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Double conversion FM radio

23rd Jan. 2014

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It applies to regional and remote mountainous areas of weak signal using a highly sensitive FM radio sensitivity reach up to 0.5μV, can received well no matter strength or weak signal from radio station. Refer Figure 1.

F1 (BP8804) is 87 ~ 108MHz ceramic band-pass filter whose center frequency is 97.3MHz, the shape shown in Figure 2a.

V1 and R1, L1 pre-release composed a high frequency, which can significantly increase the reception sensitivity. The machine uses double-conversion methods to improve the image frequency rejection ratio, which can add a multi-stage amplifier circuit high frequency without interfere by image frequency.

IC1 is dedicated FM tuner circuit TA7335, including high-level radioactive internal, local oscillator, mixer and other functions, the operating voltage is 2 ~ 6V, pin ① is the signal input; ② is the positive terminal of the power source; ③ and ④ high amp circuit; ⑤ ground; ⑥ IF output terminal; ⑦ is the oscillator circuit; ⑧ pick AFC voltage.

IC2 is dedicated FM receiver IF amplifier circuit MC3361, only a small number of its external components to complete a secondary frequency, medium, frequency, audio amplification and other functions.

IC1 of the first IF signal output from pin⑥ into MC3361’s { pin16} and X3 (10.245MHz) signal mixing again, the difference from the X3 455kHz second IF signal coupling, thus achieving a double-conversion. Inductive circuits can be self-made.

If you couldn’t buy a band-pass filter BP8804, can use figure 2 (B) circuit to instead. Lo with ∮ 0.5mm ∮ 4mm wire wound on a rod made ​​of five laps prolapsed; L1, L2, L3 wire and the same winding method and Lo, L1, L2 are 4T, L3 to 3T; T1, T2 using the tone cap IF transformer restructuring, the data shown in Figure 1, T1 wound to the attention of the first class of the same name ends. C0 is commonly FM radio variable capacitor capacitance value is generally 3.5 ~ 20pF adjustable.

Debugging machine main regulator L2, L3, so that the energy difference 10.7MHz intermediate frequency signal, and adjust T1, T2 so that the best sound far, when necessary, adjust the number of turns of the inductor to meet the requirements debugging

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