PCB Layout & Decoupling - Measuring and Fixing (Part 3)

How to fix a noise on power pin by measuring PDN impedance.

Part 1: PCB Layout & Decoupling - Explained why it's so complicated

Part 2: PCB Layout & Decoupling - Understanding Impedance

Part 3: PCB Layout & Decoupling - Measuring and Fixing

Links:

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[rogerfurer]

Thank you Robert et al. I for one, find these videos very educational. I'm trying to imagine why this is happening. The ferrite bead has a higher impedance at ~60KHz because of its inductance; would a different bead affect a different frequency? Is this due to reflection between the bead and the Vcc pin? And I guess that this happens regardless of what kind of capacitors bypass at the Vcc pin? So many questions, so little brain! But thanks again.

[helmuthschultes]

Excellent series and of considerable help to many viewers. Non comes as surprise to me, but I have near 50years in electronics behind me.

Well Robert, all is in line with my general original comments made at part 1 concerning series/parralel resonances, ignoring the ferrite. But also noting the ferrite is an absorber at high frequencies, less so and inductive element except that at lower frequencies the ferrite ceases being lossy and becomes a inductance and will form its own resonant characteristics.
The ferrite involved is intended to be functional at high MHz frequencies, where it is a lossy element absorbing HF passing through its zone. Sadly at these lower frequencies it is a typical high Mu device with low lisses and becomes a significant inductor and resonates. Here it was making a bad component creating serious noise. It was misused in this instance. Such ferrites should ONLY apply to very high frequency suppression or a different ferrite material with high losses in lower KHz range should be used. Given 60KHz maybe a mild iron or iron powder ferrite could have avoided the severe problem. But I would not have used the ferrite in the first place, given the frequency domain of interest.

[joesmith-jetq]

I watched your three episodes in full. I enjoy the interactive discussions with the people you have had on. They were all very well done. Thank you for your time and the people who helped you put this together.

[codedesigns]

This was awesome! I’m sure this video would attract niche people who have a deeper understanding of circuit resonance and analysis and their discreet component relationship. I think those interested in RF communication would find this particularly important. For myself, learning the math and putting it into a spreadsheet (as you have done) is far cheaper and helps retain the knowledge than purchasing a high cost tool (unless you are military or telecom where speed is important). I believe this is why Eric knew in part 1 what was happening before a tool was ever used. Excellent call!

[siavashtaherparvar]

You should definitely continue these series, these were great!!! and I hope you keep continuing this topic.

[balintillenyi]

Hi Robert,
Thank you for these videos about this important topic!
This topic is very actual in many design consideration. I've learned new perspective for considering the design of a power supply routing, and impedance calculation method.
I hope, this video series will be continue soon...
Regards,
Balint

[ats]

I hope you will continue these videos. They deal with real design issues that are obviously very complicated which most designers never even consider.

This particular video was very interesting (thumbs up!), but doesn't really address how to come up with an optimal design. The inductance of the ferrite bead must be interacting with a large capacitance (the bulk capacitance of the power supply?) to generate the 60 kHz resonant peak. Getting rid of the ferrite bead helps in this case, but maybe a different ferrite bead would be effective? Or maybe a small resistance (maybe 5 Ohms?) or maybe a small capacitance before the ferrite bead to generate a pi network? I have more questions after seeing the video than before! ;-)

I hope in the future you will bring on some guests who will suggest how to optimize the PDN impedance curve, and then provide simulation and measurement results!

[mdchethan]

Thank you Robert for continuing this topic and deep-diving into finer details. Ferrite beads are generally good to suppress HF noise on the power lines and I generally use so many of them in all my boards, and it's surprising to see the removal of ferrite beads in fact helps to reduce impedance peaks. I personally would like to see a few more videos related to this topic and also very curious to see hardware design-related content.

[marcgoovaerts]

Hi Robert, I've watched the 3 video's in this series with great interest. I've already studied and implemented several solution myself, as they are already mentioned below . As a first note, PDN can also be simulated with Spice in AC analysis. Its not exactly the same, less accurate, but it leads to similar results and insights. A second point would be that it's a pity to throw away the ferrite bead to solve a 66kHz problem and probably risk to have an EMI problem above 100 Mhz. Simulating LC damping solutions learns that they are indeed a solution. You basically add a large capacitance (1 order of magnitude larger) with a small series resistor. The resistor can be calculated for optimal damping but it is not very critical. They usually end up in the sub 1R range. So you can sometimes use a lossy electrolytic or (not a fan myself) tantalum or a ceramic in series with a resistor.
By all means, do continue to present a solution as it's very informative.

[xtasyk]

Thanks so much Robert . I’ll look forward to watching this after work today 🙏

[sigfreed]

Yes, please continue! Even past ep 4 which has already been released

[Graham_Wideman]

Robert -- yes please, do continue! This is a quite niche topic that is nonetheless valuable, and the video series will, I think, be found by viewers over a long period of time, rather than just a burst of views upon initial release.

[FilipMilerX]

I would like to know more about the ferrite beads. If it's a good practice or bad practice and so on.

[Gengh]

That's a really good practical demo of their product, definitely gonna try to do a similar measurement in my next board.

[guillepk]

I'm still VERY interested!!! Thank you Robert for this series!

[EDGARDOUX]

Fantastic series an decoupling and desing. Very well explained. Thank you Robert for the time and effort. Be safe!

[Jeremy-flxt]

This is both very interesting and very relevant. I'd like to filter supply side noise, as well as deliver power consistently and it seems like this is often more art than engineering. This is great content, and I hope you go even deeper with the material. It is eye opening to me.
A video on how to use ferrite beads well (I don't yet see why one would be used instead of an inductor) would also be interesting.

[shazam]

A question and a comment:
1) the FB is there to prevent noise generated by the uC from polluting the Power line and traces, to reduce conducted and radiated EMI. What are the effects of changing the FB to a Zero Ohm on the EMI of this PCB / Product?
2) This was both interesting and fascinating, BUT to paraphrase Bob Pease, my "simulator" is a soldering iron. Instead of many hours involved simulating and measuring the impedance (perhaps 5x to 10x of the actual video run time) I have many times, found a better solution in a few minutes by changing parts or the actual circuit / routing on the PCB (hint: cut out the traces and use de-soldering braid to make connections).

To the commenters below about "The Ferrite Bead": There is no One Ferrite Bead, there are many, with many different characteristics, check out Murata, TDK, and others, look at the curves of the charts.

PS I found you and originally watched this on Odysee

[danielcampanoflorido]

Thanks for the video Robert. I have found the video very interesting. It is always useful to learn how to cancel high impedance to prevent any ringing when switching could create noise. I think a video explaining more in detail how to generate a PDN graph by hand would be useful, as the Bode 100 is a bit expensive.... I find sometimes problems getting my board passed for EMC on conducted emissions for instance, explaining the relation between PDN graphs and results when running a board on conducted emissions would be very very useful. Thanks again for your videos!

[pnjunction]

Another great video, Thanks Robert and Florian. I really appreciate that you're covering this topic in this length and detail. I'd be very interested to see how to use ferrite beads correctly. I think I used them a bit willy nilly in the past.