Crystal/Resonator Noob question

Anthony Fremont wrote:
> Joerg wrote:
>> Anthony Fremont wrote:
>>> amerdsp wrote:
>>>> Greetings,
>>>>  I am having a problem understanding how to setup an SMD ceramic
>>>> crystal to work with an LPC2106 micro.  Maybe its the crystal part
>>>> that I do not get.  What is the load capacitance? Is it the
>>> It is 18pF according to the datasheet you pointed us to.  ;-)  Oddly
>>> enough though they don't bother stating whether the crystal is cut
>>> for parallel or series resonance.
>>>
>>>> capacitance that the crystal exerts on the circuit?  How do I pick
>>>> the
>>> It is the capacitance that the crystal expects the circuit to
>>> provide so that it runs on its stated frequency.
>>>
>>>> capacitor values required, and how do I know that they are needed?
>>>> The part I am looking at is
>>>> http://www.abracon.com/Resonators/abm8.pdf which is a 14.7456MHZ
>>>> miniature SMD crystal.  I tried to look at the manufacturer's
>>>> website to get some hints but was unsuccessful. Any hints or pointers is 
>>>> greatly appreciated.
>>> Look at Section 3 of the User Manual.  You will hook it up as shown
>>> in figure 7b.  I suggest two 33pF caps to start.
>>>
>> It'll usually be less than that. The OP needs to take into account
>> other capacitances: Traces to ground and to each other, pins to
>> ground, etc. For the LPC2106 the chip capacitances might be found in
>> a family spec, not necessarily in the datasheet.
> 
> I was thinking that Cstray wouldn't be all that high given the smd parts, 
> and that a pair of 22pF might not always start reliably. 
> 

I don't know what it is for the LPC because I pretty much don't use NXP 
parts anymore these days. But it has to be checked. Sometimes it is 
surprisingly high, on some uC families the burden caps can even be 
programmed.

-- 
Regards, Joerg

http://www.analogconsultants.com/

Anthony Fremont wrote:
> amerdsp wrote:
>> On Nov 15, 8:51 pm, "Anthony Fremont" <nob...@noplace.net> wrote:
> 
>>> Look at Section 3 of the User Manual.  You will hook it up as shown
>>> in figure 7b.  I suggest two 33pF caps to start.
>> I am sorry, but which user manual are you referring to?  All I could
>> find is the datasheet and a few application notes.
> 
> http://www.nxp.com/acrobat_download/usermanuals/UM_LPC2106_2105_2104_1.pdf 
> 

Page 39.

-- 
Regards, Joerg

http://www.analogconsultants.com/

Joerg wrote:
> Anthony Fremont wrote:
>> amerdsp wrote:
>>> On Nov 15, 8:51 pm, "Anthony Fremont" <nob...@noplace.net> wrote:
>>
>>>> Look at Section 3 of the User Manual.  You will hook it up as shown
>>>> in figure 7b.  I suggest two 33pF caps to start.
>>> I am sorry, but which user manual are you referring to?  All I could
>>> find is the datasheet and a few application notes.
>>
>> http://www.nxp.com/acrobat_download/usermanuals/UM_LPC2106_2105_2104_1.pdf
>>
>
> Page 39.

I guess I should have looked at figure 8.  Unless I'm missing something, 
they're speccing even more C than I suggested (2 38pF caps).  Hmmm....by my 
calculations then Cstray is ~1pF.  That would mean that the OP should use a 
pair of 34pF caps to get the extra 17pF he needs, I stand corrected.  Good 
luck finding them.  ;-)

Anthony Fremont wrote:
> Joerg wrote:
>> Anthony Fremont wrote:
>>> amerdsp wrote:
>>>> On Nov 15, 8:51 pm, "Anthony Fremont" <nob...@noplace.net> wrote:
>>>>> Look at Section 3 of the User Manual.  You will hook it up as shown
>>>>> in figure 7b.  I suggest two 33pF caps to start.
>>>> I am sorry, but which user manual are you referring to?  All I could
>>>> find is the datasheet and a few application notes.
>>> http://www.nxp.com/acrobat_download/usermanuals/UM_LPC2106_2105_2104_1.pdf
>>>
>> Page 39.
> 
> I guess I should have looked at figure 8.  Unless I'm missing something, 
> they're speccing even more C than I suggested (2 38pF caps).  Hmmm....by my 
> calculations then Cstray is ~1pF.  That would mean that the OP should use a 
> pair of 34pF caps to get the extra 17pF he needs, I stand corrected.  Good 
> luck finding them.  ;-)
> 

Interesting. But somehow 38pF doesn't feel right for a 15MHz crystal.

-- 
Regards, Joerg

http://www.analogconsultants.com/

If I change the crystal to have a load capacitance of 20pF as the
manual says, I need a 38pF C1 and C2.  It seems that these are
difficult to find.  Can I use a 39pF instead? What would be the
consequences if I do?

Thank you,

A

amerdsp <amerdsp@hotmail.com> writes:
> If I change the crystal to have a load capacitance of 20pF as the
> manual says, I need a 38pF C1 and C2.  It seems that these are
> difficult to find.  Can I use a 39pF instead? What would be the
> consequences if I do?

Slight changes in capacitance will cause slight changes in frequency.
However, note that your circuit layout also causes slight changes in
capacitance.  If you need higher precision than these variances allow,
design in a small variable cap to tune the resonant frequency.

Also, the two caps don't *have* to be the same value.  You can do some
testing with different values to try to get closer to the desired
frequency.

On 18 Nov 2007 12:36:10 -0500, I said, "Pick a card, any card" and
DJ Delorie <dj@delorie.com> instead replied:

>amerdsp <amerdsp@hotmail.com> writes:
>> If I change the crystal to have a load capacitance of 20pF as the
>> manual says, I need a 38pF C1 and C2.  It seems that these are
>> difficult to find.  Can I use a 39pF instead? What would be the
>> consequences if I do?
>
>Slight changes in capacitance will cause slight changes in frequency.
>However, note that your circuit layout also causes slight changes in
>capacitance.  If you need higher precision than these variances allow,
>design in a small variable cap to tune the resonant frequency.
>
>Also, the two caps don't *have* to be the same value.  You can do some
>testing with different values to try to get closer to the desired
>frequency.

In fact, if the caps are exactly the same, oscillation may not
occur. There needs to be a miniscule amount of difference in order
to get the crystal going. It's the physics of the thing. Fortunately
caps of the same value are never, ever precisely the same.
--
Ray

Ray Haddad wrote:

> On 18 Nov 2007 12:36:10 -0500, I said, "Pick a card, any card" and
> DJ Delorie <dj@delorie.com> instead replied:
> 
> 
>>amerdsp <amerdsp@hotmail.com> writes:
>>
>>>If I change the crystal to have a load capacitance of 20pF as the
>>>manual says, I need a 38pF C1 and C2.  It seems that these are
>>>difficult to find.  Can I use a 39pF instead? What would be the
>>>consequences if I do?
>>
>>Slight changes in capacitance will cause slight changes in frequency.
>>However, note that your circuit layout also causes slight changes in
>>capacitance.  If you need higher precision than these variances allow,
>>design in a small variable cap to tune the resonant frequency.
>>
>>Also, the two caps don't *have* to be the same value.  You can do some
>>testing with different values to try to get closer to the desired
>>frequency.
> 
> 
> In fact, if the caps are exactly the same, oscillation may not
> occur. There needs to be a miniscule amount of difference in order
> to get the crystal going. It's the physics of the thing. Fortunately
> caps of the same value are never, ever precisely the same.

Really  ?! You do realise this is electronics, not physics ?

Are you able to prove that in Spice ? (which will allow you to have 
identical Cap values)

Tip: Oscillation is all about phase.
Try it yourself, use a trimmer.

-jg

Jim Granville wrote:
> Ray Haddad wrote:
> 
>> On 18 Nov 2007 12:36:10 -0500, I said, "Pick a card, any card" and
>> DJ Delorie <dj@delorie.com> instead replied:
>>
>>
>>> amerdsp <amerdsp@hotmail.com> writes:
>>>
>>>> If I change the crystal to have a load capacitance of 20pF as the
>>>> manual says, I need a 38pF C1 and C2.  It seems that these are
>>>> difficult to find.  Can I use a 39pF instead? What would be the
>>>> consequences if I do?
>>>
>>> Slight changes in capacitance will cause slight changes in frequency.
>>> However, note that your circuit layout also causes slight changes in
>>> capacitance.  If you need higher precision than these variances allow,
>>> design in a small variable cap to tune the resonant frequency.
>>>
>>> Also, the two caps don't *have* to be the same value.  You can do some
>>> testing with different values to try to get closer to the desired
>>> frequency.
>>
>>
>> In fact, if the caps are exactly the same, oscillation may not
>> occur. There needs to be a miniscule amount of difference in order
>> to get the crystal going. It's the physics of the thing. Fortunately
>> caps of the same value are never, ever precisely the same.
> 
> Really  ?! You do realise this is electronics, not physics ?
> 
> Are you able to prove that in Spice ? (which will allow you to have 
> identical Cap values)
> 
> Tip: Oscillation is all about phase.
> Try it yourself, use a trimmer.
> 

And make sure the fung shui is ok. Might have to place some mirrors if 
not ...

-- 
SCNR, Joerg

http://www.analogconsultants.com/

On Mon, 19 Nov 2007 09:42:26 +1300, I said, "Pick a card, any card"
and Jim Granville <no.spam@designtools.maps.co.nz> instead replied:

>Ray Haddad wrote:
>
>> On 18 Nov 2007 12:36:10 -0500, I said, "Pick a card, any card" and
>> DJ Delorie <dj@delorie.com> instead replied:
>> 
>>>amerdsp <amerdsp@hotmail.com> writes:
>>>
>>>>If I change the crystal to have a load capacitance of 20pF as the
>>>>manual says, I need a 38pF C1 and C2.  It seems that these are
>>>>difficult to find.  Can I use a 39pF instead? What would be the
>>>>consequences if I do?
>>>
>>>Slight changes in capacitance will cause slight changes in frequency.
>>>However, note that your circuit layout also causes slight changes in
>>>capacitance.  If you need higher precision than these variances allow,
>>>design in a small variable cap to tune the resonant frequency.
>>>
>>>Also, the two caps don't *have* to be the same value.  You can do some
>>>testing with different values to try to get closer to the desired
>>>frequency.
>> 
>> In fact, if the caps are exactly the same, oscillation may not
>> occur. There needs to be a miniscule amount of difference in order
>> to get the crystal going. It's the physics of the thing. Fortunately
>> caps of the same value are never, ever precisely the same.
>
>Really  ?! You do realise this is electronics, not physics ?
>
>Are you able to prove that in Spice ? (which will allow you to have 
>identical Cap values)
>
>Tip: Oscillation is all about phase.
>Try it yourself, use a trimmer.

Theoretically what I stated is true. If both caps are exactly (and I
do mean exactly) the same value, there will be no oscillation. The
first oscillation requires an imbalance. Spice takes that into
account and simply allows oscillation to occur so forget proving it
in a Spice model. You do have to go back to basic physics to get to
the truth. But, it's a minor point. I mean that. It's not worth
arguing about because there can never, ever be two capacitors with
the exact same value. Period.
--
Ray