Hartley Oscillator

Discussion in 'Homework Help' started by FBorges22, Dec 2, 2008.

  1. FBorges22

    Thread Starter Active Member

    Sep 11, 2008
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    Hello there,

    I have a small question about the hartley oscillator that I am having a little trouble.

    Can I use this oscillator to generate a pure sine wave of 60Hz?

    Thanks,
    FBorges22
     
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Alas, no. You can APPROACH a sine wave with very careful adjustment of the bias and feedback levels, but never quite get there. The very nature of an oscillator requires some non -linear (limiting) action, which precludes a sine wave. If you want a really clean sine wave, you need to use something like a Wien Bridge oscillator. (Even these require careful adjustment). These are more suitable to low frequency anyway..and are the norm for low distortion audio test generators.

    Eric
     
  3. leftyretro

    Active Member

    Nov 25, 2008
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    Of course, virtually any standard oscillator design can be utilized at any frequency desired, it is determined by the various L and C values used in the oscillator. However some oscillator types work better then others at high and low frequencies.

    If you just want a simple reliable, practical and easy to build solution then you might consider the popular 555 timer chip followed by a good 60hz low pass filter.

    Lefty
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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  5. FBorges22

    Thread Starter Active Member

    Sep 11, 2008
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    The 555 timer generates a square wave signal. How exactly I can generate a sine wave signal with him using the filters? I am intersested to learn about this.
     
  6. leftyretro

    Active Member

    Nov 25, 2008
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    A square wave is equivalent to a sine wave of the fundamental frequency plus all the odd harmonics of the fundamental frequency. If you feed a 60hz square wave into a low pass filter with a cut-off frequency of say 80hz, only the fundamental sine wave will pass through it, the harmonics going to ground through the filter elements. Search on low-pass filters and you should find several filter calculators available to help you determine the value of the filter components.
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    There are many, many ways of doing this. Anything involving a RC oscillator will work (probably good enough), but stability is always a problem. Depending on what you need, you may need to go with a crystal time base, divide it down digitally to 60 hz, then filter it. This also has the advantage it is easier to adjust a fast time base to a precision number than a low one.
     
  8. steveb

    Senior Member

    Jul 3, 2008
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    A 32768 Hz quartz watch crystal can be divided down by 546 to generate a nominal 60.015 Hz which is different from 60 Hz by 0.025 percent or 250 ppm. The crystals typically have 20 ppm frequency tolerance and 0.04 ppm/deg C temperature stability.
     
  9. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Most people don't realize just how accurate the 60 Hz line frequency is of the U.S. power grid. Though it's got short term glitches, over the long term it has atomic accuracy. It's the best time reference most people have access to, outside of an atomic clock!

    Eric
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    Yep, and the CD4060 was almost designed for this function.
     
  11. steveb

    Senior Member

    Jul 3, 2008
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    Yes, good point. If the application allows tying into the local AC power, then a phase locked loop can be used.
     
  12. davebee

    Well-Known Member

    Oct 22, 2008
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    There's a circuit called a walking ring counter that you can drive with a square wave at some multiple of your desired sine, then add a few of the counter outputs through particular valued resistors to generate a stair-step output.

    The raw output looks kind of ugly, but mathematically it's very low in harmonics, so with just a simple resistor-capacitor filter you can generate a quite clean sine from a square wave without complicated filter calculations.

    Don Lancasters CMOS Cookbook has examples of this but I can't find any online examples.

    But like everyone has already said, since both the sine purity and the frequency will ways have an error, you have to decide at some point how good a sine and how close to 60 Hz you can be satisfied with.
     
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