Crystals' Shunt & Load Capacitance

Discussion in 'Electronic Basics' started by quanghoc@gmail.com, Aug 11, 2006.

  1. Guest

    , Aug 11, 2006
    #1
  2. PeteS Guest

    wrote:
    > My question refers to this spec
    > http://www.raltron.com/products/pdfspecs/crystal_rse_a_b_c_d.pdf
    >
    > What is Shunt Capacitance and Load Capacitance?
    >
    > Can you explain it to an engineer idiot by using some kinda analogy? I
    > don't know much about these stuffs since I am new. Thanks...


    The shunt capacitance is the total capacitance of the device itself
    across it's terminals.

    The load capacitance is **normally** the loading capacitance required
    to operate the device nominally.

    Once you know those numbers, you can calculate the required loading
    caps (necessary to operate the device properly).

    As circuit stray capacitance and amplifier capacitance vary, the
    crystal mfrs simply specify what the crystal has (for shunt which is
    also sometimes the static capacitance, Co).

    Note that different manufacturers have different notations, so in this
    case Cs may refer to the capacitance of the terminals when mounted on a
    circuit board, in which case it becomes part of the stray capacitance
    below, rather than being the internal static capacitance, which affects
    equivalent resistance and drive levels.

    So if:

    CL = loading capacitance from datasheet
    Cs = Circuit stray capacitance
    Cx1 and Cx2 are loading caps we have to put on the circuit, which may
    be the same value, but sometimes are not (see below)

    Then for proper operation CL = Cs + [(Cx1 * Cx2) / (Cx1 + Cx2)]

    In some cases, oscillators won't start up properly unless they are
    deliberately capacitively imbalanced, in which case the crystal loading
    caps will be different (a ratio of 1.5:1 - 2:1 is typical).

    Cheers

    PeteS
    PeteS, Aug 11, 2006
    #2
  3. Guest

    Wow, that's way more complicated than I thought. But thanks so much.

    So, I know the load capacitance and its use. I still don't get the
    shunt capacitance on its usefulness. Why do we need to know this? How
    does it affect the purchasing decision?

    Thanks.


    PeteS wrote:
    > wrote:
    > > My question refers to this spec
    > > http://www.raltron.com/products/pdfspecs/crystal_rse_a_b_c_d.pdf
    > >
    > > What is Shunt Capacitance and Load Capacitance?
    > >
    > > Can you explain it to an engineer idiot by using some kinda analogy? I
    > > don't know much about these stuffs since I am new. Thanks...

    >
    > The shunt capacitance is the total capacitance of the device itself
    > across it's terminals.
    >
    > The load capacitance is **normally** the loading capacitance required
    > to operate the device nominally.
    >
    > Once you know those numbers, you can calculate the required loading
    > caps (necessary to operate the device properly).
    >
    > As circuit stray capacitance and amplifier capacitance vary, the
    > crystal mfrs simply specify what the crystal has (for shunt which is
    > also sometimes the static capacitance, Co).
    >
    > Note that different manufacturers have different notations, so in this
    > case Cs may refer to the capacitance of the terminals when mounted on a
    > circuit board, in which case it becomes part of the stray capacitance
    > below, rather than being the internal static capacitance, which affects
    > equivalent resistance and drive levels.
    >
    > So if:
    >
    > CL = loading capacitance from datasheet
    > Cs = Circuit stray capacitance
    > Cx1 and Cx2 are loading caps we have to put on the circuit, which may
    > be the same value, but sometimes are not (see below)
    >
    > Then for proper operation CL = Cs + [(Cx1 * Cx2) / (Cx1 + Cx2)]
    >
    > In some cases, oscillators won't start up properly unless they are
    > deliberately capacitively imbalanced, in which case the crystal loading
    > caps will be different (a ratio of 1.5:1 - 2:1 is typical).
    >
    > Cheers
    >
    > PeteS
    , Aug 11, 2006
    #3
  4. PeteS Guest

    wrote:
    > Wow, that's way more complicated than I thought. But thanks so much.
    >
    > So, I know the load capacitance and its use. I still don't get the
    > shunt capacitance on its usefulness. Why do we need to know this? How
    > does it affect the purchasing decision?
    >
    > Thanks.
    >
    >
    > PeteS wrote:
    > > wrote:
    > > > My question refers to this spec
    > > > http://www.raltron.com/products/pdfspecs/crystal_rse_a_b_c_d.pdf
    > > >
    > > > What is Shunt Capacitance and Load Capacitance?
    > > >
    > > > Can you explain it to an engineer idiot by using some kinda analogy? I
    > > > don't know much about these stuffs since I am new. Thanks...

    > >
    > > The shunt capacitance is the total capacitance of the device itself
    > > across it's terminals.
    > >
    > > The load capacitance is **normally** the loading capacitance required
    > > to operate the device nominally.
    > >
    > > Once you know those numbers, you can calculate the required loading
    > > caps (necessary to operate the device properly).
    > >
    > > As circuit stray capacitance and amplifier capacitance vary, the
    > > crystal mfrs simply specify what the crystal has (for shunt which is
    > > also sometimes the static capacitance, Co).
    > >
    > > Note that different manufacturers have different notations, so in this
    > > case Cs may refer to the capacitance of the terminals when mounted on a
    > > circuit board, in which case it becomes part of the stray capacitance
    > > below, rather than being the internal static capacitance, which affects
    > > equivalent resistance and drive levels.
    > >
    > > So if:
    > >
    > > CL = loading capacitance from datasheet
    > > Cs = Circuit stray capacitance
    > > Cx1 and Cx2 are loading caps we have to put on the circuit, which may
    > > be the same value, but sometimes are not (see below)
    > >
    > > Then for proper operation CL = Cs + [(Cx1 * Cx2) / (Cx1 + Cx2)]
    > >
    > > In some cases, oscillators won't start up properly unless they are
    > > deliberately capacitively imbalanced, in which case the crystal loading
    > > caps will be different (a ratio of 1.5:1 - 2:1 is typical).
    > >
    > > Cheers
    > >
    > > PeteS


    In this particular case, the shunt capacitance is *probably* the
    capacitance of the package and must be used in the Cs (above) to
    calculate the proper loading capacitors. It's also important when
    choosing an amplifier (or choosing a crystal for a particular chip that
    has an internal oscillator).

    Cheers

    PeteS
    PeteS, Aug 11, 2006
    #4

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