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Large capacitance varicaps, where are thee?

Discussion in 'Electronic Design' started by Joerg, Oct 22, 2012.

  1. Joerg

    Joerg Guest

    Folks,

    Looked at Digikey and some others. Where are those huge capacitance
    varicaps? The ones with several hundred pF of range for AM radios. All
    gone lalaland by now?

    Even the ones I found in the 100pF range are either obsolete or not
    recommended for new designs.

    What I am trying to do: I need to control a switcher chip in frequency
    because I've got a very resonant load to deal with. Unfortunately it
    sets the frequency with a timing cap. I'll have to somehow vary that
    between 750pF and 2000pF. Can also be digital but then with a
    granularity of 5pF. The sawtooth voltage across it is 2.5Vpp.

    Doing it with caps and a mux chip or two has its own challenges. The
    ADG-series from AD is around 11pF per pin, otherwise their Rdson is too
    high. Talking about the a rock and a hard spot here.

    Oh, and cost is not very important. If a diode or two or three are
    needed that cost $5 a pop that's ok.
     
  2. Hmm.. I have no idea Joerg.
    So silly ideas, can you parallel a bunch of small ones?
    What's the frequency? Could you use a cruddie Y5/Z5 ceramic?

    (You've most likely already been down both these roads.)

    George H.
     
  3. miso

    miso Guest

    There are plenty of hoarders that have those caps. I picked them up
    years ago when I figured they would go extinct. They same goes for 10
    turn pots of the panel variety with the dials on them. (Good old Mike
    Quinn in San Leadro by the Oakland airport would save panels with those
    pots and junk the rest of the product.] Most radios are synthesized
    these days, so there is no market for those caps.

    If you are not conflating sawtooth with triangle wave, then maybe you
    could shunt some current from the osc pin, which would slow down the
    frequency. Pick your external cap for the highest frequency needed, then
    bleed with some DAC based circuit or even a resistive trimpot. The
    10-turn PCB mounted resistors still exist.

    The difficulty will be in the compliance of the current source used for
    bleeding.
     
  4. Guest

    possible to force you own triangle in?

    -Lasse
     
  5. Nico Coesel

    Nico Coesel Guest

    How about using a bad quality ceramic capacitor?
     
  6. Joerg

    Joerg Guest


    Yup :-(

    They don't make Z5U this small. Paralleling gets to be tough for space
    reasons because I'd need a boatload.
     
  7. Joerg

    Joerg Guest

    Yeah, I know but I was hoping there'd be at least one other market.
    Seems like there ain't.


    That won't be a problem but the chip immediately gets sea-sick when you
    do anything DC to that pin.
     
  8. Joerg

    Joerg Guest

    Not on this one. It issues a major reset pulse of almost 50mA and if
    that sees any obstructions it all goes bonkers. It already does when you
    tough it with a 10k resistor, like the princess on the pea.
     
  9. Joerg

    Joerg Guest

    They don't make Z5U this small. Only for larger capacitors.
     
  10. Les Cargill

    Les Cargill Guest


    Could something like this be a start?

    http://www.timeelectronics.com/products/category/decade-boxes/1071-capacitance-box

    Gets pretty close - 10 pF resolution if I read that right.
     
  11. Guest

    this too crazy? http://www.falstad.com/circuit/e-capmult.html

    -Lasse
     
  12. Joerg

    Joerg Guest

  13. Joerg

    Joerg Guest

  14. Guest

    http://www.angelfire.com/electronic2/index1/VariableCap.html

    About half way down.
    No kiddin? ;-)
    Use a regulator with an external clock. That's what we do (all of our
    switchers are synchronized).
    A regulator with an external clock shouldn't be an issue, then.
     
  15. Guest

    I'm just guessing, diode across R3 ?

    -Lasse
     
  16. Joerg

    Joerg Guest

    No, there is more or less a wire.
     
  17. OK so how 'bout a few Z5U in series?

    George H.
     
  18. Guest

    I meant add diode so discharge is fast


    -Lasse
     
  19. Joerg

    Joerg Guest

    Well, I need one from LTC because it must be simulated. The load is
    really ugly yet must be well regulated with some unorthodox loop
    elements in there. Linear only has the 3721 and 3723 for push-pull. Some
    older ones as well but they have the same engine type in them.


    If LTC had a suitable one. We have a processor so sync'ing is no big
    deal except that I usually get the looks if I request processor MIPS.
    Analog dudes aren't really entitled to those.

    Anyhow, it's not a show stopper, I can use a small varicap plus mux. The
    mux is like an umpteen-speed gear shifter, not very pretty. If big
    varicaps were still available it would become very easy. But I guess
    even if I found one the risk is too great that it's going to be
    obsoleted in a few years.
     
  20. Joerg

    Joerg Guest

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