"How to?" indeed!

[John Brown]

Oh, there are dill speed / feed calculators online if you look.

I can't stand dill, even if it's fed to me really slowly.

 

[TRITON]

cobalt drills

I've never seen a cobalt drill. Is it made by Bosch or Makita ?

 

[throbscottle]

That's the speed of light in free space (vacuum), not the speed of propagation of a signal in a wire.

The speed of light in free space (a vacuum), c, is very close to 300,000,000 m/s, i.e. 30cm/nS or about 11.8"/nS.

Real-world signal propagation speed occurs relative to c, and is defined in terms of relative permittivity, or dielectric constant known as "Er". For the FR4 material used in most common PCB boards, Er ranges from 3.8 to 4.8, depending on the glass weave style, thickness, resin content, and copper foil roughness etc.

The speed of signal propagation in a wire is c/sqrt(Er) where Er is the dielectric constant for the surrounding material.

Therefore, on an FR4 PCB, in 1nS a signal will travel between 13.7 and 15.4cm, i.e. between 5.4 and 6in.

Teflon (PTFE) has an Er of 2.1 (or thereabouts), which means that in 1nS, a signal propagates about 20.7cm.

For comparison, BS6231/Tri-rated cable has a PVC covering with an Er of around 3.2, so in 1nS a signal travels about 16.8cm, i.e. Teflon is FAR superior to PVC as an insulator or substrate at high frequencies.

The Er values for PCB substrates and wire insulation are extremely important at high frequencies where timing is important, so material choice is a critical part of the design process.

Is this where "cable velocity" comes from?

 

[throbscottle]

I can't stand dill, even if it's fed to me really slowly.

Drill!

 

[nickds1]

Is this where "cable velocity" comes from?

"Cable velocity" is simply the speed of signal propagation expressed relative to the speed of light in free space, c. Typically it's 1/sqrt(Er), i.e. a fraction of c, or 100/sqrt(Er) which is a percentage of c, where Er is the dielectric constant of the surrounding material.

 

[throbscottle]

"Cable velocity" is simply the speed of signal propagation expressed relative to the speed of light in free space, c. Typically it's 1/sqrt(Er), i.e. a fraction of c, or 100/sqrt(Er) which is a percentage of c, where Er is the dielectric constant of the surrounding material.

Yes, then. I wondered how cables had different veloicities. I was going to build a little TDR device (never got round to it) and came across it then.