# light vs

Discussion in 'General Electronics Chat' started by supermankid, May 20, 2014.

1. ### supermankid Thread Starter Member

May 26, 2013
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Speed of light vs. circuit speed

Speed of light is 3*10^8[m/s]

sometimes, the speed of the electrical signal could be at some GHz.

eg. 3*10^9 [V/sec] @ 3[GHz]

Can we somehow relate and compare those two speed?

2. ### Papabravo Expert

Feb 24, 2006
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1,850
The speed of light in a vacuum could also be called the "velocity of propagation". The velocity of propagation in other materials, for light and electrical signals, will be slower. The velocity of propagation does not depend on frequency so the two measurements are not comparable in any meaningful way.

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3. ### alfacliff Well-Known Member

Dec 13, 2013
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the rate of change in voltage isnt the speed of the propagation. if it were, then any very large swing in voltage could excede the speed of light. the 3 ghz signal in free space still travels at the speed of light. and through any medium (transmission line, air, ect.) travels slower.

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4. ### AnalogKid Distinguished Member

Aug 1, 2013
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First, 3x10^8 is the speed of light *in a vacuum*. In other media it is slower. For fibre optic cable it is around 30-40% slower. The important thing is that the speed of light in any media is finite.

Because of this, very fast signals like gigahertz serial data can have a wavelength that is shorter than the circuit board trace it is on. When this happens the nitty gritty details of how an electrical signal propagates down a length of copper become important. Source impedance, characteristic impedance, and transmission line termination don't matter when sending 10 KHz RS-232 data a few feet through a modem cable. But they all are critically important to signal integrity when sending 10 GHz signals even a few inches.

ak

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5. ### alfacliff Well-Known Member

Dec 13, 2013
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does red light travel at a different speed than blue? the frequency is all the diffefrence. the data rate does not determine the speed through the medium. the speed of light in a vacuum is the same for very long radio waves and very short light waves. you dont use a different value of C (speed of light) when determining the wavelength, or velocity of transmission.

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6. ### wayneh Expert

Sep 9, 2010
12,406
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Sure. The former is a measure of velocity. Units are distance per time.

The latter is a frequency, incorrectly referred to as a "speed". Units are inverse time (not volts per time as noted).

About the only relation between the two that is interesting is the very short wavelength associated with transmitting a GHz frequency, as AK noted.

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7. ### alfacliff Well-Known Member

Dec 13, 2013
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the pretty waveform know as a sine wave is just a representation of how electromagnetic waves travel, in reality, EM waves are electric and magnetic fields and have no shape. so the rise times and fall times cant be sed to compute their velocity, only the wave front. that catches a few every year, using the representation instead of whats actually there.

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8. ### AnalogKid Distinguished Member

Aug 1, 2013
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I do.

The permeability, permittivity, and dielectric constant all vary from one material to another. I've designed high speed circuits for five different rigid and flex printed circuit materials, and each has a different speed of light, characteristic impedance as a function of core thickness, dissipation factor, etc. This affects all aspects of signal integrity.

ak

9. ### Lestraveled Well-Known Member

May 19, 2014
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A dinosaur brains story:
I built an antenna measurement system that was installed at an outdoor range where the transmitting section was 10,000 feet downrange. It was linked to the system via fibre optic cable. During installation we told the dinosaur brained project manager that we had to determine the fibre optic delay and program it into the operating system. He laughed and said that fibre was at the speed of light and was instantaneous. When we showed him the effects of compensating for the fibre delay he said," Dang, the speed of light is not what it use to be."

I hope someone enjoyed this little story.

10. ### Lestraveled Well-Known Member

May 19, 2014
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What you are all talking about is called the "coefficient of dielectric velocity". It basically states that the speed of light is different through different materials. Printed circuit boards substrates, coax cables, fibre optic cables, etc., all have different velocities and as an engineer you have to deal with the resultant delays.

11. ### alfacliff Well-Known Member

Dec 13, 2013
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the velocity through the material is different, but not at different frequencies. the speed is still determined by the material light or rf is flowing through, not the frequency.

12. ### AnalogKid Distinguished Member

Aug 1, 2013
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I agree. You may have misunderstood my post #4. I wasn't saying that 10 KHz signals are slower than GHz signals. I was saying that because a low-frequency signal's wavelength is so much greater than the path length, propagation effects are minimal to trivial.

ak