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voltage to frequency converter circuit (VCO)

Discussion in 'Electronic Design' started by [email protected], Oct 19, 2006.

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  1. Guest

    i'm working on a design project right now to create a voltage
    controlled oscillator using only opamps and discrete components. I'm
    able to create a square and triangle waveform easily and i've found
    many methods to create a sine waveform such as piecewise breakpoint and
    jfet amplifer from the triangle wave.

    However, I'm having trouble finding information about a voltage to
    frequency interface so that i can vary the frequency of the waveforms
    using a DC voltage. There needs to be two ranges of voltages from
    0.1-0.5V which control the frequency with 200hz/V and 1khz/V (user
    selectable from one of the two).

    Any help is appreciated. thanks.
     
  2. Tom Bruhns

    Tom Bruhns Guest

    I'm a bit confused whether you need the VCO to have native sinewave
    output or not. Assuming that a fixed-amplitude sawtooth (triangle)
    output is ok, the common way to do it is to use an op-amp integrator
    which integrates between two setpoint voltages, say +1V and -1V. A
    resistor feeds current into the integrator, just a capacitor from op
    amp out to the (-) input. The voltage which feeds that resistor
    determines the ramp rate of the integrator, and thus the frequency.
    The control voltage input can be fed to an inverter to generate a
    negative version of itself, and then the integrator input resistor is
    switched between the control voltage and its negative to ramp down or
    up, respectively. Which way it goes is controlled by some bi-stable
    circuit that switches state when the integrator output limit in either
    direction is reached. You can easily change either the capacitance or
    the resistance in the integrator to change between 200Hz/V and 1kHz/V.

    If you want native sinewave output, you _could_ build a pair of higher
    frequency oscillators, controlled by varactor diodes, and mix
    (multiply) the two outputs to generate the sum and difference
    frequencies, using a simple low-pass filter to get rid of the sum
    frequency. Making such an oscillator with a very linear f vs V
    response is not easy.

    I'm curious; is this a homework assignment?

    Cheers,
    Tom
     
  3. Jamie

    Jamie Guest

    http://www.musicfromouterspace.com/analogsynth/VCO200503REV01/VCO200503REV01.html

    that mite give you something to work with.
     
  4. john jardine

    john jardine Guest

    I've been f****** programming all week and felt an extreme urge coming on
    to design something for real. (better now:)
    With smaller Cap, circuit is good to about 40kHz.

    (max 3.5V) +5V
    0 to 500mV i/p .-----o-. 10n|| Triangle
    o-o--------------------o1 14 | .-||---.6V pkpk Square
    | ___ 10k | | | || | ___
    | .-|___|-. | 2o--. | | .-|___|-.
    | | | | | | ___ |6|\| | | 15k |
    | | +5V | | IC1 | o-|___|-o-|-\ | ___ | |\ |
    | ___ |2|\| | | | | Rset | >-o|___|-o-|+\ |
    '-|___|-o-|-\ 1 | | 9o--' .-|+/ 7 10k 10| >--o-o
    10k | >--o----o8 | |5|/| .--|-/ 8 |
    .---|+/ | | | | 9|/ |
    | 3|/| | | -o- -o- |
    -o- -5V .-o3 5o-. 0V 0V |
    0V | | | | |
    .---o-o6 13o-o------------------------------'
    | | |
    .-. | | Rset=7350 ohms for 1KHz/V
    10k| | | 4 7 | Rset=36750 ohms for 200Hz/V
    | | '-o---o-' IC1 74HC4066
    '-' -5V -5V Opamp TL084 (+5V p4,-5V p11)
    |
    +5 -o- "Voltage controlled Oscillator".

    (created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)
     
  5. Yes, nice, but if you want only a square wave, and use chips anyways,
    why not use the 74HCT4046 PLL.
    It has has a VCO.

    But I think I can do this simpler with discrete components:

    ----------------------------------------- +12V
    | | |
    [ ] [ ] R1 [ ]
    | | |
    | |</ e |
    |---| PNP T2 | T3
    | |\ c |--
    T1 | --------\ | unijucntion transistor
    | | |--
    |---- d | |------------ pulse out
    ----| JFET | |
    |---- s | [ ]
    | === |
    Uin [ ] | C1 |
    | | |
    ------------------------------------------- 0V



    Capacitor C1 will be charged by constant current source T2.
    The amount of current determines how fast T3 will trigger.

    Current source T2 is controlled by T1
    You probably want top add a si diode for temp compensation of T2.

    [] are resistors (European symbol).
    Does anybody still remember UJTs? Great stuff.
    If you want a square wave add a 2 transistor flip flop :)
    OK, have to dash now.
     
  6. 2N2646, still got a copy of the GE transistor manual somewhere with a
    whole UJT chapter, (and one on tunnel diodes)


    martin
     
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