chrystal load capacitance (inbuilt?)

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.

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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.

Reply to
Quack
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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

Reply to
Leon

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.

Reply to
Quack

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.

Reply to
Wes Stewart

-snip-

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.

--
Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

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

Reply to
Rich Grise

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

Reply to
theJackal

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.

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Reg.
Reply to
Reg Edwards

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

Reply to
Wes Stewart

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.

--
Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

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.

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give a slightly different approximation for the difference from my derivation ...umm Now I wonder who gives a better approximation? . 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

Reply to
theJackal
[snip]

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:

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Reply to
Wes Stewart

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.

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theJackal

Reply to
theJackal

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