Author Topic: Crystal load capacitance  (Read 845 times)

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Offline TheHolyHorseTopic starter

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Crystal load capacitance
« on: August 30, 2019, 12:20:40 pm »
Hi!

I'm working on a project which has a MCU(STM32F373) and I'm looking at crystals when I suddenly came across a crystal that didn't specify load capacitance it just said series. So I started doing some research but I still can't wrap my head around it.
This article https://ecsxtal.com/news-resources/video-learning/126-everything-you-need-to-know-about-quartz-crystal-resonators/133-understanding-load-capacitance says I can just ignore load capacitance when dealing with series resonance crystals.
So what can I just drop it in there and be done with it, no caps no nothing, it's just gonna work?? Sounds to good to be true.

Can someone fill me in?!
 

Offline T3sl4co1l

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Re: Crystal load capacitance
« Reply #1 on: August 30, 2019, 12:23:32 pm »
ST's appnote on crystal oscillators will be more beneficial:
https://www.st.com/content/ccc/resource/technical/document/application_note/c6/eb/5e/11/e3/69/43/eb/CD00221665.pdf/files/CD00221665.pdf/jcr:content/translations/en.CD00221665.pdf

It's a parallel oscillator; the specs of a series crystal will be slightly different (may not be the desired C and ESR leading to poor startup or an error of some kHz).

More importantly, ST's oscillators tend to be weak (low transconductance), so need a relatively high ESR / low C crystal.  Shop for this. :)

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 
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Offline TheHolyHorseTopic starter

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Re: Crystal load capacitance
« Reply #2 on: August 30, 2019, 12:29:08 pm »
Tanks for the link and reply, I'll absolutely follow your advice.
 

Offline TheHolyHorseTopic starter

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Re: Crystal load capacitance
« Reply #3 on: August 30, 2019, 12:50:24 pm »
More importantly, ST's oscillators tend to be weak (low transconductance), so need a relatively high ESR / low C crystal.  Shop for this. :)

On a side note, would would be considered low load capacitance?
 

Offline T3sl4co1l

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Re: Crystal load capacitance
« Reply #4 on: August 30, 2019, 03:29:38 pm »
Your average crystal is something like, say, 8MHz, 100 ohms ESR, and 20pF specified loading.  Or something around there.  ST oscillators are more stable with like 200 ohms / 10pF.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline TheHolyHorseTopic starter

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Re: Crystal load capacitance
« Reply #5 on: August 30, 2019, 06:07:12 pm »
Your average crystal is something like, say, 8MHz, 100 ohms ESR, and 20pF specified loading.  Or something around there.  ST oscillators are more stable with like 200 ohms / 10pF.

Tim
Alright thanks, I'll stay in that ballpark.
 


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