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Comment by jagrsw | Hacker News Reader

In the spirit of times - does anybody know what room-temp supercoductors would do to the transmission lines, like DP/HDMI?

Would 0 resistance have positive influence on signal degradation/coherence (ie. increasing maximum cable lengths or their reliability), or maybe it's already limited by various forms of capacitance/inductance or other problems in cables?

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This is a really great question. If you had a perfect conductor in air, then yes you would have no attenuation (or "insertion loss", one common term used in Signal Integrity). Real transmission lines (cables, PCB traces) use dielectric insulating layers adjacent to the conductors. These dielectrics have a property called permittivity which affects what we call "dielectric loss". Essentially, transmission line loss can be broken into conductor loss (resistance, skin effect, etc.) and dielectric loss (energy lost by causing physical rotation in the dipoles of the dielectric material).

A perfect conductor would have zero conductor loss but would still have some of this dielectric loss. I covered the concept a little bit in a webinar recently [0] but SI guru Eric Bogatin really boils it down best. In this [1] equation for loss estimation in a PCB trace, you can see the conductor and dielectric portion of loss on either side of the "plus sign". The left part is the conductor loss and is proportional to the square root of the frequency. The right part is the dielectric loss and is proportional to the frequency of the signal, as well as proportional to parts of that permittivity (Dk and Df in that equation). To answer your question though, it absolutely would allow for much longer cables since you eliminate one part of the loss.

Sorry for all the ridiculous detail, if you're still interested in the topic please feel free to reach out for more info! Contact info in my profile and at [0].

[0] https://wiki.shielddigitaldesign.com/High_Speed_Design_Wiki/...

[1] https://www.edn.com/loss-in-a-channel-rule-of-thumb-9/

That man is really good at teaching/explaining SI concepts. He was my professor at my university and taught High Speed PCB design class. Lots of learning like the one you described!

The details - I feel like I learned everything I would have from a whole 3-credit university course.

Thank you.

I wonder if a coaxial cable with the outer tube made superconducting would make a difference.

Yes it definitely would :) There's conductor loss in both the signal and return path.

Line length would not be limited any more in theory, but it's not like you can do ridiculous length even with a HTSC wire. The most obvious factor is protocols, you can't get faster than light which means protocols with a tight response timeout won't work out.

The second one is capacitance, as most transmission is something very closely resembling AC, so IIRC even at zero resistance you'd still have a capacitance between the lines and a resulting reactive power.

You don't have to go that fancy, you could just use fiber optics right now.

You can get insane bandwidths out of quite cheap and manageable cable. Eg, you can put 100 Gbps down a single strand. Not enough? Try a MPO cable with 12 of those. Maximum length is effectively unlimited.

superconductors are lossy at AC (less so than regular conductors, but significantly above negligible)

a very large bulk of the loss is through dielectric loss too. that one you can't get rid of with superconductors.

No such thing as a super insulator?

vacuum :)

Vacuum is actually a horrible insulator, like in practice.

Assuming you could produce viable consumer cables that are superconductors, my understanding is that there would be no signal loss, so your hypothetical cable could have infinite length.

Digital is digital so this would be true for any data use case.

Maybe someone smarter can chime in about how electromagnetic interference affects or doesn’t affect a hypothetical “superconductor cable.”

The longer the cable, the longer duration loopback signals take. This can affect timing of any synchronous specification. If it is too long, it could be out of scope of the spec and thus stop working. Maybe you can solve this with a transceiver MITM device.

Oh right I forgot about that! So yeah… probably longer than you’d need but not infinitely long. Unless the protocol is fine with it.

Superconductors only have zero resistance when measured in DC.

AC still faces impedance, high frequency AC like DisplayPort especially so.

Im sure a plethora of high end audio vendors will come out with whole lines of superconductor digital audio/video cables with higher sound clarity and wider soundstage precision.

Note also that superconductors only have zero resistance to direct current.

AC, including and especially any form of high frequency signaling, still face impedance.

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