The radio circuit is shown in Figure 1. Its novelty is that the front-stage transistor VT1 works in a different way from super regenerative and super external, and also has four functions of mixing, local oscillator, PLL synchronous detection and low frequency amplification. L1 and C2 form a wide-band input loop of FM frequency band (87MHz-108MHZ) with a low Q value, and the center frequency is 98MHZ. L2, C5, and C6 form the LO tuning loop. The LO frequency is the input and receive frequency and the second harmonic of the LO signal. After mixing, the output intermediate frequency signal falls within the audio frequency range. Because the output conductance of VT1 is a function of collector current, it has the function of controlling the frequency of the local oscillator.
When VT1 is used as a local oscillator, it is connected to a common base circuit. Since L1 and C2 are detuned to the local oscillator frequency, the base of VT1 is equivalently grounded.
When VT1 is used as a mixer, it is a common emitter circuit.
When VT1 is used as the same frequency detector, it is also a common base circuit. This is a very large value for C3, and it has little capacitance to the audio signal. It can be considered that the base of VT1 is AC grounded. Audio at this time (ie after mixing 
The amplification of the resulting intermediate frequency) signal is approximately R3 / R2. C7 is a high-frequency bypass circuit used to bypass the carrier frequency component after detection. The local oscillator frequency of VT1 is controlled by the input signal frequency within a certain range, because when the second harmonic of the local oscillator signal is close to the transmission frequency of a FM station, the two of the integrated electrode current of VT1 will be mixed. After the audio output, the output conductance of VR1 changes accordingly, and the local oscillator frequency also changes. That is, the local oscillator frequency is synchronized with the external signal, which is completely similar to the principle of phase-locked reception and has an AFC function.
The sensitivity of this receiving circuit is comparable to that of a super regenerative circuit, but there is no super regenerative noise. Due to the large difference between the local oscillator frequency and the input signal frequency, the local oscillator radiation in the FM section is small. When rotating C6 to tune to an FM radio station, the amplitude of the audio signal generated on R3 can reach tens of millivolts, which is basically independent of the strength of the received signal. This audio signal is fully coupled to a simple low-frequency amplifier composed of VT2 and VT3 via C8, which amplifies the signal and drives a low-impedance earphone (8Ω) to sound. The long wire of the earphone here cleverly acts as a receiving antenna to improve the receiving sensitivity. L3 and L4 are high-frequency choke coils, which use audio signals to pass through early and unimpeded, but prevent high-frequency signals from flowing and prevent the FM broadcast signal received by the headphone cable from being bypassed by current and C9. C9 is the power supply filter capacitor to avoid low frequency self-excitation from the circuit.
In order to improve the receiving sensitivity at a higher frequency, you can also solder a piece of 2M long flexible wire as the antenna as shown by the dotted line in Figure 1.
At the time of production, C6 uses 7 / 270PF small sealed double variable capacitors (such as CMB-202), and only one of them is used. VT1 selects FT≤800MHZ UHF tube 9018, β> 80. VT2 selects high magnification transistor 9014. VT3 selects low-power germanium tube 3AX31, and the leakage current should be small. L1 uses φ0.5MM enameled wire to wind 5 turns (turns) on the φ4mm drill bit 
(The distance is 1MM) After being reborn into a hollow coil, there is a center tap.L2 is wound with 15 turns of φ0.5MM enameled wire on the φ4MM drill bit (the pitch is also 1MM) and then reborn into a hollow coil.L3, L4 can be purchased from commercially available color codes inductance. The resistors use 1 / 8W four-color ring carbon film resistors. The withstand voltage of the electrolytic capacitor should be greater than 6V. Other non-polar capacitors use small ceramic capacitors. Use 1 5 # battery for power supply. The headphone socket can bend the inner reed a little inward according to Figure 2, so that the radio is powered on when the headphone plug is plugged in, and the radio is powered off when dialed out, and also functions as a power switch.
The whole machine is installed on the 66 * 50MM single-chip printed circuit board shown in Figure 3. The height is relatively simple. After plugging in the headset, the radio is powered on. Rotate C6 to receive the FM radio station. At this time, fine-tune R1 to make the sound pure and loud. Then fine-tune L2 (pull or compress the pitch) so that the position of the radio basically matches the dial. Then fine-tune L1 to make the sensitivity of the high and low-end circuits even. If you feel the sound is too loud, you can adjust R3 to make the volume appropriate. After adjustment, wax can be sealed for L1 and L2 to prevent the inductance from changing after shock. Finally, the machine is equipped with a casing made of colored organic glass.
The sensitivity of this machine in Shanghai area, without additional antenna, only use the headphone lead as the receiving antenna, you can listen to all the FM broadcasts of Shanghai Radio and Oriental Radio satisfactorily. When receiving, the background is quiet, and there is no "sloppy" noise of the super regenerative radio, which is comparable to the mid-range radio with noise reduction circuit. Zhou Juhua 
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