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Crystal Capacitor Formula (Parallel Mode)

Discussion in 'Electronic Basics' started by Costas Vlachos, Jul 3, 2003.

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  1. Hi all,

    I'm looking for the formula to calculate the value of the two capacitors C1
    and C2 used in crystal oscillators for microcontrollers (you know, the ones
    you put between the Xtal terminals and GND). I've found this on the net:

    Crystal Capacitor Formula (Parallel Mode)
    -----------------------------------------
    Let: CL = Crystal's load capacitance
    CP = Circuit's parasitic capacitance
    CI = Chip's input capacitance
    C1 = Crystal oscillator capacitor 1
    C2 = Crystal oscillator capacitor 2

    Then: C1 = C2 = 2 * CL - (CP + CI)

    Example: For a crystal with 12pF load capacitance (CL)
    and assuming CP + CI = 5pF (typical), we have
    C1 = C2 = 2 * 12pF - 5pF = 19pF. We can then
    use two 18pF capacitors (closest standard
    value).

    Is this correct? Can someone please confirm?
    Many thanks.

    Costas
     
  2. Yes.

    Best regards,
    Spehro Pefhany
     
  3. Larry Hatch

    Larry Hatch Guest

    On a side note. There is a fellow on the internet (Ricardo Richi, or
    something like that). He shows how to make pf caps with wirewrap. I
    have done about 10 PIC and AVR projects, with a xtal and two little
    twisted wires to run it. And no errors in the uP running ever. One is
    on a motorcycle that does 150MPH wheelies and viberates a lot. Only
    a cell phone too close have cause a problem, and I do not believe it is
    in the wire caps, but the lack of caps and the long wire from the back
    on the bike to the front.

    Anyway, I though the wire twisting was interesting, now to have a good
    meter to tune them. Any ideas on that? Good way to measure 33pf, etc caps.
     
  4. The trick to measuring low capacitance is to build a small insulated
    test jig that plugs directly into your meter to minimize stray
    capacitance, and to keep the value stable. Pomona, Keystone, and other
    companies make banana plugs with either a threaded stud, or threaded
    hole for a 6-32 screw. A small piece of PC board with a v shaped
    section of the copper clad scored and peeled off gives you a good
    surface to check surface mount capacitors, or leaded parts. You can
    tack solder gimmicks or leaded caps to the board for critical
    measurements, or use a cheap, self wiping IC socket cut in half to let
    you plug leaded parts in for test. Just remember that the sockets wear
    quickly, and you need to make sure you can replace them easily.

    Yes, but that is a pretty cheap method for test equipment.
     
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