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L-C oscillator topology

Discussion in 'Electronic Design' started by Tom, Oct 3, 2005.

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  1. Tom

    Tom Guest


    I have a varying inductance L say 20..100nH, which I need to measure in
    a very small footprint (<2mm wide x a few mm long) and with a low
    supply voltage (3V). Because the wiring to the device will have much
    larger stray inductance & capacitance, the electronics should be local
    (hence the need for the small footprint).

    I thought about building an oscillator with known capacitor(s) and the
    unknown inductance, such as a Colpitts oscillator, and with a tiny
    bipolar transistor such as the Panasonics in SS mini (1.6 mm) or SSS
    Mini (1.2mm) packages that DigiKey carries. If I then put the resulting
    signal through a schmitt trigger I get a nice square wave of which I
    can measure the frequency digitally e.g. using a CPLD or
    special-purpose IC.

    However, I would like to keep the frequency relatively low (<20MHz if
    possible), and with such a small inductor, that implies relatively
    large capacitance, which the Colpitts topology does not seem to like. I
    cannot tap my inductor so Hartley seems less suitable.

    Any suggestions for an appropriate oscillator topology? I also looked
    at some JFET-based oscillator circuits (including the one in AoE) but
    JFETs in ultra small packages seem to be difficult to find, so I prefer
    to stick with bipolar or MOSFET.

  2. Tom wrote...
    Whew! 20nH of inductance resonates with a huge 3166pF of capacitance
    at 20MHz, and the resonant impedance is very low, Q times 2.5 ohms.
    I'd advise using a higher frequency and dividing down if necessary.
  3. Ken Smith

    Ken Smith Guest

    (at 20MHz)

    No matter haw you attack this, the low Z of the 20nH at 20MHz will be
    trouble. I suggest you go up to at least 100MHz. The semi-isolated
    Colpits oscillator would be a good way to go. You could use a, lets say,
    50 Ohm, collector resistor to match to a coax directly.
  4. Phil Allison

    Phil Allison Guest


    ** To keep the frequency lowish and the Q high with practical capacitors,
    why not used a small fixed inductance in series with your tiny variable one.
    Seeing as you intend to use a frequency meter, the range of frequency change
    need not be large.

    If you use say, 500 nH in series, then the total L value to be resonated
    varies from 520nH to 620 nH - which is far more manageable than 20 nH to
    100 nH.

    With a parallel C of say 1000pF, the frequency will then vary from 7.0 MHz
    down to 6.4 MHz.

    Should be easy enough to get Mr Colpitts to co-operate with you then.

    ........... Phil
  5. Clapp.
  6. Jim Thompson

    Jim Thompson Guest

    Is a venereal disease ;-)

    ...Jim Thompson
  7. Joerg

    Joerg Guest

    Hello Tom,
    As Win said 20MHz won't really cut it here. Go to a few hundred MHz and
    place the oscillator on board. The BFS17A comes to mind which is a SOT23
    pocket rocket for UHF. If you don't go too high in frequency you may be
    able to divide it down at reasonable cost with newer logic families.

    In case it absolutely has to be low frequency you could consider a
    crystal oscillator that is "pulled" by that variable inductance. You'd
    have to think about the circuitry, I have only done that to measure
    minute capacitance variations but not inductors (yet). The nice thing
    about this narrow bandwidth scheme is that it can be done in an ISM band
    to avoid potential EMC scuffles with the FCC.

    Regards, Joerg
  8. Go buy an LCR meter and reverse-engineer it.

    Good Luck!
  9. OK, then, how about Pierce? ;-P


    (for those who are just tuning in, the joke is, "Isn't that how you
    get the Clapp in the first place?")
  10. Robert Baer

    Robert Baer Guest

    You have the *what*????
  11. Robert Baer

    Robert Baer Guest

    ...already hav too many things pierced!
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