Secondary frequency conversion FM radio circuit diagram

The machine is suitable for high-sensitivity FM radios used in weak signals and remote mountainous areas. The sensitivity is up to 0.5μV, and the receiving effect of the strong and weak stations is better.

See Figure 1. F1 (BP8804) is a 87-108MHz ceramic bandpass filter with a center frequency of 97.3MHz and its appearance is shown in Figure 2a. V1 and R1, L1 form a first-level high-frequency preamplifier, which can significantly increase the receiving sensitivity. The machine adopts a secondary frequency conversion method to increase the image frequency rejection ratio, so that a multi-stage high-level circuit can be added without interference from the image frequency. IC1 is FM dedicated high-frequency head circuit TA7335, which includes high-level, local oscillator, mixing and other functions. The working voltage is 2~6V, pin 1 is the signal input terminal; 2 is the positive terminal of the power supply; 3 and 4 are high. Put the circuit; 5 ground; 6 is the intermediate frequency output; 7 is the local oscillator circuit; 8 is connected to the AFC voltage. IC2 is an intermediate frequency amplifying circuit MC3361 dedicated to the FM receiver. It only needs to connect a small number of components to complete the functions of secondary frequency conversion, middle frequency, frequency discrimination and audio amplification. The first intermediate frequency signal output from the 6th pin of IC1 enters the {16} pin of the MC3361, and is again mixed with the X3 (10.245MHz) signal, and the second intermediate frequency signal of 455kHz is coupled by X3, thus realizing the secondary frequency conversion.
The inductance of the circuit can be made by itself. If you can't buy the bandpass filter BP8804, you can use the circuit of Figure 2(b) instead. The Lo in the figure is made with ∮0.5mm enameled wire on the ∮4mm round bar for 5 turns; L1, L2, L3 wire The winding method is the same as Lo. L1 and L2 are both 4T and L3 is 3T. T1 and T2 are modified by the cap-type intermediate frequency transformer. The data is shown in Figure 1. When winding T1, pay attention to the same name of the primary and secondary. C0 is a commonly used variable capacitor for FM radio. The capacitance value is generally adjustable from 3.5 to 20pF.
The whole machine debugging mainly adjusts L2 and L3, enables the difference of 10.7MHz IF signal, and then adjusts T1 and T2 to make the sound optimal. If necessary, the number of inductors can be adjusted to meet the debugging requirements.