Could someone explain the physics of this circuit?

  1. Jul 17, 2014 #1
    To the best of my knowledge, radio waves are generated by an oscillating (or alternating?) electric current, which produces either an alternating electromagnetic field or an alternating magnetic field (can someone confirm which one?) I've heard that the collapse or alternation (can someone confirm this also?) of this generates a radio wave (obviously, with no data).

    I found this circuit online, and I was just wondering whether it would work, and also how it works (e.g the function of the components, role of the LC circuit).
  2. jcsd
  3. Jul 17, 2014 #2


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    Radio waves are electromagnetic. They consist of both a varying magnetic and electric field. A varying (pure) magnetic field will not radiate energy.
    The circuit is basically an oscillator circuit, with the frequency being governed mainly by the resonance of the LC parallel resonating circuit.
    All such oscillators have feedback and amplifier gain. A proportion of the output signal is fed back to the input (like microphone howl round) and the amplifier will enhance the level of the output signal to cause oscillation. The inherent resonance of the LC circuit is modified by the capacitors C1 and C3 (plus the capacitance of the microphone), which provide feedback and they govern the precise frequency of the oscillation. As sound strikes the electret microphone, its capacitance varies, which alters (modulates) the natural frequency of the oscillation. The circuit is somewhat crude and the microphone lead needs to be short but it should work.
    Note, if you wanted to construct one like that, you would need 'green fingers' or be given precise information on the coil construction and the layout of the circuit.
    There is an old saying :"Many Oscillators only amplify and many Amplifiers Oscillate".
  4. Jul 17, 2014 #3
    Yes radio waves and more generally electromagnetic waves are generated by oscillating electric current. Oscillating or alternating means practically the same thing, it means a quantity that varies in space and time according to some function f(x-vt). An electromagnetic wave is an oscillating electric field, together with an oscillating magnetic field.

    I cant analyze the circuit in detail but i believe:
    1) The L1C2 circuit role is to provice the carrier signal frequency. That is an oscillating current with frequency [tex]f=\frac{1}{2\pi\sqrt{L1C2}}[/tex] which will be modulated by the microphone's oscillating current to produce the final signal that will go as input to the antenna.
    2) The role of the transistor Q12N2222 is to modulate the carrier signal with the microphone's signal and produces the final current which goes to the antenna via the capacitor C3.
  5. Jul 17, 2014 #4
    It's a crude circuit.
    The electret mic actually generates a bit of current in response to sound. So M1, Q1, R2 is a circuit. Current created by M1 will pass across the base and emitter of Q1 and that will allow current to pass from the collector to the emitter. This is assisted with current from +3V through R1, Q1, R2 to ground - so as M1 vibrates, it will add to that existing current. The purpose there is to allow M1 to influence Q1 during its full waveform.

    C2 and L1 are the electronic equivalent of an echo chamber. I don't know what frequency they ring at, but I'm sure it's RF. Anything that happens to the Q1 will cause C2 and L1 to ring. That ringing is passed through C3 to R2 where it is dampened. C3 is important because it helps move RF energy Q1 to C2/L1 and from there to the antenna - and it changes the RF ring frequency only when Q1 is off. When Q1 is on, C3 is shunted and cannot lower the ring frequency. This is the key to the "FM".

    C1 simply keeps stray RF from the affecting the Q1 base current.
  6. Jul 17, 2014 #5
    Thank you so much, both of your answers were really helpful!
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