When a Signal Hits The End of a PCB Track - What happens? | Reflections by Eric Bogatin

[@bissoruben]

One of the PCB Design Lord King👑! Thank you once again!!

[@ericmc6482]

It is useful to conceptualise electrical pulses as having mass and inertia. Recall playing in a bathtub as a child sending waves and pulses to the end wall of the bathtub and the reflections and splashings that resulted.

[@Stewi1014]

Thank you again Eric! Your knowledge is allowing me to feel comfortable with more complex boards - I'm building things I couldn't have dreamt of a year ago. (although right now I'm having a very easy time with a simple board, and these 600Hz PWM signals are going to be the cleanest signals these motor controllers have ever seen haha!!!)

[@Jeremy-fl2xt]

Really great content. Very important concepts (like the waves traveling), along with important takeaways (like measure signals at the receiver) clearly stated.

[@xenofontzaras2741]

Danke!

[@RobertFeranec]

Thank you very much @xenofontzaras2741

[@ionix2000]

Thanks!

[@RobertFeranec]

Thank you very much

[@dmitry.shpakov]

Thanks Robert and Eric! Love your videos!

[@remy-]

Very interesting thx!

[@RobertFeranec]

Thank you very much @remy-

[@Ghost572]

I was just theoristing reasons for why reflections occur with my new found knowlege of how signals propergate and then I find this video. Let's hope the universe doesn't decide to stop reflecting or it will blow up.

[@chromatec-video]

Thanks for the link Eric - looks like we should avoid stubs at all cost!

[@oseyedian]

Thank you! You’ve answered my questions I’ve been carrying for years!

[@irpheus]

Tak!

[@RobertFeranec]

Thank you very much

[@Bl00drav3nz]

Wow really nice explanation thank you. The bit about source termination surprised me and I had to stop and really think about it, but it makes sense! The input side sees the sum of the input signal and the reflected signal, but it takes a whole roundtrip for it to settle.

[@metalafterlife666]

very good content as always, thank you!

[@veyoncecreations7251]

This kind teacher i want in my life. Oh god he nailed it ❤❤❤❤❤❤.

[@dmssultan5118]

Wow! A very useful tools!

[@slicer95]

Always great to list to Eric

[@uccoskun]

very educational video. Thank you for sharing your knowledge.

[@maxmonterocarreras6164]

why is there a voltage divider when the signal is launched to block 1? (input impedance and trace impedace). min: 17:28

[@ionix2000]

This video is fantastic!

[@richardrudek01]

I was thinking that the tool should perhaps show (an interpretation\comprehension) of the interaction between the forward and reflected signals as a vortex rather than a diagonal edge - similar to a 'pipeline' beach wave. With that comprehension, it allowed me to understand how the reflected signal combines with the source.

[@LaserFur]

It's better to say that it's about conservation of energy. The sender can't know what the receiver is doing due to the speed of light (in the cable)

[@WinChester_Ltd]

Thanks 👍👍👍

[@ashwin372]

really informative

[@WinChester_Ltd]

Hi, can you please explain, where do I have to add a resistor to increase an impedance of a source? Do I have to add it parallel to a pin or how?

[@pochuanpeng1729]

Great walkthrough. My question is what if the output resistance of drive is 50 ohm, it seems termination resistor is not required. But what is the downside?

[@UnbornWarrior123]

high driving current

[@董晨-t7b]

That’s so great, thank you again Eric, by the way, I don’t know how to use FEDSI to get a 3 months free, could you help me?

[@rkriisk]

And thats why you had 50 ohm bnc terminators on both ends on old coax network cable.

[@electronics.unmessed]

👍👍 Nice lesson! I did something similar by calculating the waveguide impedance and the reflection factors. In case somebody is interested to complement his know-how ..

[@ericmc6482]

Isn't this just all standard transmission line theory.... ie matched send build out resistance, line impedance and load impedence required to retain signal integrity !.

[@ahmeta6367]

50 ohm impedance is desired from end to end but this gives half the voltage of the output. Isnt that a problem?

[@RobertFeranec]

Watch the source termination 39:25 Notice the voltage on the input.

[@ahmeta6367]

Seems that load should be higher impedance to cause reflections to build up to the full voltage of the reciever. So load being same impedance with source and line is not a good thing.

[@tomasbergh]

Why dont you terminate with 50ohm with capacitor to 0V to avoid halfing the voltage level?

[@RobertFeranec]

Signal edge is built from many different frequencies and we need 50 ohm for every frequency. An ideal capacitor's impedance for low frequencies is very high (DC doesn't flow through a capacitor), an ideal capacitor's impedance for high frequencies is very low (capacitor is a short circuit for high frequencies). For real capacitors it is a little bit different, but still, a standard capacitor will not have 50OHM impedance for wide range frequencies - that is why resistor is a simpler solution. PS: watch 39:25 about halfling the level

[@tomasbergh]

@@RobertFeranec Yes, I think I was unclear about what I mean. Ringing is often rather high frequent. I want to avoid 50ohm loading on logic DC-levels (and low frequent digital pulses) and still maintain the logic levels without halfing it. Placing a capacitor in series with a resistor to 0V will give zero current (i.e. low power consumption) at DC and low frequencies. Problematic ringing is only a troublesome issue in the fast signal transitions. My intention is that the capacitor shall be a short circuit at high frequencies and thus the 50 resistor terminates high frequencies well and att low frequencies the capacitor is high impedant and thus disconnects the terminating resistor. This is how we sometimes handled emc-issues on high frequency databus-es when I was working at Flextronics. Thank you for great videos Robert, (a little bit toooo long videos, but very interesting videos! :-) )

[@LaserFur]

Yes you can do a AC termination system if the change of the signal is low enough compared to the capacitance of the wire. you need a capacitor at least 10 times the capacitance of the entire trace. and the signal can't change a a rate faster than the RC time constant. I use AC termination in many places where I want sharp edges without reflections, but my application meets the requirements. Edit: this is a Capacitor and a resistor in series. The frequencies only see the resistor and the capacitor only needs to hold on till the energy is absorbed.

[@tomasbergh]

@@LaserFur Yes, ofcourse... AC-temination is the word for it! Thanks

[@vincei4252]

Wait, people still use Adobe Flash in 2022? 😂 Jokes aside, another great video, Robert. Thanks!

[@thomasanderson4490]

🙏 𝓟Ř𝔬𝓂𝔬𝐒ϻ