20 mhz oscillator please help

Discussion in 'General Electronics Chat' started by matbymay, Jul 29, 2009.

  1. matbymay

    Thread Starter Member

    Jul 14, 2009
    27
    0
    hi everbody

    i need sinewave output oscillator which operates 20 mhz or more.
    crystal oscillator will be preffered
    please help i have many oscillator design but i need very stable.

    thank you
     
  2. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
    How pure does the sine wave need to be?

    How stable does it need to be, and over what temperature range?

    Does it need to be tunable, or is it at one frequency always?
     
  3. matbymay

    Thread Starter Member

    Jul 14, 2009
    27
    0
    it is just one frequency not adjustable and i need sinewave very clear.
    if there is some IC sinewave generator you can suggest me or have you got good tested one oscillator will be ok

    thank you
     
  4. davebee

    Well-Known Member

    Oct 22, 2008
    539
    46
    You could get an inexpensive canned crystal oscillator, like this one -

    http://www.jameco.com/webapp/wcs/st...storeId=10001&catalogId=10001&productId=27932&

    to generate a stable 20 mHz square wave, then pass the signal through a bandpass filter to clean it up into a sine.

    I don't know of any off-the-shelf 20 mHz bandpass filters that you could just buy - you might have to make this yourself from discrete parts. But there are plenty of filter design tutorials on the web that can help with this design.
     
  5. matbymay

    Thread Starter Member

    Jul 14, 2009
    27
    0
    thank you for yor solution

    but any other ideas quick and clear.
    when i searh to internet i found max038 IC but it is not being manufactured anymore. do you know any equivalent ?

    thank you
     
  6. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469

    I was thinking the same thing when I asked those questions. So far, the simple answers seem to indicate this may work. However, we really need more detailed quantitative specifications for harmonic purity and temperature stability to be sure.

    Hopefully, the OP can look at his requirements and generate some real specifications to go by.
     
  7. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
    You haven't given any real specifications or details for how you will use it, or size or anything. What about cost?

    Can you use a benchtop signal generator? This will be designed to be very pure and reasonably stable, depending on how much you are willing to spend.
     
  8. matbymay

    Thread Starter Member

    Jul 14, 2009
    27
    0
    temperture is up to max 65C and it would be pure sine wave as i can as. frequency and amplitude accuarcy should be 1% at whole performance.

    i implement some colpitts oscillator but it is not usefull actually i need really good one thats why i m asking IC type generator

    thanks for your attention. please keep answer me, i really stuck to implement oscillator
     
  9. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    Can you post a schematic of the oscillator you have now?

    The stability of the oscillator strongly depends on the type of capacitors used.

    Greetings,
    Bertus
     
  10. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
    Looking at the MAX038 specifications and the ones you just gave, the stability and purity specifications do not seem that stringent. An LC based oscillator may meet those specs if you design carefully. Certainly, a fixed frequency crystal oscillator generating a square wave followed by a buffered LC bandpass filter stage will be able to meet those specifications.

    The MAX038 says the following:
    > 100 PPM/C frequency stability (note that's per Celcius degree!)
    2% harmonic distortion

    Compare these to crystal specifications.
    http://microcontrollershop.com/product_info.php?products_id=2061
     
    Last edited: Jul 29, 2009
  11. matbymay

    Thread Starter Member

    Jul 14, 2009
    27
    0
    i have and implemented this circuit below but it didnt work what you think about this circuit
    i used 20 mhz crytal oscillator on this circuit
     
  12. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    What kind of crystal is used?
    Is it an overtone crystal?
    Did you get some frequency out of the circuit?

    Greetings,
    Bertus
     
  13. matbymay

    Thread Starter Member

    Jul 14, 2009
    27
    0
  14. matbymay

    Thread Starter Member

    Jul 14, 2009
    27
    0
    could you check for me please it is usefull or should i use different type so it should be different could you suggest any ?

    thank you
     
  15. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
    1,758
    98
    matbymay have you found an LED or Phototransistor that runs at 20MHz yet. I would worry about that part of your design more than this?
    (from another forum about the same thread)

    PS you can just buy a sine out OCXO or oscillator rather than build it yourself.
     
  16. matbymay

    Thread Starter Member

    Jul 14, 2009
    27
    0
    thank you blueroom but i sent list to you did you check it? i think it is alright i checked datasheet and slew times more than 20 mhz. but if suggest any others i can buy them.
    on the other hand could you suggest specific OCXO or oscillator because you know my project
     
  17. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
    1,758
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    Although I'm not an expert in PLL design and optics but your design appeared to be based solely on your imagination / wishful thinking vs any real prior designs.
    You provided no references to any similar design whatsoever.

    Why not post it here for other opinions.

    Google OCXO, they're expensive but accurate and both HCMOS & sine outputs available.
    http://www.conwin.com/
     
  18. matbymay

    Thread Starter Member

    Jul 14, 2009
    27
    0
    thanks blueroom
    but first of all i have to do driver circuit (for that i need osc.) and photodetector circuit with opamp. actually the key role is oscillator which should be stable. other step i m goingto ask here step by step. this is project and every information i ll explain.
     
  19. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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    Again you don't provide any information on what you're basing your design on. I'm trying to spare you the grief of building something based on no solid plan whatsoever. You had a block diagram on the other forums, why not post it here.
     
  20. matbymay

    Thread Starter Member

    Jul 14, 2009
    27
    0
    ok i m sorry i just understood you. i have sent diagram but still same thing i didnt draw good one
     
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