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LC oscillator

Discussion in 'Electronic Design' started by Johnson F., Oct 8, 2004.

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  1. Johnson F.

    Johnson F. Guest


    consider this CMOS gate "LC" oscillator circuit :

    I want to use L1=1uH and C1=1500pF. The circuit must oscillate close to
    4Mhz. It will be used as part of a simple inductance meter, the frequency
    being measured to find an unknown the inductance value (the inductance to be
    measured, that can range from 1uH to 100mH, is placed in series with L1).

    *But* should C2 also be increased to 1500p, that is, be the same value as

    Thanks for any hints!
  2. Your changes lower the impedance of the resonator enough that the CMOS
    inverter may not be a good match. Take a look at a somewhat similar
    oscillator based on a comparator (lower output impedance and lots more
    gain). It was designed as part of an inductance/capacitance meter.
  3. Andrew Holme

    Andrew Holme Guest

    L1, C1 and C2 form a capacitively tapped tuned circuit:

    +------+ Equal capacitors in series
    | | C = C1/2 = C2/2
    --- |
    C1 --- ) Tuned circuit f = 1/(2.pi.sqrt(LC))
    | L )
    --- ) f = 0.23/sqrt(L.C1)
    C2 --- |
    | | This is the equation in the PDF
  4. Johnson F.

    Johnson F. Guest

    Mmmh! So I would have to use something like 3000pF for both C1 and C2 to get
    the desired 1500pF!!!
    As John says in the other reply, the CMOS gate may have a hard time to
    I forgot to mention I want to use the 74HC4060 with its built-in oscillator

    (sorry for my english...)
  5. Tim Shoppa

    Tim Shoppa Guest

    Not disputing your points, but C1 = C2 is not a necessity. C2
    in combination with the 10K resistor and the output impedance of the
    CMOS gate form a RC low-pass filter. Making C2 bigger moves the
    cutoff frequency down, and this can take away the loop gain and
    take the circuit out of oscillation.

    In the PDF schematic, RC is 4.7uS (f = 212000 Hz) which is fine for a 125
    kHz oscillator. Making C be 1500 pF without changing the 10K will
    make RC be 15uS (f = 66kHz) which is way too low for an oscillator at
    4MHz. The "right" thing to do is use a much smaller C. Making the 10K
    resistor smaller will help to an extent but quickly you'll be at the
    limit of the output impedance of the CMOS gate.

    I suspect that with 10K and 1500pF, if the thing oscillates, it will
    be a RC relaxation oscillator at around 66kHz.

  6. Tam/WB2TT

    Tam/WB2TT Guest

    L - C 1 - C2 forms a pi network. The C1/C2 ratio determines the "gain" from
    output to input. I would keep C1 and C2 the same value, or you run the risk
    of either not enough gain, or burning out the CMOS input. As has been
    pointed out, 1 uH is way smaller than what is typically used at 4 MHz. It
    will probably oscillate if you make L= 10 uH, C1 = C2 = 300 PF, and change
    the 10K to 1 K.

    I really don't think this is the circuit you want to use for an inductance
    meter, and vote for plan-B. Search the net for inductance meter or
    inductance measurement. I would look for some kind of RL circuit. For one
    thing, frequency would vary linearly with L. Whatever you use, you can't
    measure 1 uH to 100 mH all in one range. Also, the 4XXX stuff is way too
    slow to run at 4 MHz, your error might be 50 % of the calculated frequency.
    How about 74AC, or better.

  7. Johnson F.

    Johnson F. Guest

    Thanks for your advices!
    Obviously, I have to reconsider the oscillator.
  8. Ban

    Ban Guest

    Why don't you copy or buy this inexpensive instrument? Works very well. I
    have one standing here and I'm happy with it.
  9. Johnson F.

    Johnson F. Guest

    Just for the fun of it!
    I was more or less copying from this one.

    Measures from ~100nH to ~1H using 74HC gate and a BF245A JFET for the
    oscillator followed by a 74HC4040 to divide the frequency by 32 then fed
    into a computer port. Maximum frequency is ~4Mhz at the oscillator and
    ~125Khz at the computer port.
    I was trying to simplify the oscillator using only a 74HC4060. Simplifying
    and modifying circuit is part of the fun I got from a project like this and,
    so far, it was a successfull method with most other projects I've done. I
    have realized that some circuits from magazines and other sources *can* be
    simplified without drawbacks (or very little) and I also learn a great deal
    during the process.

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