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epp
Joined: 04 Mar 2004
Posts: 265
Helped: 8
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 03 Jul 2006 20:10 Two crystals at one PCB !??? |
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Hi!
I am designing digital AC motor controller with dsPIC.
I have two crystals (2pin, HC49) in my pcb file: one for dsPIC (14.7x MHz), and other is for USB chip (6MHz). One is in horisontal and other is in vertical axis.
What is reccomended minimum distance between them?
Now, I have distance roughly about 18-20mm and I am afraid that they might have influence
at each other.
Please help.
Regards.
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IanP
Joined: 05 Oct 2004
Posts: 6361
Helped: 1509
Location: West Coast
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| 04 Jul 2006 2:26 Re: Two crystals at one PCB !??? |
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I don't think there is minimum recommended distance between two crystals ..
My gut feeling tells me that 20mm, or so, between them should be just fine ..
Regards,
IanP
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xxargs
Joined: 28 Jul 2002
Posts: 231
Helped: 44
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| 04 Jul 2006 9:34 Re: Two crystals at one PCB !??? |
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| epp wrote: |
Hi!
I am designing digital AC motor controller with dsPIC.
I have two crystals (2pin, HC49) in my pcb file: one for dsPIC (14.7x MHz), and other is for USB chip (6MHz). One is in horisontal and other is in vertical axis.
What is reccomended minimum distance between them?
Now, I have distance roughly about 18-20mm and I am afraid that they might have influence
at each other.
Please help.
Regards. |
Crytals is very very, stable in frequency depend of very high internal Q-value.
If right driven (not overdriven) you have no problem with influeces even if crystal places side by side. But, make holes and/or solder area in solder mask for possibly to grounding crystal case to ground plane with soldering of wire sling or directly between metallic crystal case and ground plane. You not want free floating crystal case working as antenna and make trouble in EMI/EMC-testing, plus any mark of interferences between crystal supresses further if crystal case is grounded...
most crystals using in RF-equipment have crystal case soldered to ground.
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Verba
Joined: 11 Apr 2003
Posts: 97
Helped: 7
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| 10 Jul 2006 22:50 Two crystals at one PCB !??? |
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yes, I agree with xxargs, there should be no problem. You just need to plase them a couple of mm apart to reduce capacitive coupling.
To reduce the phase noise, if your circuit is sensitive to it, you should groung your oscillators properly.
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