# Crystals' Shunt & Load Capacitance

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

1. ### Guest

, Aug 11, 2006

2. ### PeteSGuest

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.

to operate the device nominally.

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:

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
caps will be different (a ratio of 1.5:1 - 2:1 is typical).

Cheers

PeteS

PeteS, Aug 11, 2006

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.
>
> 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:
>
> 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
> caps will be different (a ratio of 1.5:1 - 2:1 is typical).
>
> Cheers
>
> PeteS

, Aug 11, 2006
4. ### PeteSGuest

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.
> >
> > 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:
> >
> > 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
> > 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