chrystal load capacitance (inbuilt?)

[Quack]

Hi,

This might seem like a silly question, but before i order my board and
components, this question is just nagging me.

In the following PDF, and in the digikey description, this oscillator
mentions an XXpf load capacitance.
http://www.quack.cc/CM309S.pdf
digikey part: 300-2049-1-ND

My question is, is this already 'inbuilt' into the chrystal, or are
they recommended values i need to add myself ?

Also, considering its a CM309 package, 2 of the pins remain unused, am
i understanding the document correctly ?

Currently i have drawn my layout to use pins 1 and 4 only, and external
load capacitance pads, so atleast if i ommit the external ones or not
it will work either way.

Thanks!
Alex.

 

[Leon]

You need external capacitors, Something like 22pF should be OK, but it
depends on the oscillator and crystal. With those packages, two of the
pins are usually unconnected.

Leon

 

[Quack]

Thats what i thought, but the digikey discription mentioned
specifically 18pf, and the PDF another value, and no mention of either
'recommended' or 'inbuilt' so i was not sure.

Thanks

Alex.

 

[Wes Stewart]

Hi,

This might seem like a silly question, but before i order my board and
components, this question is just nagging me.

In the following PDF, and in the digikey description, this oscillator
mentions an XXpf load capacitance.
http://www.quack.cc/CM309S.pdf
digikey part: 300-2049-1-ND

My question is, is this already 'inbuilt' into the chrystal, or are
they recommended values i need to add myself ?

Also, considering its a CM309 package, 2 of the pins remain unused, am
i understanding the document correctly ?

Currently i have drawn my layout to use pins 1 and 4 only, and external
load capacitance pads, so atleast if i ommit the external ones or not
it will work either way.

A quartz crystal can be operated at its series resonant point, where
is behaves as a low value resistor (the equivalent series resistance)
or above this frequency, where it provides an inductive reactance.

When it is used as an inductor (anti-resonant), it must be resonated
at the marked frequency by a "load" capacitance. The specified load
capacitance is simply the value that makes the crystal oscillate on
its specified frequency.

The topology of the oscillator circuit determines which mode is used
and the same crystal can work in either circuit, the only difference
being the frequency of operation.

What the data sheet is saying is that if you want the crystal to
operate on the specified frequency then you need an anti-resonant
circuit that provides 16 pF of load capacitance (if that's what you
specify) or a series resonant oscillator with a 16 pF capacitor in
series with the crystal.

As to the mounting, Pins 1 and 4 are all that need to be connected.

 

[Tim Wescott]

-snip-
What the data sheet is saying is that if you want the crystal to
operate on the specified frequency then you need an anti-resonant
circuit that provides 16 pF of load capacitance (if that's what you
specify) or a series resonant oscillator with a 16 pF capacitor in
series with the crystal.

The load capacitance is the equivalent capacitance as seen by the
crystal including both loading capacitors _and_ the capacitance of the
oscillator circuit. I suggest doing a web search on "crystal
oscillator". Better yet, check the app notes for the device you're
planning on using to see if it has any guidance for selecting capacitors.

 

[theJackal]

Hi,

This might seem like a silly question, but before i order my board and
components, this question is just nagging me.

In the following PDF, and in the digikey description, this oscillator
mentions an XXpf load capacitance.
http://www.quack.cc/CM309S.pdf
digikey part: 300-2049-1-ND

My question is, is this already 'inbuilt' into the chrystal, or are
they recommended values i need to add myself ?

Also, considering its a CM309 package, 2 of the pins remain unused, am
i understanding the document correctly ?

Currently i have drawn my layout to use pins 1 and 4 only, and external
load capacitance pads, so atleast if i ommit the external ones or not
it will work either way.

Thanks!
Alex.

You don't have to add the external capacitor if you are going to use
your oscillator in a "Series mode oscillator" or in other words as a
replacement for a RLC circuit in your oscillator design.
The resonant frequency is Fs = (2*pi *L1*C1)^-1/2
If you need it in a parallel mode oscillator (as a replacement for an
inductor) then "Yes" you have to add the load capacitor.
The resonating frequency is different ... not usually by much in the
two cases.
Doing all the hectic math leads you to the following approximation
for large values of Cl
Fs-Fp= delta F = (C1/2*Co ) * Fs

"Go easy with the whisky"

theJackal

 

[Rich Grise]

Thats what i thought, but the digikey discription mentioned
specifically 18pf, and the PDF another value, and no mention of either
'recommended' or 'inbuilt' so i was not sure.

You want the total capacitance of your circuit that the crystal sees
in parallel with it to equal 16 pf. You don't say what kind of circuit
it's in, but in general, the uPs I've seen have an Xtal Out pin and
and Xtal In pin, and ISTR they have typical parasitics of about 3 pf.
And each crystal pin has a cap to ground. You want the series-parallel
combination of those capacitances to "add up" to 16 pf, like 2x 27 or
33 pF, since from the crystal's POV the two caps (and their parallel
parasitics) are in series, so you get C/2.

Sometimes they specify different caps on the input and output side,
to adjust the drive and stuff like that.

Have Fun!
Rich

 

[Reg Edwards]

The frequency of oscillation in an oscillator circuit depends on the
load capacitance across the Xtl.

The load capacitance, in pF, is the nominal value which when connected
in parallel with the Xtl will cause the circuit to oscillate at the
required frequency.

It is the value used by the Xtl manufacturer during the grinding and
polishing process during which the Xtl is being adjusted precisely to
the required frequency.

In practice, the oscillator circuit will inevitably provide its own
uncertain capacitance across the Xtl which will usually be different
from the nominal value.

So what is required in the oscillator circuit is a variable preset
capacitor which allows the total capacitance across the Xtl to be set
precisely to the nominal value, and the oscillator frequency to be set
precisely to the required frequency.

The preset capacitor should have a minimum value such that the total
shunt capacitance is less than the nominal value required by the Xtl.
When the preset capacitor is set to its maximum value the total
capacitance will be greater than the nominal value.

As a guide, if the intrinsic circuit capacitance is 10 pF, and the
required nominal value is 20 pF, then the preset capacitor should be
adjustable between 1 and 20 pF.

The required osillator resonant frequency will then occur when the
preset capacitor is set to about half of its maximum value.

The whole design and adjustment procedure is very non-critical. Just
twiddle the preset capacitor until the oscillation frequency is
exactly at the required value.

 

[Wes Stewart]

The load capacitance is the equivalent capacitance as seen by the
crystal including both loading capacitors _and_ the capacitance of the
oscillator circuit.

In my mind, "load capacitance" -is- the total capacitance across the
crystal, regardless of its source. It includes strays and the
feedback capacitors.

 

[Tim Wescott]

In my mind, "load capacitance" -is- the total capacitance across the
crystal, regardless of its source. It includes strays and the
feedback capacitors.

Mine, too. But the OP appears to be a newbie, and I've seen newbies not
understand this point, so I wanted to clarify it.

 

[theJackal]

With a load capacitance a Quartz crystal will operate in "Parallel
resonance" in which case the crystal operates at a frequency greater
then the series resonating frequency where a Crystal operates without
a load capacitor the difference given approximately by the amount
given in my earlier post.

http://www.ecsxtal.com/pdf/quar_des.pdf give a slightly different
approximation for the difference from my derivation ...umm Now I
wonder who gives a better approximation? <grin> . It actually depends
on the value of the external capacitor. They give
the difference between the 2 frequencies as
C1/2*(Co+ Cl) . C1 is the static inbuilt capacitance of the crystal
Cl is the external load capacitance and Co the capacitance of the
electrode arrangement. The same document gives the Series resonance
frequency as a function only of C1 showing it is completely
independent of external capacitances , which are either those of the
load or of the electrodes.

One advantage of operating a Crystal at its series resonant frequency
is it acts as a short circuit at the oscillating frequency ( in
reality the imaginary part of its impedance is null wheras the real
part is at its minimum possible value in the permissible range of the
resonating frequencies of the crystal) a condition not possible with
the parallel resonant frequency mode.

They are other advantages of the SR mode, possibly even more
important, especially when designing vacuum tube oscillators but i'll
stop here.

"Go easy with the whisky"

theJackal

 

[Wes Stewart]

On Thu, 15 Dec 2005 11:31:35 +0100, theJackal

[snip]

One advantage of operating a Crystal at its series resonant frequency
is it acts as a short circuit at the oscillating frequency ( in
reality the imaginary part of its impedance is null wheras the real
part is at its minimum possible value in the permissible range of the
resonating frequencies of the crystal) a condition not possible with
the parallel resonant frequency mode.

They are other advantages of the SR mode, possibly even more
important, especially when designing vacuum tube oscillators but i'll
stop here.

There are both advantages and disadvantages to either mode of
operation. Neither is inherently superior to the other for all
purposes.

One of the better, most readable and easily obtained references is
here:

http://ww1.microchip.com/downloads/en/AppNotes/00826a.pdf

 

[theJackal]

On Thu, 15 Dec 2005 11:31:35 +0100, theJackal

[snip]

One advantage of operating a Crystal at its series resonant frequency
is it acts as a short circuit at the oscillating frequency ( in
reality the imaginary part of its impedance is null wheras the real
part is at its minimum possible value in the permissible range of the
resonating frequencies of the crystal) a condition not possible with
the parallel resonant frequency mode.

They are other advantages of the SR mode, possibly even more
important, especially when designing vacuum tube oscillators but i'll
stop here.

There are both advantages and disadvantages to either mode of
operation. Neither is inherently superior to the other for all
purposes.

One of the better, most readable and easily obtained references is
here:

http://ww1.microchip.com/downloads/en/AppNotes/00826a.pdf

Yes thats for sure ... one just can't mention everything there is
to say on a topic .

Nice though it doesn't mention a thing about tunability of series
resonance mode crystal oscillators, Parallel mode versus series mode
at high frequencies and so on. I guess the subject is too broad.

The most single complete qualitative notes I've found on Quartz
crystals is the following HP application note.

http://my.ece.ucsb.edu/yorklab/Useful Stuff/Tutorials/QuartzCrystalOscillators.pdf

theJackal