Novel FM receiver

The FM receiver introduced in this article uses the principle of super-regenerative FM reception. Because it uses a high-gain micro-integrated circuit, the circuit is simple and novel. The receiving effect reaches the level of general FM receivers, at the same time it overcomes the shortcomings of poor selectivity and high noise of super regenerative receivers, while maintaining the advantages of high sensitivity, low power consumption, simple circuit and low cost (component cost is less than 5 yuan). Suitable for electronics enthusiasts.
The circuit schematic diagram of the machine is shown in the figure.

It is composed of super regenerative FM reception, FM-AM conversion part, amplitude modulation detection and low-amplifier circuit. The super-regenerative reception of the FM wave actually converts the FM wave into an AM wave, and at the same time performs envelope detection on the AM wave to obtain a low-frequency signal. The transistor VTl and peripheral components in the figure form a typical super-regenerative FM receiving circuit, and convert the FM wave signal into an AM signal and perform envelope detection to output the audio signal. If the audio signal after envelope detection is directly taken from the R3 terminal and amplified, the obtained audio noise is relatively large, but the selectivity of the receiver is deteriorated.
Therefore, the method of high-frequency amplification and detection of the audio signal from the emitter of VT1 through the high-frequency choke coil in the series loop and then outputting the audio signal is adopted to overcome the above deficiencies. When VT1 works, an amplitude modulation signal modulated by a frequency modulation program will be formed on the high frequency choke. This signal is coupled to the special amplitude modulation receiving micro IC1 7642 through the transformer T1 to demodulate the amplitude modulation wave.
This integrated circuit contains the whole process of one-stage high-impedance input, three-stage high-frequency amplification and detection output, and the gain is greater than 70dB. The audio signal output by the detection is coupled to the transistor VT2 by the capacitor C9 for low frequency amplification, and output to the load (headphone) through the headphone socket CZ to listen to the broadcast program. The function of the high-frequency choke T2 is to prevent the high-frequency signal from forming a loop with the battery and other parts to be attenuated, but it does not hinder the audio signal.
Capacitor C6 is a small ceramic dielectric fine-tuning capacitor. When welding, it is required to connect the moving piece to the A end of the figure. The purpose is to reduce the influence of human body induction on the tuning circuit when tuning. The high-frequency inductance L1 is made of Φ1.0mm enameled wire wound around 3 circles on a Φ5.0mm round bar. The high-frequency choke transformer T1 is selected from the AM-IFT mini mid-circumferential winding that was removed from the old machine, and the enameled wire that was originally wound on the I-shaped core is removed, and then weighed with a high-strength enameled wire of ΦO.07mm Winding, the primary high-frequency choke part is about 50 turns, and the secondary induction part is about 150 turns, and then the adjustment magnetic cap and the outer shield can be added. The high-frequency choke T2 is made of double-hole magnetic ring, which is made of Φ0.2mm enameled wire around each hole for 10 turns.
First adjust the collector current of VT1 to 0.3mA-0.5mA by adjusting R1, and adjust the resistor R7 to make the collector current of VT2 about 2mA. At this time, you can listen to the sound of "Sisi" running water (electric noise) with headphones, and listen to the FM radio program by adjusting the capacitance of C6. Fine-tune the L1 pitch and
The magnetic cap of T1 makes the sound quality volume the best.

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