kHz oscillator - output amplitude insensitive to temperature

Discussion in 'The Projects Forum' started by ChateauduChillon, Jul 25, 2014.

  1. ChateauduChillon

    Thread Starter New Member

    Jan 28, 2014
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    Hi all,

    I'm working on designing a kHz sine wave oscillator whose output amplitude is quite insensitive to temperature. Frequency drift is really not a concern; the nominal target will be near 30kHz but even 1kHz drift over temperature would be OK. Harmonic distortion is not a huge concern, having the 3rd harmonic at -40dB down would be acceptable. Anyone have a topology I should try out?

    Thanks in advance for any advice!

    Thanks,

    nick
     
  2. Experimentonomen

    Member

    Feb 16, 2011
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    The classic wien bridge oscillator perhaps ?
     
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  3. ChateauduChillon

    Thread Starter New Member

    Jan 28, 2014
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    Was looking into the wein bridge. My concern is that most of the amplitude methods I've seen (e.g. zener diode, thermistor, etc.) seem like they would make the output amplitude pretty temperature dependent which would be a problem for me.

    Also looked at a 2 op-amp quadrature oscillator, but given that the amplitude depends on the R's and C's I didn't think it would do very well either.
     
  4. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    And the range of REQUIRED temperatures is.... what?
     
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  5. ChateauduChillon

    Thread Starter New Member

    Jan 28, 2014
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    ~ambient to 150C, hot, sorry should have specified that. Will most likely use OPA170 amplifiers to deal with this temperature range.
     
  6. ronv

    AAC Fanatic!

    Nov 12, 2008
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  7. crutschow

    Expert

    Mar 14, 2008
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    My offhand suggestion for a stable oscillator is to use a Wein bridge oscillator with a FET amplitude control. Use an op amp peak detector or precision rectifier (to remove the diode temperature effect) at the output voltage and compare that with the voltage from a precision voltage reference. The output of the comparator then goes to the FET input. The amplitude will thus be mostly determined by the stability of the voltage reference, which can be quite good.

    One problem is finding a voltage reference for that temperature since most only go to 125°C but I found this one which goes to 210°C(!).

    Incidentally TI has a whole series of high temperature semiconductors which may be of interest to you.
     
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Why start with bad design specs?

    Why does the circuit need to be subjected to 150'C (which is above the max rated temp for the majority of electronic components)?

    And why is the sine output amplitude so critical? Just how critical? Can it tolerate 1% amplitude change? More? Less?

    If you explain more about the project maybe we can solve these weird issues at the source (with a better design!) instead of trying to work around a bad design spec.
     
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  9. ChateauduChillon

    Thread Starter New Member

    Jan 28, 2014
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    Thanks, yeah 150C is not fun. Makes my life difficult.

    Thanks, I like this servo-like idea. I will have to experiment with this.

    I know the 150C part sucks, believe me. I can't really motivate it, but that's the way it will have to be. Output amplitude within 1% would be my initial target.
     
  10. ChateauduChillon

    Thread Starter New Member

    Jan 28, 2014
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    thanks everyone who gave advice here! I've gotten a 2-op amp quadrature oscillator (needed quadrature output now) with peak detector and integral controller feeding JFET working fairly well. just battling second harmonic distortion but the stability is good! Didn't take me all these months don't worry just been working on other stuff
     
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