Missing crystal - what happens?

Discussion in 'General Electronics Chat' started by yoyo42, Mar 27, 2011.

  1. yoyo42

    Thread Starter New Member

    Mar 27, 2011
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    Hi all, I'm new here and joined up to get an expert response to a question that cropped upon another forum. Inexpert responses are also welcome ;)

    If a radio control transmitter, in this case 35MHz, is run with the crystal removed, what happens? Will it still transmit but at some uncertain frequency, still transmit but drift around all over the spectrum, or not transmit anything?

    The question arises because the transmitter in question has a separate 2.4GHz module fitted which is doing the actual controlling, but it doesn't have any easy way to disable the 35MHz output stage. The 35MHz antenna has been removed and the crystal taken out, but I have a feeling that may not be enough to avoid interfering with other 35MHz receivers that may be in the area...
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
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    A crystal oscillator does not oscillate without the crystal. The 35MHz crystal might be the reference for a 2.4GHz PLL circuit.

    The transmitter for my RC airplanes and helicopter operate only at 2.4GHz with a very short antenna. The crystal is so small that I can't read its frequency.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    Any decent design (and these are pretty highly regulated) would shut down without a crystal. The FCC take a dim view of uncontrolled transmissions.
     
  4. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,672
    899
    35 MHz is used in Europe and England, I think, like we use 72 MHz. The crystal is only for the original band and does not affect the 2.4 GHz module.

    On my radios, one simply removes the complete RF module/PCB to convert to 2.4 GHz, so the crystal-only question has never come up on RC groups in which I participate.

    You may find this link helpful: http://www.rcgroups.com/diy-electronics-199/

    John
     
  5. yoyo42

    Thread Starter New Member

    Mar 27, 2011
    3
    0
    Thanks for the replies.

    On my transmitter, the module does automatically shut down the 35MHz RF stage (which you're right about, it is the pre-2.4G model aircraft band here in the UK).

    The question was referring to someone else's tx which didn't have any obvious way to disable the RF stage, so he just pulled the crystal and assumed it would be enough... I was just looking around to see if it was or not.

    It's looking likely, but I guess the only real way to tell is with a spectrum analyser.
     
  6. K7GUH

    Member

    Jan 28, 2011
    191
    23
    One does not need a spectrum analyzer to apply elementary logic and common sense. A crystal oscillator depends on the crystal to oscillate; without the crystal or without primary power, the circuit cannot oscillate. If you still have doubts, use an RF probe and an old fashioned VTVM, remove the crystal or cut the primary power, and see what happens. If you have super duper up to date measuring equipment, you may use that instead, but I predict that the results will be the same.
     
  7. yoyo42

    Thread Starter New Member

    Mar 27, 2011
    3
    0
    Fair enough. I was thinking that it might be that there was a roughly 35MHz signal being generated to stimulate the crystal, which would then lock up to it's exact resonant frequency - so without the crystal that hypothetical oscillator would just free run.

    But I guess not.

    Someone else on the thread has used an RC 'channel checker' - essentially a specialised RF probe simplified for RC use - and confirmed that the transmitter stops completely when the crystal is removed.

    Thanks for the responses, it's been helpful.
     
  8. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,740
    759
    Mostly for transmitter frequency disabling, the Crystal is taken out.

    Try this and check. I don't think you will harm anything by doing this
     
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