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How do electronics generate frequencies/signals ?

  1. Jan 1, 2012 #1
    Hi, I was just wondering how these electronic components connected together on a PCB with an antenna produce frequencies.

    1. I found schematics for a circuit that transmits FM but it doesnt actually explain anything, you basically connect a microphone, a transister, a variable resistor, some fixed resistors, some capacitors which are connected to an antenna along with a coil of copper wire(whats this for ?). I just dont understand how these things produce a frequency that can transmit my voice. Also further reading shows that you can take the microphone out and emit noise which if at the right frequency would disrupt the signal of a nearby radio tuned at that frequency now I know why this would happen because those noise will cancel out the signal received by the radio something I learnt in A-Level physics.

    2. Why not just connect a capacitor to a copper coil around iron to emit frequencies much like EMPs but at low energy so it does fry anything just emit certain frequencies ? Again I am also confused about EMPs, do they work because the electrons discharged from the capacitor go round in a coil and just jump out in the air ? if so why not just use a straight wire why do we have to use a coil ?

    I would appreciate if someone could explain to me the concept behind these devices. I am getting more and more interested in RF engineering even though I am doing mechanical. Cheers.
  2. jcsd
  3. Jan 1, 2012 #2
    You must have missed an oscillator somewhere in the schematic. It generate a higher frequency carrier that the voice signal modulates on it.
  4. Jan 1, 2012 #3


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    There's enough questions there to write a book-length answer (or even several books)...

    Google will find plenty of tutorials on basic electronics.

    If you don't know anything about electronic components, start by learning about resistors (OK, you probably know Ohms law already!) capacitors and inductors.

    Then look at series and paralllel RLC circuits. You should be able to relate those to what you know about mechanical vibrations in ME.

    The look at transistor amplifiers, feedback, and oscillators.

    Now you know enough to design working circuits that will transmit and receive Morse code....

    I'm going to stop there, because if you do start studying this, you will almost certainly come back with some more specific questions about what you don't understand (and PF is a good place to ask them).
    Last edited by a moderator: Jan 6, 2012
  5. Jan 1, 2012 #4


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    You can look here for some oscillator circuits:


    Depending on the circuit, these oscillators may produce sinewave or square wave output, however the output is repetitive and will normally continue until the power is removed.

    Oscillator circuits with inductance and capacitance in a tuned circuit will produce sinusoidal output which can be varied in frequency by varying the values of the inductance or capacitance or both.
  6. Jan 1, 2012 #5
    You have to eat this large pizza in small chunks. :)
    1. All those discrete components are performing some block level functions of FM modulator.
    In the heart of the FM is the Local Oscillator (L.O.) generated by positive feedback (unstable) circuits.
    In NBFM. L.O. may be generated by Colpitt or Hartley Oscillator or Crystal oscillator.
    The baseband signal is integrated and multiplied with -90 degree phase shifted L.O. which is amplified and mixed with the carrier which is the L.O. itself.
    The expression of FM becomes,
    [itex]x_{NBFM}[/itex](t) = Acos [itex]\omega_{c}[/itex]t - A[itex]k_{f}[/itex]sin [itex]\omega_{c}[/itex]t * [itex]\int[/itex]m(t)dt
    the spectrum of FM has many discrete frequency components shown below. The first one is NBFM.
    http://www.radio-electronics.com/info/rf-technology-design/fm-frequency-modulation/fm-spectrum-02.gif [Broken]

    Now until we see the circuit I cant tell you which part does what.

    2. EMP acts much more like an antenna emits EM wave. Only more powerful. A very good thing to study the usefulness of powerful EM wave is the magnetron in microwave which makes the water molecules in food dance in its own tune. hehe. However, if the same thing is aimed at us we will be nuked.
    Last edited by a moderator: May 5, 2017
  7. Jan 3, 2012 #6
    Hi thanks for the reply.

    Yungman- It does not have an osc in it: http://www.circuitdiagram.org/best_fm_transmitter.html

    Alephzero-I do know about the components like resistors and capacitors and thats why I am confused because I do understand that capacitors will be used to discharge in bigger amounts but why need more than one in a circuit ? How do you know that you need more ?

    I know what resistors are, I can use the V=IR (ohm's eq) to change the voltage by alternating the resistor but how does this help at all ? I read about the RLC circuits but again how does connecting a capacitor to a coil produce harmonic oscillations. In my head everytime someone says "oscillations" I think of pendulums I just dont understand this concept in electronics.

    vk6kro- http://www.circuitdiagram.org/best_fm_transmitter.html the circuit there does have variable resistors and capacitors so basically thats how you change the frequencies right ? but then how do you know what capacitance sets a certain frequency can you give me an equation ?
  8. Jan 3, 2012 #7
    Hey thanks for your reply. Can you tell me what the A,k and the function m(t) is in the equation ? Also what is the angular velocity w(c) for ?

    this is the circuit: http://www.circuitdiagram.org/best_fm_transmitter.html. It does not have an amplifier or a L.O. Also I dont understand what the "positive" feedback is. I did study about it as there is a small module about electronics in my degree but I never understood what this feedback actually is. All I understand is that a device communicates with another device and the device sends some data/information back which I think is feedback.

    For the emp you said it is much like an antenna which produces EM waves but then is the coil to send the EM waves like a tsunami wave ? LOL I am confused sorry for all the questions. Also do mobile phones operate on FM modulation aswell or are they in a league of their own ? haha.
    Last edited by a moderator: May 5, 2017
  9. Jan 3, 2012 #8


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    You asked about the FM transmitter.

    The transistor forms an oscillator with the components around it.

    The frequency is controlled mostly by the inductance of L and the combined capacitance of the 28 pF and the variable capacitor TR.
    The frequency is about 100 MHz as mentioned in the text.

    The 5pF capacitor is what turns this circuit into an oscillator as it provides feedback from output back to the input.

    Variable resistor 1-10K does not affect frequency. It provides current to operate the microphone.
  10. Jan 6, 2012 #9
    Cheers for that mate. That is the sort of info I am looking for because it tells me how something works or why it is in the circuit in the first place. Can you tell me why the capacitor provides feedback ? is it because it is discharging ?
  11. Jan 6, 2012 #10


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    Capacitors can conduct AC signals while stopping DC voltages.

    That is what is happening here.

    The AC signal at the collector of the transistor is connected to the emitter of the transistor and this allows some of the output signal to go back to the emitter where the transistor amplifies it and sends a bigger version of it to the output.

    So, you may be able to see that this sort of behaviour will result in a circuit that generates its own signal by oscillating
  12. Jan 7, 2012 #11
    Knight, I think you understand a lot less than you think you do. That sort of overconfidence will not help you learn. You really should have at least some basic understanding of what capacitors, inductors, and resistors do if you want to understand how an oscillator works. I would recommend the allaboutcircuits web site ebook, The Art of Electronics and maybe Practical Electronics for Inventors if you ignore all the example problems. They are wrong in ways that can screw up your understanding of the concepts he explains so well. The explanation for some electronic concepts are pretty good depending on how you feel about water analogies. Someone also recently recommended Electronic Principles by Malvino which looks promising to me. I plan to get myself a copy to check out. For a start try googling for LC or tank circuits. Then look at RLC circuits, which I believed are just a damped version of the same. All practical circuits will have some resistance. So even an LC circuit is really an RLC circuit in real life. A capacitor stores electrical energy. An inductor converts electrical energy to magnetic energy and stores it . When you connect them together in the right way you get a flux that continually converts between electric and magnetic and there's your oscillator. Connect an antenna to it and you will radiate those oscillations out into space as an electromagnetic wave. If you modulate that wave you can transmit information over it. Note that LC circuits are not the only way to generate EM waves. Other methods are used particularly at high frequencies.
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