[SI-LIST] Re: Square wave harmonics

> "Zhou, Xingling (Mick)" wrote:
> 
> Second, in reality, there is no "frequency domain", everything is in
> time domain for us (human beings).  

The major problem I've noted with people "getting their brain around"
the duality of time and frequency domain is that one does not exist
without the other.  You can't describe time without a frequency derived
time base.  Sixty seconds per minute is still a number of events per
time, aka frequency!  Time is defined by the number of atomic
vibrations, also frequency!  But, you can't describe frequency without
waiting a "period" of time.

They're the same thing, just front and back.  Actually, the front and
side views is a better description!

> However, we still live in time domain. 

But I hear and see in frequency!

> Back to electrical engineering, people have invented many "frequency"
> devices like resonators etc. But we should say they are still in time domain
> even we can approximately describe them using frequency domain terminologies.
> If the approximation is good enough and practical, we always accept it.

Sometimes it's much easier to accurately describe the phenomenon in
frequency domain than time domain.  Phase noise can be described as edge
jitter with a certain statistical distribution in time, or as an
increase in noise level above ambient versus frequency offset.  Which
one is the correct representation depends on the application.  In my
line of work, the amount of noise in adjacent frequency channels tends
to be the most important, not the timing inaccuracies.  However, in RF
systems transmitting digital data, the timing also becomes important
because of noise induced in the "data constellation".

<PONTIFICATION ON>
The most important part of being an engineer isn't the knowledge, though
it is certainly important.  It's being able to use the knowledge as
appropriate to the job at hand.  Avoiding the "hammer and nail" trap <1>
is a constant battle.  

<1> "If all you have is a hammer, every problem looks like a nail."  It
means one tool doesn't fix everything.  It seems obvious, until you've
done it.
<PONTIFICATION OFF>
-- 
Mark Randol, RF Evaluation Engineer
Motorola SPS, Inc.
M/S EL536
2100 E. Elliot Road
Tempe, AZ 85284
(480)413-8052 Voice
(480)413-8690 FAX
m.randol@xxxxxxxxxxxx
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