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Can 14.31818Mhz Modem Xtals be used in OverTone mode

Discussion in 'Electronic Design' started by Kevin Doyle, Jul 15, 2005.

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  1. Kevin Doyle

    Kevin Doyle Guest

    Hi all,
    I have two crystals that I salvaged from two old 56K modems. Im not sure if
    they are cut for use in over tone mode.
    Is there some way of knowing or is it dependant on the circuit elements to
    get the Xtal to oscillate at over tone mode.

    The two Xtals have markings as follows PRE 14.31818Mhz, 14.31818 TQG.
    And both were spot welded or soldered on the top of the casing to the
    circuit board, is this an earth connection?

    Kevin.
     
  2. From experience, I can say they will work well at third overtone - close to
    3 times the fundamental. If I wanted 5 times, I'd probably buy something
    cut for the job, but I bet it could be made to work.

    Roger
     
  3. Guest

    It's very hard to manufacture a crystal without any overtone modes. A
    crystal advertised as "overtone" has been cut to emphasize them. It's very
    likely that a carefully adjusted overtone oscillator will make any crystal into
    an overtone device.
    That connection is both electrical (probably not needed) and mechanical
    (to take the strain off the fragile leads on the other end).

    Jim
     
  4. Kevin Doyle

    Kevin Doyle Guest

    OK Thanks guys!!
    Kevin.
     
  5. SioL

    SioL Guest

    One word of caution, usually 3rd overtone is not exactly 3x the prime frequency.

    However, the difference is very small and probably irrelevant in your case.

    Its also usually more difficult to pull the oscillator up or down at 3rd overtone.
     
  6. Of course, if he wants a third overtone of that specific frequency, it
    may be that he needs it because of that frequency. In other words, he
    wants to do some video work and needs a multiple of 14.31818MHz.

    The solution in that case is to look through more junk. Commercial
    stuff justs multiples of the frequency, so one could either pull
    a crystal or a full oscillator off some sort of board.

    Michael
     
  7. Tam/WB2TT

    Tam/WB2TT Guest

    This frequency is too low to be an overtone crystal. To operate it as an
    overtone crystal at 3X the frequency, the oscillator would have to include a
    tuned circuit at something like 43 MHz. It will not be an exact multiple of
    the fundamental though. If you actually need 2X or 3X that frequency, run
    the oscilator at the fundamental, and then through a frequency multiplier
    ( single transistor)

    Tam
     
  8. Joerg

    Joerg Guest

    Hello Tam,
    Yes, I'll second that. Just run it through a stage that is deliberately
    non-linear like a transistor without bias but driven hard enough.

    Regards, Joerg
     
  9. Tam/WB2TT

    Tam/WB2TT Guest

    For those who don't know what this means, you still need to have the
    transistor draw base current. You do this by connecting a resistor or RF
    choke from base to ground. This also assumes the signal is sinusoidal and a
    couple of volts p-p. For a square wave output, since a square wave already
    contains a third harmonic, you could just add an LC filter tuned to 3X the
    oscillator frequency. For a frequency doubler, run the 1X square wave, and a
    replica of it delayed 1/4 cycle by and RC network into an Exclusive OR
    gate. The XOR output will be 2X the frequency. Clean that up with a tuned
    circuit.

    Tam
     
  10. Joerg

    Joerg Guest

    Hello Tam,
    In case the drive signal is a bit on the wimpy side you can bias the
    base just a wee bit higher. Just so that the transistor still doesn't
    draw any significant current over the expected temperature range. Or
    remains in class C as the RF guys say.

    Regards, Joerg
     
  11. budgie

    budgie Guest

    which is probably better defined as a conduction angle of <180 deg with
    sinusoidal drive. I'll let others do the maths for optimum content of any
    specific odd harmonic in the output.

    Peter
    RF guy (mainly)
     
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