Characteristic impedance - low vs high frequencies

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skyline1 19 years ago

Hi

Why characteristic impedance of transmission line isn't important at low frequencies (1000kHz) and it is one of the main parameters in RF? At high frequencies if a generator isn't matched with the impedance of the line the reflection of the power can damage the genarator, why isn't that the case at low frequencies? Or am I wrong about the destroying the generator? (example: If we connect high power audio amp to speaker we don't have to worry about the characteristic impedance of the cable) Can somebody explain this?

Thanks

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Andrew Holme 19 years ago

Transmission line effects are negligible if the line length is a tiny fraction of a wavelength.

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Stanislaw Flatto 19 years ago

Totally wrong. Just ask a 'serious' electic company engineer on matching generators to loads with and without reflections. It is the physics that is involved, not beliefs or frequencies.

HTH

Stanislaw

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Eeyore 19 years ago

Who says it isn't ? That's the upper end of the DSL frequencies on a phone line and it's very important to use transmissin line thinking for that to work.

Graham

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Eeyore 19 years ago

Audio only goes to 20kHz. The wavelength @ 20kHz is 1.5 km. A speaker cable is maybe 10m. There's no transmission line effect to consider here.

In long-distance telephony, the long length of the cables make transmission line thinking important even at these audio frequncies.

Graham

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skyline1 19 years ago

Sorry, I meant to say 1000Hz not 1000kHz

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Phil Allison 19 years ago

"Andrew Holme"

** WRONG !!

Same old ham radio bollocks trotted out as fact about a different area of electronics.

....... Phil

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Phil Allison 19 years ago

"Eeysore F****ng Pommy IDIOT "

** NOT AGAIN !!!

Can't the Graham Stevenson MORON ever get a

F****NG decimal point right ?????

Or any other point right ??

....... Phil

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Phil Allison 19 years ago

** That is not a correct statement - just another myth from the mouths and keyboards of fools. All signal and power cables have a characteristic impedance and it can matter at low frequencies.

The most important fact about characteristic impedance is that IF the load on the end matches the cable's characteristic impedance - the parallel capacitance & series inductance of the cable no longer has any effect.

Capacitance in a mic cable 100 metres long can have a significant effect on the high frequency output of a mic.

The inductance of a twin wire speaker cable of only 10 metres long will cause a drop in level at 20kHz of several dB if the load impedance is 1 ohm.

...... Phil

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Eeyore 19 years ago

It's that damn Windows calculator !

Graham

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John Fields 19 years ago

A poor workman blames his tools.

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John Larkin 19 years ago

Why?

John

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Phil Allison 19 years ago

"John Fields"

** However, in this case the man's " tool " is clearly the problem.

....... Phil

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Jasen 19 years ago

??? how do you get that? I get 10km for free space so probably 5 or 6 km for twisted pair - what propagation rate does a twisted-pair give?

Bye. Jasen

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skyline1 19 years ago

Can someone recommend me a good book about that topic or a web site!

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John Larkin 19 years ago

I'd vote for 15 km.

Figure twisted pair at roughly 0.7, depending.

John

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Eeyore 19 years ago

I get it using the MS calculator which I find very easy to accidentally move the decimal point one place.

Graham

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Rich Grise 19 years ago

Because in Phil's universe, everything is wrong? ;-)

Cheers! Rich

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Rich Grise 19 years ago

It's lame to blame the calculator. Man up, say, "I made a mistake", and get on with your life.

Cheers! Rich

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John Larkin 19 years ago

umm, good point.

John

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Characteristic impedance - low vs high frequencies

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