Need Help in modifying this circuit

Discussion in 'Homework Help' started by ojou-san, Apr 23, 2013.

  1. ojou-san

    Thread Starter New Member

    Jan 7, 2013
    20
    0
    Hello. I am currently taking BS ECE and we are given a task to make a wireless microphone. I found this schematic while surfing on the internet: http://www.reconnsworld.com/transmit/fm_trasm.gif
    Please help me modify this circuit. Our instructor wanted us to make our very own design and just use that as a reference. Please help me. I don't know how do I know what computations are needed to determine the capacitance's value, resistor's value, and how come that is the transistor being use? My Instructor wanted us to show him the computation of those parts when we have modified it. He wanted us to tell him how did we modify it and what computation etc.:confused: Please help me. I would gladly appreciate your help. Thanks in advance!
     
  2. #12

    Expert

    Nov 30, 2010
    16,304
    6,814
    I think you should ask your instructor to point out the sections of your text books where he taught you how to do this. Then ask him how he wants it modified in terms of frequency, range, distortion, or whatever he really wants.
     
    ojou-san likes this.
  3. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
    675
    The first thing to do is understand what the circuit is doing, then you can understand how to modify it.


    You can see that the first transistor is an AC coupled amplifier stage - so you can modify the gain of this stage.


    The second transistor is doing the modulating of the amplified AC signal from the microphone.

    Have you tried stimulating this circuit? That way, you can make adjustments without risk of damaging things. Just make sure to account for losses when designing the actual circuit.
     
    ojou-san likes this.
  4. aws505

    Member

    Mar 11, 2013
    59
    7
    Hey,

    It looks like the amplifier is a simple two-stage audio amplifier and mixer. The microphone picks up sound and then A/C couples the sound signal through C1 to the first stage.The first stage, Q1, seems to be implemented as a Class-A (or maybe just on the verge of Class-AB -- it's DC bias is at 0.8V) amplifier for the audio signal. The audio signal is then AC coupled thru C2 to the second stage, Q2. It looks like Q2 is an oscillator whose oscillation frequency is determined by L1 and C4.

    Unfortunately, I don't exactly understand the specifics of the calculations necessary to make intelligent changes. A little googling suggests that the second stage my be a Colpitts oscillator. What exactly does your instructor wish you to alter? Are you supposed to get the transmitter to work at a specific frequency? With a specific antenna? At a certain gain? The more information you provide, the more we can help.
     
    ojou-san likes this.
  5. Sue_AF6LJ

    Member

    Mar 16, 2013
    45
    27
    The circuit in question isn't really a straight forward example of an FM transmitter.
    Read this, when you get done you will see how that circuit works.
    http://facultad.bayamon.inter.edu/rflores/ELEN4360/Labs/Com1_Lab4.pdf

    By the way there are more straight forward ways of generating an FM signal.
    This method is less than desirable since there is also a lot of unwanted amplitude modulation created by the modulation process.
    Here is a useful search page with useful links.
    https://www.google.com/#hl=en&sclie...7b9547a1aefef82f&biw=868&bih=413&pf=p&pdl=300

    There are about a half dozen ways to generate an FM modulated signal.
    Some are really interesting....
     
    ojou-san and aws505 like this.
  6. ojou-san

    Thread Starter New Member

    Jan 7, 2013
    20
    0
    Well we can modified in anything we want as long as it is not the same as that reference. btw thanks for replying!:)
     
  7. ojou-san

    Thread Starter New Member

    Jan 7, 2013
    20
    0
    Thanks~! Well I am trying to simulate it in multisim but I cannot find all the parts. -.-
     
  8. ojou-san

    Thread Starter New Member

    Jan 7, 2013
    20
    0
    Thanks for helping :D Hmm Well our professor wanted us to modify the circuit but it is up to us on how to modify it. well base on what you said I think I wanted it to work in a frequency lower that 88MHz because that circuit works at 88MHz. Well if you want me to state a specific range well maybe in I wanted it to work in a frequency in the range of AM radio.
     
  9. Sue_AF6LJ

    Member

    Mar 16, 2013
    45
    27
    That circuit might not work that low and produce FM modulation.
     
    ojou-san likes this.
  10. ojou-san

    Thread Starter New Member

    Jan 7, 2013
    20
    0
    Yeah I just realized that it will just work in the range of 88MHz-108MHz. Sorry for not realizing it. I am still thinking of what to modify in that circuit and what calculations do i need to do.BTW Thanks! :)
     
Loading...