Novel FM receiver circuit

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 reception effect reaches the level of general FM receivers, while overcoming the shortcomings of super-regenerative receivers such as poor selectivity and high noise, while maintaining the advantages of high sensitivity, low power consumption, simple circuits and low cost (component cost is less than 5 yuan). Suitable for electronic enthusiasts. The circuit schematic diagram of the machine is shown in the figure. 　　It consists of super regenerative FM reception, FM-AM conversion part, AM detection and low amplifier circuit. Super-regenerative reception of FM waves actually converts FM waves into AM waves, and at the same time performs envelope detection on the AM waves to obtain low-frequency signals. The transistor VTl and peripheral components in the figure form a typical super-regenerative FM receiving circuit, which converts the FM wave signal into an AM signal and performs envelope detection to output an audio signal. If the audio signal after envelope detection is directly taken out from the R3 terminal and amplified, the resulting audio noise will be relatively large, but the selectivity of the receiver will be deteriorated. Therefore, here we adopt the method of amplifying and detecting the amplitude modulation signal induced from the emitter of VT1 through the high-frequency choke coil in the series circuit to output the audio signal at high frequency to overcome the above shortcomings. When VT1 is working, an AM signal modulated by the FM program will be formed on the high-frequency choke. This signal is coupled to the AM dedicated receiving micro IC1 7642 through the transformer T1 for demodulation of the AM wave. This integrated circuit includes the entire 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 detector is coupled to the transistor VT2 by the capacitor C9 for low-frequency amplification, and is output to the load (headphones) through the headphone socket CZ to listen to the radio 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 and being attenuated, but it has no hindrance to the audio signal. Capacitor C6 is a small ceramic trimming capacitor. When welding, the moving plate is required to be connected to the A terminal in the figure. The purpose is to reduce the impact of human body induction on the tuning loop when adjusting the station. The high-frequency inductor L1 is made of Φ1.0mm enameled wire wound three times around a Φ5.0mm round rod. The high-frequency choke transformer T1 is wound using the AM-IFT micro mid-circle removed from the old machine. The original enameled wire wound on the "I"-shaped core is removed, and then reused with ΦO.07mm high-strength enameled wire. Winding, the primary high-frequency choke part is wound about 50 times, the secondary induction part is wound about 150 times, and then the adjusting magnetic cap and outer shielding are added. The high-frequency choke T2 uses a double-hole magnetic ring, which is made of Φ0.2mm enameled wire wound 10 times in each hole. First adjust the collector current of VT1 to 0.3mA-0.5mA by adjusting R1, and adjust the resistor R7 so ​​that the collector current of VT2 is about 2mA. At this time, you can listen to the "silk" sound of running water (electrical noise) with headphones, and listen to the FM radio program by adjusting the capacitance of C6. Finely adjust the L1 turn pitch and T1 magnetic cap to achieve the best sound quality and volume. Guangdong Huang Shaoji was originally published in the bound edition of "Electronic News" in 2003