3 ms·

Surprised this didn’t mention the reason for all of this: that a high-speed signal runs not in the trace, but in the field between the trace and the nearest gro

Surprised this didn’t mention the reason for all of this: that a high-speed signal runs not in the trace, but in the field between the trace and the nearest ground plane. And in differential pairs, also between trace and the virtual ground plane between the two traces. Keeping this in mind makes the rules and potential mistakes clearer.

sbaiddn 4y ago
At what frequency does the signal travel between traces and not on it? Is it a feature size thing, the macroscopic size of the traces that are effectively transmission lines (that cant be, most pcbs are smaller than \lambda?). Or is it a skin effect of the conductor?
- planede 4y ago
  Between 0 Hz and infinite Hz.
- azzentys 4y ago
  This is probably a good video for understanding - https://www.youtube.com/watch?v=icRzEZF3eZo https://www.youtube.com/watch?v=icRzEZF3eZo
  - thrtythreeforty 4y ago
    Basically, go watch every Rick Hartley video on YouTube. He is awesome at explaining. It will become intuitive instead of being a bunch of cargo cult advice. (Normally I dislike long videos but this is absolutely worth your time if you design PCBs.) https://youtu.be/QG0Apol-oj0 https://youtu.be/QG0Apol-oj0
  - azzentys 4y ago
    ADD: in short, it happens for "any" frequency. This is how a voltage signal propagates. However, for DC it happens once because you see "change" in voltage only once and that's it. As you increase the frequency, you'll end up seeing more voltage waves due to "change" in voltage. That's when "reflections" come into picture - happens due to discontinuity in impedance.
- pulvinar 4y ago
  Technically the signal always also travels between, but it's not a concern until roughly 50 MHz and above. It's actually the rise time that matters, and how long the signal takes to rise relative to the trace length and propagation speed.
- deleted 4y ago
  [deleted]
deleted 4y ago
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potatofrenzy 4y ago
This, in itself, is an oversimplification simplification. Signals travel through traces. It's just that the motion of charges in the conductor is mediated by electromagnetic forces, and electric and magnetic fields are not constrained to the conductor. Indeed, electrochemical batteries aside, most of the energy storage in an electrical circuit is mediated by fields interacting with the surrounding space. At low switching speeds, these effects are inconsequential outside discrete components specifically designed to exploit them (capacitors, inductors). At high speeds, these small capacitances and inductances begin to have perceptible effects, for example turning the PCB a non-negligible capacitor that induces voltages and currents on other layers. A related problem is that when trace length is no longer negligible in function of wavelength, you can no longer treat signals as changing the potential across the entire conductor in an instant. You get different voltages in different places, and all of sudden, you have to worry about wave propagation - reflections, etc. Some of the advice in this article isn't entirely sound; for example, you certainly don't need to worry about sharp trace angles or vias at 50 MHz. Similarly to the fetish of (often discontinuous!) ground planes, there's a lot of PCB design advice that matters in specific use cases (and even there, needs to be done well) that over time morphed into a bit of a "you absolutely need to do this in your Arduino project or else".
- GeorgeTirebiter 4y ago
  Although primarily an EMC Consultant, Henry Ott (before he retired) had a nice collection of tips on his website. Fortunately, archive.org had crawled his pages before they were taken down: https://web.archive.org/web/20160308031350/http://hottconsultants.com/ https://web.archive.org/web/20160308031350/http://hottconsul... (Bookmark this page, you WILL need it later if you do any serious EE stuff....) If you click on the "Tech Tips" link on the left side of that page, you will get to his discussion of multi-layer PCBs. ("PCB Stackup"). Read This Section, and Become Enlightened! ;-)
  - jbay808 4y ago
    To echo this, I highly recommend his book "Electromagnetic Compatibility Engineering".

sbaiddn 4y ago

At what frequency does the signal travel between traces and not on it? Is it a feature size thing, the macroscopic size of the traces that are effectively transmission lines (that cant be, most pcbs are smaller than \lambda?). Or is it a skin effect of the conductor?

planede 4y ago
Between 0 Hz and infinite Hz.
azzentys 4y ago
This is probably a good video for understanding - https://www.youtube.com/watch?v=icRzEZF3eZo https://www.youtube.com/watch?v=icRzEZF3eZo
- thrtythreeforty 4y ago
  Basically, go watch every Rick Hartley video on YouTube. He is awesome at explaining. It will become intuitive instead of being a bunch of cargo cult advice. (Normally I dislike long videos but this is absolutely worth your time if you design PCBs.) https://youtu.be/QG0Apol-oj0 https://youtu.be/QG0Apol-oj0
- azzentys 4y ago
  ADD: in short, it happens for "any" frequency. This is how a voltage signal propagates. However, for DC it happens once because you see "change" in voltage only once and that's it. As you increase the frequency, you'll end up seeing more voltage waves due to "change" in voltage. That's when "reflections" come into picture - happens due to discontinuity in impedance.
pulvinar 4y ago
Technically the signal always also travels between, but it's not a concern until roughly 50 MHz and above. It's actually the rise time that matters, and how long the signal takes to rise relative to the trace length and propagation speed.
deleted 4y ago
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deleted 4y ago

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potatofrenzy 4y ago

This, in itself, is an oversimplification simplification. Signals travel through traces. It's just that the motion of charges in the conductor is mediated by electromagnetic forces, and electric and magnetic fields are not constrained to the conductor. Indeed, electrochemical batteries aside, most of the energy storage in an electrical circuit is mediated by fields interacting with the surrounding space. At low switching speeds, these effects are inconsequential outside discrete components specifically designed to exploit them (capacitors, inductors). At high speeds, these small capacitances and inductances begin to have perceptible effects, for example turning the PCB a non-negligible capacitor that induces voltages and currents on other layers. A related problem is that when trace length is no longer negligible in function of wavelength, you can no longer treat signals as changing the potential across the entire conductor in an instant. You get different voltages in different places, and all of sudden, you have to worry about wave propagation - reflections, etc. Some of the advice in this article isn't entirely sound; for example, you certainly don't need to worry about sharp trace angles or vias at 50 MHz. Similarly to the fetish of (often discontinuous!) ground planes, there's a lot of PCB design advice that matters in specific use cases (and even there, needs to be done well) that over time morphed into a bit of a "you absolutely need to do this in your Arduino project or else".

GeorgeTirebiter 4y ago
Although primarily an EMC Consultant, Henry Ott (before he retired) had a nice collection of tips on his website. Fortunately, archive.org had crawled his pages before they were taken down: https://web.archive.org/web/20160308031350/http://hottconsultants.com/ https://web.archive.org/web/20160308031350/http://hottconsul... (Bookmark this page, you WILL need it later if you do any serious EE stuff....) If you click on the "Tech Tips" link on the left side of that page, you will get to his discussion of multi-layer PCBs. ("PCB Stackup"). Read This Section, and Become Enlightened! ;-)
- jbay808 4y ago
  To echo this, I highly recommend his book "Electromagnetic Compatibility Engineering".