Crystals' Shunt & Load Capacitance

[[email protected]]

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...

 

[PeteS]

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

 

[[email protected]]

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.

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]

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.

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