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AM Radio Circuit Design

Discussion in 'Electronic Design' started by Franklin, Nov 17, 2003.

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  1. Franklin

    Franklin Guest

    I'm quite new to Electronics, and have been working on understanding
    an AM radio circuit. I think I've gotten most of it, but I'm stuck at
    a few points.

    The schematic can be seen at
    http://support.radioshack.com/support_games/doc60/60245.htm , or, for
    a better picture, at
    http://support.radioshack.com/support_games/doc64/64800.pdf .

    Here are my questions:

    A) Block (1) - I recognize the tuned circuit, but its not in any
    configuration that I'm familiar with. Normally, I've seen the signal
    come in, and a tuned circuit bypass to ground, and then signal out.
    Here, I don't see any signal in (where does the antenna connect to?)
    or signal out - only to ground. How is this working?


    B) Why is an inductor used to couple to Q1?

    C) There are several capacitors which bypass to ground along the way -
    from right to left, a 0.1uF, a 0.01uF, another 0.01uF, and a 0.05uF.
    Are these all used to detect the AM - that is, to let the HF drop to
    ground while the audio carries through? Why do you need so many?

    D) Block (4) - How can you couple with only a 0.022uF capacitor -
    won't this block all but the lowest frequencies?

    Can anyone help me with any of these questions?
     
  2. Luhan Monat

    Luhan Monat Guest

    AM radios use a coil on a large ferrite rod as the antenna.
    The extra winding is to match the lower impedence of Q1.
    They help keep residual RF from getting to the audio stage.
    No, its in series, it passes all but the lowest frequencies. This helps
    the radio play louder by keeping low frequencies from saturating the
    audio output.
     
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