frequency stability

Discussion in 'General Electronics Chat' started by shreyas_bhat, Feb 9, 2005.

  1. shreyas_bhat

    Thread Starter Active Member

    Jul 26, 2004
    47
    0
    Hello All
    I have built an oscillator (Twin T oscillator, with just R and C components). However, there seems to be an appreciable drift in frequency. Is there some additional circuitry that could help me stabilize the frequency.

    Thanks
     
  2. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
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    What type of capacitors are you using?
     
  3. shreyas_bhat

    Thread Starter Active Member

    Jul 26, 2004
    47
    0
    Normal mica capacitors (not electrolytic), (47nF)..


     
  4. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    hi

    can you show your schem?
     
  5. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    You don't normally get mica capacitors of that size. Are you sure they are bot ceramic?
     
  6. shreyas_bhat

    Thread Starter Active Member

    Jul 26, 2004
    47
    0
    OH THAT WAS A AN ERROR. THEY'RE IN FACT FLAT DISC CERAMIC CAPACITORS.



     
  7. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    Then change them to 'poly' types, ie. polycarbonate, polyester, etc. Ceramic caps don't give good stability because of their poor temperature coefficient. The types known as High K are the worst. They use a ceramic with a high dielectric constant to get the required capacity into a smaller space. They are OK for decoupling but not for your application.
     
  8. torpedopudding

    Member

    Feb 6, 2005
    28
    0


    You are correct however there are actually many types of ceramic capacitors. First question is how much capacitance do you need?

    Generally you have a tradeoff between stability and (cost and size together) and capacitance. My numbers below are rough......

    NP0 ceramic is rock solid but will become large and expensive above 1 nanofarad.

    Mica can be excellent but is not so common these days; and I think there may be good and bad mica. Mica is mined and mainly comes from India and Africa (??)

    Another very stable type of capacitor is polystyrene; excellent electrical characteristics but this is destroyed by temp over 85 degrees C. I think this becomes huge and expensive above maybe 10 nF.

    Polycarbonate is not as stable as polystyrene but is not too bad and you can get more capacitance, maybe up to a few uF although it becomes a large honker. Also polycarbonate can handle higher temp.

    Polypropylene, then polyester are the next steps down from polycarbonate although are still much better than "high K" ceramics such as X7R, Z5U.
     
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