasing the number of turns on the coil on the magnet to ensure that the radio is within the proper frequency reception range.
Note: (1) CD4011: 4-2 input NAND digital integrated circuit.
(2) The variable capacitor is selected as CBM-233P, and the capacitance varies between 5 and 141p. The inductor L is wound around 100 turns with a 5:3*55 mm magnetic bar of 0.18 mm diameter enameled wire. The triode uses 9012. The power supply uses 4 No. 5 batteries. The headset uses a 32 ohm stereo headset.
3. Analysis and introduction of the working principle of each module
(1) Tuning loop module
Figure 4 high frequency amplification and voltage doubler detection circuit diagram
As shown in FIG. 4, the NAND gate CD4011 performs high frequency amplification, and then the diodes D1, D2, the capacitor C3, the resistor R2, and the like are used to detect the voltage. When the signal voltage is positive for half a cycle, the signal voltage is charged through D1 to charge C3, and the voltage on C3 is left positive and negative. When the signal voltage is negative half cycle, the signal voltage is connected in series with C3 and flows through diode D2 and resistor R2. The detection load resistor R2 can obtain about 2 times the output voltage of the signal voltage. The function of capacitor C4 is to filter out the detection output signal. The high frequency component in the middle gets the audio signal.
Figure 5 low frequency amplifier circuit diagram
As shown in Figure 5, the CMOS analog amplifier circuit adds a suitable bias voltage to the CMOS gate to operate in a linearly amplified state. A feedback resistor R4 is connected between the output terminal and the input terminal of the NAND gate CD4011 to offset the operating point of the NOT gate in the middle of the transfer characteristic curve, which constitutes a linear analog amplifier, and the amplification factor is equal to the feedback resistor R4. The ratio to the input resistor R3.
(4) Power amplification module
As shown in FIG. 6, since the output current of the CMOS circuit is small, in order to make the radio have sufficient volume, the circuit VT is composed of a current amplifier, and R5 is the bias resistor of the transistor VT. The current amplification module is essentially an emitter follower that amplifies the output current of the CMOS circuit.
Fourth, [analysis and simulation results analysis]
As shown in Figure 7, the overall circuit simulation diagram, after the simulation, hears the humming sound of the speaker. According to the principle, the size of the inductor can receive the radio in the middle band of 535kHz to 1065kHz, but it has not been accepted in the effective frequency band after adjustment. sound. Therefore, the design module is simulated. (1) Low frequency amplification simulation
As shown in Figure 8, the left side is connected to an AC signal source, and the other end of the amplifier circuit is connected to the oscilloscope. From Figure 9, it can be seen that the input signal (the first waveform) is amplified.
(2) Power amplification simulation
As shown in Figure 10, an AC signal source is connected to the left side, and an oscilloscope is connected to the other end of the amplifying circuit. From Fig. 11, the input signal (first waveform) and the output signal (fourth waveform) can be seen. ) The gap between.
Figure 11 Simulation waveform before power amplification
(3) High frequency amplification simulation
The simulation circuit diagram is shown in Figure 12. An AC signal source is connected to the left side, and two oscilloscopes are connected to both ends of the circuit. The signal shown in Figure 13 is amplified by the amplifier circuit as shown in Figure 14. It can be seen that the amplitude of the signal is significantly increased after being amplified.
Five, [component list]
to sum up
The design of the simple radio was made by CD4011 chip, the working principle of the radio was understood, the process of modulation and demodulation was truly understood, and the function and usage of the pins of the CD4011 chip were also known.
Improved thinking
In the simulation, the circuit component CD4011 in the circuit diagram was not found in Proteus, so the NAND gate 4011 was used instead. Since the CD4011 is a CMOS circuit containing four NAND gates, each NAND gate has two inputs. One output. When two inputs have an input of 0, the output is 0. The output is only 1 when the input is 1. When both inputs are 0, the output is 1. So CD4011 is replaced by 4011 in the figure. The radio frequency signal obtained from the receiving antenna of the simple radio is generally very weak. Even if the high frequency amplifier has been added, the power of the detection output is usually only a few milliwatts. It can be heard with headphones, but it is too small to use the speaker. After detecting the output, add a current amplifier to push the speaker to work. If a superheterodyne circuit is adopted, the intermediate frequency of the radio local oscillator frequency and the frequency of the received signal are different. The selection circuit and the local oscillator before the mixer adopt a unified tuning line, because the intermediate frequency is fixed, and the frequency is higher than the high frequency. The signal is low, the gain of the Chinese side can be made larger, the work is more stable, and the passband characteristics can also be made ideally, so that the detector can obtain a sufficiently large signal, so that the sound of the whole simple radio is better. audio signal.
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