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FM transmitter

Discussion in 'Electronic Basics' started by svetoslav belchev, Feb 12, 2005.

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  1. Hi.So I try several years to build a smal FM transmittet. I built mani
    small transmitters, but no one of them can't work stable. For example
    when I touch the device or move it, its frequency deviate (the
    frequency is not stable).
    The other problem is that there is many harmonics. Does anybody occupy
    in these kind of electronics ? Thanx.
  2. Andrew Holme

    Andrew Holme Guest

    I presume you're building single tansistor free-running VHF oscillators.

    You won't get decent stability from a free-running oscillator much above
    5MHz. Even at 5MHz, you still need to take great care, use high quality
    components - especially the inductor - and mount it in a screened metal box.
    Temperature variation is a major cause of drift. Even at 5MHz, designers
    need to select components that cause drift in opposite directions and
    thereby partially cancel.

    For stability at VHF: either use a PLL synthesizer; or a crystal oscillator
    and frequency multiplier chain. Hand-capacity effects are more significant
    at VHF because circuit capacitances are so small.
  3. Yes the circuit have only one transistor. I don't want to use PLL
    sinteser because it need complex (difficult)circuit and components
    like IC. The other variant is to use quartz cristal oscillator at
    about 10-20 MHz, but it requires multipler and band filter on its
    output. I need some simple decision.
  4. SioL

    SioL Guest

  5. Any siplier circuit ?
    Did you think, that power amp with one transistor next to the
    oscillator, may enchance frequency stability ?
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