super simple FM transmitter ---need explanation :)

Discussion in 'The Projects Forum' started by Undral, Dec 10, 2014.

  1. Undral

    Thread Starter New Member

    Dec 10, 2014
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    I'm freshman . I don't know what is exactly going on here but I made this fm transmitter.
    Could anyone please explain to me the different stages of this circuit ? Like, what is required by a transmitter, and where it is in the circuit... This will help me and understand the circuit a bit better..

    http://makezine.com/projects/super-simple-fm-transmitter/

    Thanks
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    C2, c1, r1 form a band pass filter to cut out the low and high frequencies (above and below audio frequencies). The capacitors also mix the audio frequencies (audio in) with the rf frequencies of the oscillator.
    Additionally, the two resistors (r1and r2) form a voltage divider to bias the transistor.

    The middle (capacitor, inductor and resistor) form an LCR tank, the transistor maintains the tank. The tank allows an oscillation to develop and center on a frequency unique to the component values.

    The capacitors on the right side are essentially a voltage divider and remove the DC component before sending the signal to the antenna.
     
  3. DickCappels

    Moderator

    Aug 21, 2008
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    It is a very common circuit that was drawn very strangely.

    A couple more aspects of operation can be mentioned.

    It is basically a Colpitts oscillator, with capacitance that provides feedback necessary for oscillation being the transistor's internal emitter-to-collector capacitance. C2 shunts radio frequency signals to ground, making the circuit a common base configuration.
    https://en.wikipedia.org/wiki/Colpitts_oscillator

    The resonant frequency of the oscillator is affected by the transistor's collector-to-base capacitance which by means of C2 and C3 shunts L1. The audio signal to the base of the transistor varies the capacitance of the collector-to-base junction (by modulating the depletion region in the junction; it is the same principle at work in a varicap -a voltage controlled capacitor), which results in the oscillator frequency being modulated by the audio signal, making FM.
    https://en.wikipedia.org/wiki/Varicap
     
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  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Presumably the 10pF of C5 as well as passing the RF signal to the output, also complements (as a parallel element) the transistor's internal collector-to-emitter capacitance. I would also think C4 (10pF) plays a role in determining the resonant frequency. C4 probably can't be too big or it will swamp the collector-base capacitance modulation necessary to produce FM.
    It's also possible that the transistor bias settles to Class 'C' operation.
     
    Last edited: Dec 11, 2014
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    That was one of the more interesting questions asked on this site recently, thanks!
     
  6. Undral

    Thread Starter New Member

    Dec 10, 2014
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    thank you
     
  7. Undral

    Thread Starter New Member

    Dec 10, 2014
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    thank you sir,

    I found this circuit (attached) it is almost same circuit but there is no c4 . what is role of c4. As I mentioned I'm freshman in electronics so I don't know most of the topics but I'm reading and studying your explained things... but I still can't analyze this circuit . If it is possible, can you please help me with analyzing this circuit ?
     
  8. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the oscilator is almost the same. c4 on the upper one is to resonate the coil on or near the fm broadcast band. the 10 pf cap on th second is to resonate the coil also, and provide a series resonant circuit for feedback on the common base oscilator colpits oacilators dont have common base circuits. the common base uses positive in phase feedback provided by the series tuned l/c tank.
     
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  9. DickCappels

    Moderator

    Aug 21, 2008
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    If those are your words you understand the circuit pretty well. Good going!
     
  10. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Your circuit does a lot of different things with a very few number of components. It is called "super-simple" but that means, in this "simple" means simple to assemble and few components. This type of "simple" means that each component must do more than one function in some cases. The band pass of these also uses the 27k resistor to the battery as the second resistor (filters can go to ground or positive supply, or essentially any stable voltage point).

    The audio input benefits from a band pass filter to remove sub-sonic and ultra-sonic frequencies.
     
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  11. uka

    New Member

    Dec 22, 2014
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    ok thanks I wanna simulate this circuit can u offer me which simulator i should use ? in order to give audio input what should i use (AC voltage source) ?
     
  12. crutschow

    Expert

    Mar 14, 2008
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    If you want a fairly good, free Spice simulator many on these forums us LTspice from Linear Technology.

    To simulate the FM modulation you need a Transient simulation. Thus you would use the SINE voltage source. But note that the large difference in frequency between the modulation frequency and the carrier frequency means that the simulation may take a long time to run, and the output may be difficult to interpret if you don't add a demodulator circuit to few the recovered modulation frequency from the carrier
     
  13. uka

    New Member

    Dec 22, 2014
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    can anyone explain to me role of circuits each component one last time ?? now my thought is first audio signal goes to band pass filter which is( c1, c2, r1,r2) then goes to transistors base . tuned circuit(L C? C? R?) takes dc voltage so it oscillates and come out carrier. then audio signal which is in the base of transistor and carrier signal which is in the emitter of the transistor modulates then what am I doing good or not ? this looks class c power amplifier. i"m just read lot of things and confused if it is possible please explain to me one by one (detailed) ??
     
  14. Undral

    Thread Starter New Member

    Dec 10, 2014
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    tor
    thanks İ attached the circuit much better drawn u said LCR tank but i think in this schematic tuned circuit is (L and trimmer cap). is transistor doing many function(amplification and in order to get carrier signal) ? as u said this circuit is doing lot of function using few components.
     
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