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Oscillator survey

  1. Oct 3, 2007 #1

    Gokul43201

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    I'm doing a literature survey on high quality oscillators, and since this is peripheral to my area of specialization, I'd appreciate some help.

    I'm looking for a low frequency oscillator (~10Hz and up) with a frequency stability of about 30ppm/C, amplitude stability of about 3ppm/C and low harmonic distortion (all higher harmonics at least 90dB below fundamental).

    Is anyone aware of anything in the market or in literature that has the above specs? If you know of something that comes close, please tell me where to look it up. And if you are not aware of anything that's been built with these specs, I'd like to hear about that too (and any related wisdom) - negative responses are valuable as well.
     
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  3. Oct 3, 2007 #2

    berkeman

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    To get that low of a frequency and good accuracy, I believe you will need to divide down a higher frequency oscillator. The best way that comes to mind would be to start with a 32kHz watch oscillator, and use an HC4060 or equivalent to divide it down. One of the 4060 series contains an unbuffered inverter to use for the oscillator section -- I'll look it up in the morning when I get to work and post info on it. You should be able to make what you want with just a jellybean 32kHz watch crystal and a 4060-like chip. It will generate discrete frequencies, though. Do you need variable frequencies?
     
  4. Oct 3, 2007 #3

    Gokul43201

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    I'm looking for an analog circuit where I can change frequencies trivially, by swapping a small number of resistors or capacitors.

    I've had to build such an oscillator for low-temperature thermometry, and the project turned out to be non-trivial, in terms of time and effort. I want to know if I've just been reinventing the wheel.
     
    Last edited: Oct 3, 2007
  5. Oct 3, 2007 #4

    berkeman

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    Instead of swapping resistors and capacitors, it would be better to swap divide ratios on a PLL's input and feedback taps. I'll post more tomorrow morning. It would help if you could post more in terms of the specs you want to acheive for output freqency steps, granularity, stability over time and temperature, etc.
     
  6. Oct 4, 2007 #5

    NoTime

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    If you don't mind spending $$$ then something like this should do it.

    http://www.bkprecision.com/www/np_pdf.asp?m=4001

    Or if you want to roll your own then a good DAC and Up with a little programming.
     
  7. Oct 5, 2007 #6

    berkeman

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    Here is more info on the frequency synthesizer technique that I was alluding to:

    http://en.wikipedia.org/wiki/Frequency_synthesizer
     
  8. Oct 5, 2007 #7

    Gokul43201

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    TTLs (and other digital creatures) are completely off-limits. The switching noise radiated by a TTL circuit can kill our measurement capability. We measure currents to an accuracy of the order of femtoAmps.
     
  9. Oct 5, 2007 #8

    Gokul43201

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    Berke, can you point me to a purely analog, low-frequency PLL synth that has specs comparable to those in the OP and a long term amplitude drift that's less that 5ppm? I really don't know anything about these beasts so there's a steepish learning curve I'm climbing.

    The wiki article mentions frequencies in the several kHz and up range. Can you reliably dial this down all the way to ~10Hz with losing quality and stability?
     
    Last edited: Oct 5, 2007
  10. Oct 5, 2007 #9

    berkeman

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    How high do you need the frequency to go (sorry if I missed it in your posts)? For low-frequency sine wave oscillators of variable frequency, I agree with NoTime that you should play digital data through a DAC. For 90dB harmonic distortion, you would need about a 15-16 bit DAC, plus some lowpass filtering of the output. I think you can meet your amplitude stability specs, as long as your power supply has that stability.

    You could base the design on a microcontroller, or you could just base it on a 16 bit wide PROM and use a CPLD to do the addressing and frequency adjustments. You can still use a frequency synthesizer to generate the sine wave data addressing clock, in order to get your overall frequency adjustments.
     
  11. Oct 5, 2007 #10

    NoTime

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    Your specs are a bit of a challenge for any purely analog solution.
    Good shielding techniques and some low pass filtering should take care of radiated noise.
    Plus the generator does not need to be local.
    For example you could use fiber optic cable driving a local instrument op-amp for your setup.

    Chances are that your measurement device is at least partialy digital :smile:
     
    Last edited: Oct 5, 2007
  12. Oct 7, 2007 #11

    Gokul43201

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    Berke, I'd like to be able to have frequencies from ~10Hz to about 1kHz.

    NoTime, our measurement system is entirely analog.

    The oscillator I've built cost me about 50 bucks in components (plus about $30 for laying out the board). I'm thinking about writing up a paper for either the Review of Scientific Instruments or for the Journal of Measurement Science and Technology (or a similar journal).
     
    Last edited: Oct 7, 2007
  13. Oct 8, 2007 #12
    what does ppm mean??
     
  14. Oct 8, 2007 #13

    NoTime

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    Parts per million. It's a measure of stability.
     
  15. Oct 8, 2007 #14

    NoTime

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    Interesting. It's been a while since I've seen anything with a moving pointer on it.

    Your oscillator goes from 10Hz to 1kHz variable?
    I might think that you would need a variable inductor to do that.
    Plus coils in general might induce unwanted currents elsewhere.
     
  16. Oct 8, 2007 #15

    Gokul43201

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    Not continuously. I have to switch between different sets of resistors and capacitors. My board goes inside a box with rotary selector knobs in the front panel.

    I doubt you will find a variable inductor (or a variable capacitor) with a 30ppm/C stability in inductance (capacitance).
     
  17. Oct 8, 2007 #16

    NoTime

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    No argument here.
    So what did you use?
    Some Wein bridge variant?
     
  18. Oct 8, 2007 #17
    I figured that from chemistry but how does parts per million relate to oscillators? I've seen it on crystal specs, but I'm not sure what "parts" per million is representing.
     
  19. Oct 8, 2007 #18

    NoTime

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    How much the frequency changes.
    While Gokul specified degrees C, in electronics this also applies with time from component aging effects.
     
  20. Oct 8, 2007 #19

    Gokul43201

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    Yup.
     
  21. Oct 8, 2007 #20

    Gokul43201

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    If the oscillator is making a signal with an amplitude of 1V at room temperature and the amplitude increases by 10 microvolts upon heating by 10C, then the mean thermal drift in the amplitude is 1ppm/C.
     
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