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Sinusoidal VCO

Discussion in 'Electronic Basics' started by Pooya Monajemi, May 9, 2004.

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  1. Hi...
    I need a sinusoidal VCO in the megahertz range... like 1-2 MHz. Can't find
    any. Everything I find is square wave output, and it's not easy to filter
    that square to get a sinusoid.

  2. John Larkin

    John Larkin Guest

    Consider an Analog Devices DDS chip.

  3. Because you're looking from the wrong viewpoint.

    If you were coming form an RF viewpoint, you'd take any LC oscillator
    and add a varactor for voltage control of the frequency. In other
    words, you start with an oscillator that works well at RF, and happens
    to put out a sinewave, and then you merely change the variable capacitor
    to one that can be controlled via voltage.

    Of course, you will not see the range that one expects from an RC
    based oscillator.

  4. Bob

    Bob Guest

  5. Rich Grise

    Rich Grise Guest

    I don't know why not - if you've got a VCO from 10-11MHz, and an ordinary
    osc. at, say, 12 MHz, then their beat freq. would be 1-2 MHz. And fairly
    easy to filter.

  6. But that's adding to the circuitry. It is an obvious solution to
    the issue of not being able to cover a wider range.

    Considering he posted to basics, and didn't actually specify exactly
    what he needed, I did not go into that.

    I was really trying to address the issue of someone coming from the
    audio world, where wide range RC VCOs are pretty easy and common. They
    get to RF, and are puzzled over how to get a sinewave, or why the waveform
    is not particularly good. The answer is simple, they are using a circuit
    that is fine for audio, but is being stretched at RF. There was a time
    when RC VCOs were a new thing, and they were the novelty, but now they've
    become the common vantage point. I don't see the limited range of a varactor
    tuned LC oscillator as a disadvantage compared to a wide range RC VCO, merely
    the result of different design schemes. Any disadvantage of a too narrow
    tuning range LC VCO is because you need a wider range.

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