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Questions relating to designing PCBs
User avatar
By leon_heller
I'm talking about a few mm, not several cm. The additional capacitance will only be a fraction of a pF. The capacitor tolerances will have a much larger effect.

User avatar
By bigglez
Greetings Leon,
leon_heller wrote:I'm talking about a few mm, not several cm. The additional capacitance will only be a fraction of a pF. The capacitor tolerances will have a much larger effect.
Well, I had read your last three posts to this thread and
I must have missed your point...

Fri Jun 20, 2008 12:19 pm
leon_heller wrote:Impossible to say.
Mon Jun 23, 2008 6:00 am
leon_heller wrote:Differences in track length won't have any effect - try calculating the wavelength of the clock frequency.
Tue Jun 24, 2008 2:40 am
leon_heller wrote:That's more to do with impedance and VSWR. It doesn't matter with oscillators.
Comments Welcome!
User avatar
By leon_heller
I don't see any inconsistency there. No-one in their right mind puts a crystal more than a cm or so from the MCU.

Unlike the connections to an Ethernet transformer the connections to a crystal do *not* constitute a transmission line.

By Lucien
winston wrote:
bigglez wrote: The capacitors should be NPO types. If the PCB has a
ground or power plane that should be removed under the
crystal and associated traces.
Taking a straw poll of various bits of hardware lying around here, I notice some do and some don't. The 3com 3c905 ethernet card has the ground plane removed under the crystal (actually, I think it's an oscillator, a 4 pin device) for both the 25MHz and 20MHz devices.

However, the Intel ethernet card I have here *does not* have the ground plane removed around the timing device (which I think again is an oscillator rather than a crystal).

I've seen other designs where people have a ground plane under a crystal, and other designs where the ground plane was removed.

On reading up about it, when I designed my ethernet board, I removed the ground plane under the crystal/traces to the MAC/PHY. In the layout guide from the chip manufacturer, they admonished you to keep the xtal lines away from the analogue power supply, but they put a 1.8v analogue supply pin *right next* to the crystal input, forcing you to bring the 1.8v supply close to the traces from the crystal! (The best I could do is steer the 1.8Va trace away from the xtal traces as soon as it left the pad). The board works well, so I suppose I must have got it right enough :-)

Presumably, in the case of a 4-pin crystal oscillator in a can, there's no need to drop out the ground plane around the output trace from the oscillator - why might the designer of the 3c905 PCB have dropped the ground plane out around the oscillator, but the Intel designer not? (Surely an oscillator will have a fairly low impedance output).
And if that wasn't enough, I've been looking at some crystals myself, in SMD packages. They're four terminal devices, two for the crystal, and the other two are connected to the package's metal "shield" and according to the datasheet, are meant to be grounded.

So even if I remove the ground plane from under the crystal, I'm adding (some of?) it back by connecting the ground pins.

I do remember Microchip has a few app notes on oscillators. I have to go through those again, and I think one of them provided a rough guideline that stray capacitance on the pins and traces could contribute ~1pF to the loading of the crystal. I don't remember if that's with or without a ground plane though.
User avatar
By leon_heller
The grounding improves the stability of the crystal and minimises emissions, the cases of two terminal crystals are often grounded.

It probably would add to the capacitance, as you suggest.

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