reasons why frequency different in 3 devices

Discussion in 'Wireless & RF Design' started by lemonjuice, Dec 1, 2016.

  1. lemonjuice

    Thread Starter New Member

    Nov 26, 2016
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    Hi, I had build a transmitter. I check its transmission frequency at antenna using my smartphone's radio and it shows 97MHz. However, when I test again using oscilloscope, it shows 50MHz and another 107MHz in spectrum analyser. Using the same song as input, but why 3 different results?
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    The scope and analyser, or the inductance and capacitance of their test leads, are presumably loading the circuit too much and affecting the tuning.
     
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  3. DickCappels

    Moderator

    Aug 21, 2008
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    Alec_t has probably guessed correctly.

    For deeper insight, post a schematic of your transmitter.
     
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  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    I see from another forum that your circuit is on a breadboard with straggly wiring. Yet another reason why the frequency is unstable.
     
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  5. lemonjuice

    Thread Starter New Member

    Nov 26, 2016
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    @Alec_t @DickCappels Thanks for your reply. Just like what you said, too much wiring... I will try to make it a PCB and try again.
     
  6. Sensacell

    Moderator

    Jun 19, 2012
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    Assembling it on a PCB will help- but not cure the problem.

    A stable transmitter needs multiple stages of buffering and isolation to prevent the load from pulling the output frequency.
    Good designs typically use crystal oscillators as a frequency standard, either as a direct source, or via a phase-locked-loop frequency synthesizer.
     
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