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Signal Chain Basics

Signal Chain Basics #82: Crosstalk Between Outputs in Clocking Devices

Signal Chain Basics
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Dean Banerjee
Dean Banerjee
10/29/2013 3:35:08 PM
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Re: Improvement of PCB
Good Discussion...

Indeed, a complete ground is essential for good performance.  From personal experience, I did a board one time where I broke it up and attempted to minimize spurs and I got very erratic and unpredictable spurs that were sensitive to things like touching the part with my finger.

That being said, power supply pins can also generate soyrs that can be transferred to the ground plane and that can show up on other outputs.  For your idea o making plane for each supply voltage, then if two supply pins to different clock outputs have the same voltage, but different frequencies, then they will inject spurs onto each other.

If capacitors are placed between the power pin and ground, then the intention is to have a low impedance and kill any noise.  But often seems that instead of killing the noise, it more like redirects it and this spur energy gets on the ground plane.

On the LMK04828 evaluation board, (http://www.ti.com/lit/ug/snau145a/snau145a.pdf), on page 34, we actually put an inductor on the power supply of the VCXO because we found it was injecting noise to the ground plane.  For this board, we also put a 0 ohm resistor and we went back and forth on which one was better for this spur.

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Netcrawl
Netcrawl
10/28/2013 9:07:02 PM
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Master
Re: Improvement of PCB
A complete ground is important and essential,  the best solkutions is to have its own layer and ground planes for every voltage, we need to take a first look at this one.

I think it depends on ESr(equivalent series resistance) and ESL(equivalent series inductance , a good combination of capacitors could lead to a low impedance.

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Dean Banerjee
Dean Banerjee
10/28/2013 5:57:29 PM
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Newbie
Re: Improvement of PCB
I agree with this and seen it on many on the EVM boards I have created for TI where power supply of one block injects noise on another block.   This phenomenon can manifest itself as crosstalk in clocking devices, fractional spurs, or crosstalk between two VCOs on a dual PLL synthesizer.  At some point, the layout does become important, but before that, if the power supplies for the critical blocks producing different frequencies are not isolated, then this is an invitation for spurs to mix and match.

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Dean Banerjee
Dean Banerjee
10/28/2013 5:48:09 PM
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Newbie
Re: Improvement of PCB
Thank you for your feedback.   With vias, the problem that I have seen is that there is a tendency to put the ground and power planes too close to them and this is a great way for noise energy to couple.  Now this is talking about vias that go to the ground and power planes, so I don't know how critical impedance of this is, because it's DC.  But for other higher frequency signals, like clock outputs, then the via inductance is definitely a much bigger consideration.

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yalanand
yalanand
10/27/2013 7:15:33 AM
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Master
Re : Signal Chain Basics #82: Crosstalk Between Outputs in Clocking Devices
To raise the value of the conversion from analog to digital, we need to describe a few terms exclusive to the digital zone. These concepts form the substance of the analog-to-digital convert and are self-determining of the technique usage to digitize the indicator. 

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goafrit2
goafrit2
10/14/2013 9:54:14 PM
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Master
Re: Improvement of PCB
>> It sounds like "event-driven" or "quasi-delay insensitive" approaches to CMOS and Chip level.

The point is that if your design is using asynchronous methodology, cross talk is practically immaterial as the signal degradation will have limited impact.

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goafrit2
goafrit2
10/14/2013 9:52:45 PM
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Master
Re: Signal Chain Basics . worth to be produced as a pdf file
>> Why not to make a kind of a .pdf file out the existing contributions and make easy integration of future features

You can copy the files and go to pdfonline.com and easily get a PDF in seconds. It is going to be hard to change this as this seems to be the adopted model for all UBM sites.

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goafrit2
goafrit2
10/14/2013 9:51:10 PM
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Master
Re: Improvement of PCB
>> A good PCB layout design starts with circuit design

Yet over the years, I have noticed that the most important factor may the quality of the material that the final PCB is made. Have a great design, use lousy material, you have no chance.

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DaeJ
DaeJ
10/13/2013 3:46:21 PM
User Rank
Master
Re: Improvement of PCB
It sounds like "event-driven" or "quasi-delay insensitive" approaches to CMOS and Chip level.

I think that cross talk is never-end topics like jitter analysis.  PicoScope 9300 scope is one of tools, providing eye-diagram with crosstalk. Based on engineering experiment, signal line location & via size and location would be reviewed to improve the signal integrity (SI).

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ptessmer
ptessmer
10/12/2013 4:00:47 PM
User Rank
Newbie
Signal Chain Basics . worth to be produced as a pdf file
Dear All, I read the contributions to the Signal Chain Basics series from the very first article. It is going to be 100, soon.  Many students and colegues of mine keep collection of hard copies of the articles. Why not to make a kind of a .pdf file out the existing contributions and make easy integration of future features  ? A kind of living book would result.  Downloading and eventual printing of a "private" copy of it would be less cumbersome as it is today. I hope it is doable.

Tessmer

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