FM TX and RX design need help

Discussion in 'Wireless & RF Design' started by ericyeoh, May 18, 2011.

  1. ericyeoh

    Thread Starter Active Member

    Aug 2, 2009
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    0
    Hi all,

    I'm doing a project FM TX and RX design without using any help of IC.
    The frequency band is choosen to in 40.xx Mhz for the project. This is the ISM band.

    Please purpose if you have any.

    TQ
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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  3. bertus

    Administrator

    Apr 5, 2008
    15,647
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    Hello,

    Did you already have a look in the RF related links thread?

    Basicaly the change to an other frequency is changing the tuned circuit of the transmitter and reciever.
    For better stability take a look at some transmitters using an X-tal oscillator.

    Bertus
     
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  4. ericyeoh

    Thread Starter Active Member

    Aug 2, 2009
    58
    0
    Tq for quick reply bertus, i was not sure about changing the "inductance" tank circuit could actually achieve the desire frequency.

    Thx for your hint, i will focus into that area.
     
  5. radiohead

    Active Member

    May 28, 2009
    474
    31
    You can build a transmitter using a crystal to lock your frequency. Google is your friend. There are hundreds of thousands of hits for crystal transmitters. It is up to you to choose one depending on the complexity of the circuit you want to build.
     
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  6. ericyeoh

    Thread Starter Active Member

    Aug 2, 2009
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    Radiohead, i hv no idea which 1 is more compact and stable and i;ve no idea choosing any.
     
  7. radiohead

    Active Member

    May 28, 2009
    474
    31
    I built this one. It works pretty good. With a little tweaking and maybe adding a PA stage with a matched antenna, you can achieve a mile or more (line of sight).
     
  8. ericyeoh

    Thread Starter Active Member

    Aug 2, 2009
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    0
    Good info, thx radiohead
     
  9. ericyeoh

    Thread Starter Active Member

    Aug 2, 2009
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    0
    Hi all,
    I got updated from my supervisor, he requests me to use VCO to build the transmitter and fix the output frequency to 40.xMhz.

    Can any 1 share the concept or idea?
    i'm still fresh
     
  10. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    In wich way must the frequency be fixed?
    Can you use a PLL?

    Bertus
     
  11. ericyeoh

    Thread Starter Active Member

    Aug 2, 2009
    58
    0
    Hi bertus,

    Yes, PLL can be included. Anyway, how could i fix the FM TX center frequency to 40.68 Mhz using VCO + PLL. Sounds like the TX getting complex.
     
  12. ericyeoh

    Thread Starter Active Member

    Aug 2, 2009
    58
    0
    Hi all,

    I would like to make clear on something. I just updated from my supervisor, he has told me to try achieved the modulation index, B ~ 5.0 (if possible).

    My project is basically design a FM transceiver.
    Input : microphone (human voice)
    Output (TX) : able to produce the (human speech)

    Design requirement
    TX :1.Frequency must be ISM band ~ 40.6Mhz.
    2.The design must stick with VCO + PLL.
    3.Intergrated circuit is restricted for designing the VCO.

    RX : The design could use IC to implement and must be able to receive the correct signal.
     
  13. ericyeoh

    Thread Starter Active Member

    Aug 2, 2009
    58
    0
    Hai all,

    How can i control the frequency deviation?
    What is the max modulating frequency if the input is human voice to micophone?
     
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