Re: gEDA-user: Question for DJ about his alarm clock

[John Doty <jpd@xxxxxxxxx>]

On Aug 15, 2008, at 5:02 PM, DJ Delorie wrote:

>
>> Did you use a ground plane?  If so, then I'd expect that the plane is
>> quiet, and it would be better to connect directly.  Or am I wrong?
>
> I used a plane.  Look up "ground bounce" ;-)

What, here, is bouncing with respect to what? Voltage is *relative*.

>
> Mostly, though, the app note said to do it, I did it.
>
> The only thing I didn't do was ground the PLL's filter on the PVSS
> side of that resistor.

And I'd guess that you lost all of the benefit of the isolation by  
doing that. One way to make sense of the recommendation is that some  
of the ground current from the digital circuitry tends to flow in the  
substrate under the PLL circuit, perturbing its operation. So, you  
block that path with your 4.7 ohms. But then, you are going to get  
*more* bounce on PVSS relative to your ground plane, and that's the  
ground the PLL cares about. So, you've connected the loop filter to a  
place that is noisy relative to PVSS.

I bet if you short R304, the spectral purity of the PLL output  
improves. That could improve the noise floor of your audio output. Of  
course, it might not be enough to notice.

>
>
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John Doty              Noqsi Aerospace, Ltd.
http://www.noqsi.com/
jpd@xxxxxxxxx




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