Medium Wave VCO

Discussion in 'General Electronics Chat' started by packrat, Feb 25, 2009.

  1. packrat

    Thread Starter Member

    Feb 25, 2009
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    Hello folks, got a problem for you, and I am willing to award five (5) internets to someone who can help me out here.

    I'm an EE intern working on a strain gauge product for my company, and one of the parts of my solution requires the use of a VCO to wirelessly transmit the strain gauge voltage.

    I have a few constraints on the system.
    -The power supply is only 0-5V
    -The output has to be a sine wave
    -I want the output frequency to range from 500kHz to 1.5 or 2MHz
    -It has to be compact, so the fewer components the better, and they must all be surface mount (though what isn't these days?)

    I was looking at using a Wein Bridge oscillator like at this website, however due to the above constraints, I can't really put in a lightbulb for stabilization.
    Also, because of the power source limitations, I am not sure if the Wein Bridge will output a sine wave centered at +2.5V, or will even work at all.

    The final issue I have is dynamically controlling the frequency itself. I was thinking about replacing the caps in the RC networks with two identical varactors controlled by the strain gauge output, but I cannot find good information on how to design them, and I can't find any prepackaged varactors that have a high enough capacitance.

    The strain gauge output will be amplified so that it typically ranges from 1-4V, with 2.5V representing no load.

    Alternatively, if anyone is aware of Medium-Wave VCO ICs out there, then that saves us all a lot of trouble.

    Sorry for the essay, but I like to be as thorough as possible.

    (as an aside, I think this may be better suited in the Projects Forum :I )
     
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,040
    287

    Are you familiar with "grain of wheat" lamps? They are just as small as any discrete component you're going to find!

    Eric
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Does the frequency need to be a linear function of the input voltage?
    Do you plan to transmit that frequency range directly, or will it be used to modulate a higher frequency carrier?
    Are you going to transmit/receive with antennas, or are you planning to use a light beam?
     
  4. packrat

    Thread Starter Member

    Feb 25, 2009
    12
    0
    The frequency doesn't exactly have to be linear, but it has to be very predictable. The frequency output must be transmitted over a rotary transformer, which will then be sampled over a ~15 microsecond period every ~37 microseconds, and the recorded frequency of that period will be translated into a strain value.
    And no, there will be no further modulation because the frequency response of the transformer drops off rapidly at frequencies higher than 2MHz.

    Also thanks for that Eric. I'll take a look at those.
     
    Last edited: Feb 25, 2009
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    657
    I didn't consider a transformer. :rolleyes:
     
  6. packrat

    Thread Starter Member

    Feb 25, 2009
    12
    0
    I'm... sorry?
     
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