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cheap 120Khz sine generator

Discussion in 'Electronic Design' started by Johnny Chang, Nov 11, 2007.

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  1. Johnny Chang

    Johnny Chang Guest

    i'm trying to find a very inexpensive way to generate a 120Khz sine
    wave to act as a carrier wave for a cheap system. browsing around the
    internet gives me way too many options.. function generators, op amps,
    integrators, etc.

    what is the best and cheapest option? i'm new to circuit design but
    i've taken a lot of classes and am a pretty fast learner
     
  2. +12
    |
    |
    |--
    ---------->| BF254B
    | | |--
    || C1 |-------- out
    |L1 |--------|
    || | |
    | C2 R1 1K
    | | |
    ------------------------
    |
    ///

    C2 = 2 x C1

    L1 together with C1 C2 should be tuned to 120kHz
    It is not a perfect sinewave, a bit flattened on one end, but it should do.
     
  3. go to analog.com, search dds, sort of $5 products, needs a bit of
    harwdare on top.
    disclaimer, only looked at the stuff cos i was interested, and I
    have'nt used their dds products


    Martin
     
  4. SBS

    SBS Guest

    Johnny Chang () ha scritto:

    :: i'm trying to find a very inexpensive way to generate a 120Khz sine
    :: wave to act as a carrier wave for a cheap system.

    Would you like to use that in a powerline communication system?
     
  5. D from BC

    D from BC Guest

    Just your luck I tripped across recently...
    http://www.ti.com/sc/docs/apps/msp/journal/aug2000/aug_07.pdf
    D from BC
     
  6. Johnny Chang

    Johnny Chang Guest

    yes, actually
     
  7. Johnny Chang

    Johnny Chang Guest

    there is also a need for a 20Mhz clock for the processor. Would there
    be a way to convert this crystal oscillator's square wave into a
    120Khz sine wave or am i better off building my own circuit?
     
  8. Phil Allison

    Phil Allison Guest

    "Johnny Chang"

    ** Is this "carrier" to be FM or AM modulated ??

    Makes a big difference to the choice of oscillator.



    ........ Phil
     
  9. Johnny Chang

    Johnny Chang Guest

    BPSK
     
  10. Johnny Chang

    Johnny Chang Guest

    We plan to use BPSK, why does the oscillator type matter? some other
    circuit will be handling the modulation and just wants a wave for it.
     
  11. Johnny Chang

    Johnny Chang Guest

    We plan to use BPSK, why does the oscillator type matter? some other
    circuit will be handling the modulation and just wants a wave for it.
    i think he is doing something with a PLL
     
  12. Phil Allison

    Phil Allison Guest

    "Johnny Chang"

    ** You did not post that info previously.

    The problem is YOU cannot post a proper spec for this " carrier "
    scillator - except that it must be 120 kHz , sine & cheap.

    No spec = no design.




    ........ Phil
     
  13. Guest

    There are a number. Unfortunately you can't divide 20MHz down to
    exactly 120kHz - the ratio is 166.66 : 1 which is not an integer
    ratio. 18MHz divided by 150 does give you 120kHz and you can buy 18MHz
    crystals off the shelf from Farnell.

    Once you've got a 120kHz square wave you can low pass filter out the
    higher harmonics (360kHz, 600kHz etc) or you can feed the edge through
    a tapped delay line clocked at some high frequency and us the taps to
    build a finite impulse response filter, which is an easier way of
    reducing the lower frequency harmonic content, though you'll still
    need some kind of analog low pass filter to get rid of the higher
    harmonics.

    Hope this helps.
     
  14. Jamie

    Jamie Guest

  15. D from BC

    D from BC Guest

    I devoted 30 seconds of Googling to find Mr. Bubba.
    No luck :)


    D from BC
     
  16. Rich Webb

    Rich Webb Guest

    Since you have a processor, you could use the DDS technique of a phase
    accumulator to a lookup table driving a DAC. Without knowing what your
    instruction cycle time is and what kind of timer peripherals there are
    it's not possible to say how clean the output would be. You'd probably
    need at least enough of a filter to remove the phase accumulator
    switching time.

    Advantage is that you can change the phase and frequency on the fly
    from the processor.
     
  17. Tim Wescott

    Tim Wescott Guest

    It does if you want it FM modulated -- deriving it from your main clock
    is easy if it can't be modulated, but way hard if it must be.

    With BPSK (or AM, or SSB, or even QPSK) you don't need it modulated.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  18. Tim Wescott

    Tim Wescott Guest

    Tapped delay line?!? I do hope you mean one that's clocked!

    You could use a 4017 and a pack of resistors to get your first
    approximation of a sine wave with several of the lower harmonics knocked
    out -- this would ease the filtering considerably.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  19. Guest

    Sorry about that - I meant to type "shift register" rather than
    ""delay line". After all, tapped delay lines aren't clocked. You can
    do something vaguely similar with a tapped delay line and a pack of
    resistors - I've done it a couple of times - but it wouldn't make much
    sense to do it here, as tapped delay lines cost quite a bit more than
    CMOS shift registers

    The 4017 is - in fact - a shift-register based "Johnson" counter

    http://www.nxp.com/acrobat_download/datasheets/74HC_HCT4017_CNV_2.pdf

    For this application you don't really need the decoding gates, though
    it makes the circuit a little easier to understand.
     
  20. I think I'd use a "phase shift oscillator", fast, cheap
    and easy, but I haven't gone to 120kHz with that type,
    I'd don't know the practical upper limit on them,
    anyone else know?
    Ken
     
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