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Homework Help: FM Transmitter Course work

  1. Jul 23, 2012 #1
    1. The problem statement, all variables and given/known data
    Hi I am weak at circuit, but I need to analyse how this circuit works and to determine the values of C4, C5 ,C6,C7, and L1.
    I need help for the understanding function of the whole component and determine the values of these component

    2. Relevant equations


    3. The attempt at a solution

    Basically, from what I've tried to research over the internet, I make a speculation of the circuit.

    First of all, the circuit starts with the electret microphone picks up sounds and produces a certain millivolt signal. The c1 couples the microphone to the input of the first stage which is the oscillator circuit form by a transistor ,R2 and R3.The R3 and R2 bias signal to the transistor and being amplified here. C1 also together with C2 form an Ac coupling to block the DC. The c2 passes the audio signal to the RF oscillator(Q2 transistor) meanwhile the R6 limits the input to the base of Q2.

    Q2 ,C5 and L1 form tank circuit which act as tuning to govern the frequency of operation.
    The c6 on the emitter taps off a small portion of the signal and passes it to the output stage. From here, we can determine the value of C5 and L1 by using the equation mentioned above.
    Let say I want to tune the frequency at 101MHz, and the value of the inductor is set to 3.6uH, I will be getting capacitor value of 0.68pF.

    So far ,this is what I have got, nothing much.
    Please correct me if I analyse the circuit wrongly.
    and help me with the values of the rest of the components.
  2. jcsd
  3. Jul 23, 2012 #2


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    Staff: Mentor

    Hi alexi0313. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    Q1 is an audio amplifier. There is no tuned circuit here, so no obvious components to cause it to be an oscillator. Otherwise your description seems accurate. I´m not sure how to calculate the necessary value for C6, sorry.
    Last edited by a moderator: May 6, 2017
  4. Jul 25, 2012 #3
    Thanks NascentOxygen.

    But any chance you know what its function as well as the C4 and C7?
  5. Jul 25, 2012 #4


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    Staff: Mentor

    I´d say C6 might be the feedback that causes that stage to oscillate; does it look like positive feedback? C4 looks like it will make sure the base is at ground for RF; though it may also form a low-pass filter (with R6//R8) to limit the audio spectrum and thus limit the bandwidth of the RF output. Which is its real purpose, I don´t know. I´ve not had much to do with designing oscillators, sorry.

    Your stage 2 appears to have a topology similar to that of the Hartley in Fig 6-1 (c) here, if you swap emitter with collector: http://dev.emcelettronica.com/fundamentals-transistors-transistor-oscillators [Broken]
    Last edited by a moderator: May 6, 2017
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