Re: Lightning and Fourier transform of an impulse

On Aug 8, 4:24 pm, Jerry Avins <j...@xxxxxxxx> wrote:
Chris Bore wrote:
On Aug 6, 10:21 pm, Jerry Avins <j...@xxxxxxxx> wrote:
Chris Bore wrote:
But the frequency spectrum exists over all time, not just during the
impulse.
So I don't think this can be used to explain why the lightning
disrupts radio for a short time.
Chris,

The lightning isn't really an impulse. Some have durations of
milliseconds, few less than tens of microseconds. That's more than
enough to localize the interference in time.

   ...

Jerry
--
Engineering is the art of making what you want from things you can get..
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Jerry,

I agree but..

Frequency, as defined by a Fourier Transform, is a steady state
frequency spectrum whose components exist at fixed amplitude over all
time.

So the Prof can't use the broadband nature of the frequency spectrum
to explain a time-limited event. He is mixing orthogonal domains.

He can say the spectrum is broadband. But he can't say the spectrum is
somehow broadband for a short time.

I think the Prof should have accepted that lightning interferes
because it is a bloody great spark, and left it at that.

Chris
================
Chris Bore
BORES Signal Processing
www.bores.com

An impulse creates all frequencies, and they exist for all time. The
interference caused by lightning is brief, showing that lightning isn't
a true impulse. All well and good, but we knew that all along, didn't
we? If you like, think of a lightning stroke as abrupt keying, producing
lots of splatter. We've all heard the splatter from an SCR motor
controller excited at the mains rate extending well up into the AM
broadcast spectrum. Long ago, listening to the FM tuner I had built with
less-than-perfect limiters, I heard a noise like !!!!!!! !!!!!!! !!!!!!!
and ran out to the street shouting, "Dad! It's misfiring on one
cylinder!" Quasi-impulses make splatter that is only quasi-eternal. :-)

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯- Hide quoted text -

- Show quoted text -

The frequency spectrum of ANY signal exists for all time.

So you cant logically base a claim on the frequency spectrum, to
explain something (like interference) that only lasts a short time.

If you work in the frequency domain, then you cannot invoke or involve
time. The domains are orthogonal.

What the Prof did is wrong. He explained short duration (broadband)
interference based on an eternal, unchanging, frequency spectrum.

The spectrum is always the same, so he cant claim it somehow comes and
goes.


Chris
.

Relevant Pages

  • Re: Lightning and Fourier transform of an impulse
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