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Can I replace an LC circuit with a crystal?

  1. Dec 5, 2012 #1
    Hello all,

    I am trying to make an FM transmitter that operates at only 1 frequency. I need the frequency tolerance to be very high, so I'd like to choose a crystal instead of an LC circuit.

    This is the circuit I'm building: http://cappels.org/dproj/FMdist/fmdis.htm

    If I were to replace the tank circuit with a crystal, would it operate the same way?
     
  2. jcsd
  3. Dec 5, 2012 #2
    Hello

    Please take note of the warning notice about the legality of transmitters that appears at the top of your linked-to article.

    Such a transmitter, if legal in your country, would work if you used a ceramic resonator of the appropriate frquency, rather than a crystal.
     
    Last edited: Dec 5, 2012
  4. Dec 5, 2012 #3
    I checked the FCC's regulations. They outline a specific section for hobbyists making low power unlicensed transmitters. I'm perfectly within my rights to make this transmitter.

    And thank you for the advice about the resonator.
     
  5. Dec 5, 2012 #4

    vk6kro

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    Crystals and ceramic resonators are both insulators to DC so you could not just remove the tuned circuit and substitute a crystal or a ceramic resonator.

    However, there are plenty of crystal oscillator circuits you could use. Not all of them would work at the frequency that this circuit operates at, though.

    Operation in the commercial FM radio band would require a 90+ MHz overtone crystal which would be expensive if you have to order one especially made.

    Also, it is quite difficult to get wideband FM from a crystal oscillator.

    Wireless microphones get around this problem by generating FM at a low frequency (like 8 MHz or so) and then mixing this with a crystal oscillator to get to the required output frequency (sometimes 45 MHz or so)
     
  6. Dec 5, 2012 #5

    sophiecentaur

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    If you want frequency stability then you are right; LC is certainly not the best way to go. If you want a suitable circuit then look in the Radio Ham and constructor pages. Trying to take that circuit and to modify it would be difficult as the impedances would be all wrong. There are dozens of tried and tested circuit designs for crystal and ceramic resonators.
    If this is one of your first projects then you don't want to be doing any design mods - you want it to be as easy as possible and VHF frequencies are not like audio.
    I found this link amongst others
     
  7. Dec 5, 2012 #6
    Here's a crystal oscillator circuit that shouldn't be too hard to incorporate into your circuit. As for the unconnected lead from C2, I'm guessing that goes to the collector of the next stage in order to increase positive feedback.
     

    Attached Files:

  8. Dec 5, 2012 #7
    How are you going to frequency modulate the crystal? Often the frequency of the crystal is multiplied a number of times in order to increase the deviation. You can also modulate the tank circuit with varactor diodes which will give you an even more stable carrier but technically it's phase modulation instead of frequency modulation. It has an advantage of providing natural pre-emphasis to audio frequencies. A phase modulated signal must also be multiplied up multiple times in order to get enough deviation.
     
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