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Can you build an FM transmitter with a crystal oscillator

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  1. Feb 14, 2019 at 12:43 AM #1
    Can you build a fm transmitter with
    A crystal oscillator? I already have a
    100 megahertz crystal oscillator. I can't find instructions on how to do
    That on the internet.
     
  2. jcsd
  3. Feb 14, 2019 at 1:14 AM #2

    Baluncore

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    Yes you might, but it would certainly be very difficult. It would be a lot easier to build an LC oscillator with a variable capacitance diode, then use an input voltage to change the diode capacitance and so the frequency of the oscillator.

    With a fixed crystal oscillator you would need to vary the phase of the transmitted signal to fake an FM signal by phase shift.It would have a very narrow FM and be low volume and hard to hear on a normal FM receiver.
     
  4. Feb 14, 2019 at 3:14 AM #3

    Borek

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    Can you transmit at 100 MHz without permit?
     
  5. Feb 14, 2019 at 3:41 AM #4

    Baluncore

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    That depends on your location and the power of your transmitter. It is in the FM band here.
     
  6. Feb 14, 2019 at 7:07 AM #5

    anorlunda

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    I seem to remember some old-fashioned walkie talkie radios that had crystals to lock them into the desired channel. Is that correct?
     
  7. Feb 14, 2019 at 11:54 AM #6

    berkeman

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    https://en.wikipedia.org/wiki/Frequency_synthesizer

    upload_2019-2-14_9-54-40.png
     
  8. Feb 14, 2019 at 2:36 PM #7

    davenn

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    Crystal oscillators are still in common use today. Mostly, as always, for single frequency operation
     
  9. Feb 14, 2019 at 3:04 PM #8

    tech99

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    It is possible to make a crystal oscillator which can be tuned over a limited range of frequency, and then it is possible to obtain FM. However, the variable oscillator is not as stable as the original.
    A quartz crystal is a mechanical resonator but electrically it looks essentially like a series LC circuit. By adding reactance in series, the frequency can be altered. Large values of reactance are required, because the crystal might have a inductance of a Henry or something like that. One method is to use a parallel LC circuit in series with the crystal to obtain a large reactance. The performance is completely hit-and-miss and depends on the precise crystal being used. By placing a variable capacitance diode across the LC circuit it is possible to obtain frequency modulation. The final oscillator will be less stable than the original.
     
  10. Feb 16, 2019 at 7:48 AM #9

    NascentOxygen

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    There are plenty of circuits, though I recommend not trying to build one because trying to get these VHF circuits working without test equipment (and at frequencies where the proximity of your hand can upset operation) is invariably an exercise in frustration.

    This two-transistor schematic has the appearance of being straightforward enough, but it needs a crystal of under 20 MHz. The output tank is tuned to 6 times the frequency of the crystal. Also, crystals even of a particular frequency do differ markedly, you can't just plug in one of the desired frequency and expect it will work.

    EDIT I believe I misread what you seek. I see now that you already have the oscillator, and want to use its 100MHz sinewave to drive a modulator to generate an FM signal? Sorry, I can't help there.

    100MHz_FM.JPG
     
  11. Feb 16, 2019 at 8:06 AM #10

    Baluncore

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    I am afraid that circuit uses a discrete crystal, which is quite different to a crystal oscillator module.

    A crystal is a resonator with a very high Q, typically about 1 million. What can be done with a crystal is decided by the external circuit.

    A crystal oscillator module is designed to be extremely stable in frequency. Crystal oscillator modules have insufficient bandwidth, or external frequency adjustment, to allow audio FM modulation. Oscillator modules usually produce a clock signal, which is a rectangular wave with high harmonic content.

    A voltage controlled oscillator, VCO, is designed to be frequency modulated by an external voltage.

    A good engineer can design a crystal oscillator that dampens the high Q of a crystal, which can then be FM modulated. That makes a VCO and defeats the reason a high Q crystal was used in the first place.
    The circuit you provided uses that technique to generate an FM signal.
     
  12. Feb 17, 2019 at 12:24 AM #11

    davenn

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    As is common with many posts on PF, there is a severe lack of info given by the OP


    That is true, but I see no specific mention of a crystal oscillator module.

    The OP states a 100MHz crystal oscillator which could mean several things

    @David lopez it is well past time that you actually state what you have at the moment
    photo(s) would be great, part numbers would be helpful as well if not visible in the photo(s)


    Dave
     
  13. Feb 17, 2019 at 7:39 AM #12

    sophiecentaur

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    Absolutely. Not long ago, we (impoverished experimenters) didn't have frequency synthesisers and it was common practice to modify a VHF AM Transceiver by pulling the frequency of the driver crystal with an audio signal, using a varactor diode. In my very brief craze with Amateur Radio, I used one (can't remember the make or model) and I also offset the IF frequency so that I could use the AM receiver as a slope detecting FM receiver. As I remember, the crystal was fairly low frequency (say 10MHz) and that signal was multiplied up to the 2m frequency band this multiplication also increased the frequency deviation to a useable amount. Not tuneable but a few different frequencies were selectable with a 'turret tuner' style switch.
     
  14. Feb 17, 2019 at 11:14 AM #13

    NascentOxygen

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    Yes, immediately I posted that circuit I realised I was actually not answering the poster's exact question. But since his question doesn't have a good answer I left my post hoping it might prompt a re-appraisal of his quest. OP might settle for building a simple VHF transmitter that does use a crystal oscillator, just not his crystal oscillator.
     
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