power & energy signals

Started by shikha April 16, 2005
pls can any body make me understand difference between power signals &
Energy signals.
why  r random signals called power signals.

		
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geeez i wish the chat-room lingo would stay in the chat room.

in article VeidnVJa8dkOkfzfRVn-uw@giganews.com, shikha at
shikhadrdo@yahoo.co.in wrote on 04/16/2005 09:01:

> > pls can any body make me understand difference between power signals & > Energy signals.
a finite energy signal: +inf total energy = integral{ x(t)^2 dt} < infinity -inf a finite power signal: +T/2 average power = lim 1/T integral{ x(t)^2 dt} < infinity T->inf -T/2
> why r random signals called power signals.
because, like a sine wave, if they are left turned on forever, they will deliver an infinite amount of energy. but their average power is finite. -- r b-j rbj@audioimagination.com "Imagination is more important than knowledge."
shikha wrote:
> pls can any body make me understand difference between power signals
&
> Energy signals. > why r random signals called power signals.
Trying to make it easy: An energy signal has a finite energy. Signals of a limited length also carry a finite energy, and so they are energy signals. A signal that decays exponentially, for example, also has a finite energy. A power signal is not limited in time (it is *always* on, from the Big-Bang to Judgement Day and beyond), and has an *infinite* energy. Since an infinite energy has no meaning for us, then we use the energy per unit of time, i.e., power. Examples: A square pulse is an energy signal. A square wave of infinite length is a power signal.
"shikha" <shikhadrdo@yahoo.co.in> wrote in message 
news:VeidnVJa8dkOkfzfRVn-uw@giganews.com...
> > pls can any body make me understand difference between power signals & > Energy signals. > why r random signals called power signals. >
Well, first of all, this is the first time I can recall ever hearing of signals labeled like this. So, I would suspect the labeling "energy signals" and "power signals". Signals are signals as functions are functions as sequences are sequences. But maybe I've missed something along the way. Robert suggested something in adding "finite" to his definitions. Now that changes things a lot! Mind well. Oh darn! Now I find that folks have been using these terms. Here is one from Stanford at: http://www.stanford.edu/class/ee179/lecture7.pdf VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV 1. Energy and Power Signals An energy signal x(t) has 0 < E < 1 for average energy inf E = int[|x(t)|^2]dt -inf A power signal x(t) has 0 < P < 1 for average power inf P = lim [1/2T]*int[|x(t)|^2]dt T>inf -inf - Can think of average power as average energy/time. - An energy signal has zero average power. A power signal has infinite average energy. Power signals are generally not integrable so don’t necessarily have a Fourier transform. - We use power spectral density to characterize power signals that don’t have a Fourier transform. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Oh my ... and where did those 1's come from? I guess I'd have to do a bit of studying on this because it doesn't make any sense to me right off. I see an equation for E that doesn't involve time - OK. Then I read a description that says "average". I always thought that power was energy per unit time. Now we have "average power as average energy/time" Well the two aren't exactly inconsistent but power=energy per unit time is already an expression of an average isn't it? So now we have?: average power is average average energy??? For this to be the case, there have to be two time frames otherwise average average energy (aae) is what? aae=[1/T]*sum[(1/T)*sum|x(t)|^2] or [1/T^2]*sumsum[|x(t)|^2] T T T T I'm supposed to know better than to waste time on silly things. Here's another description from: http://www.signal.uu.se/Courses/CourseDirs/ModDemKod/2005/Lectures/lecture1/slides.ppt#14 A signal is an "energy signal" if, and only if, it has nonzero but finite energy for all time 0<Ex<inf: T/2 inf Ex =lim int[|x(t)|^2]dt = int[|x(t)|^2]dt T>inf -T/2 -inf A signal is a "power signal" if, and only if, it has finite but nonzero power for all time 0 < Px < inf: T/2 Px =lim (1/T)int[|x(t)|^2]dt T>inf -T/2 General rule: Periodic and random signals are power signals. Signals that are both deterministic and non-periodic are energy signals. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ It seems this is supposed to help when looking at the autocorrelation and spectral densities. So, OK. I learned something, I hope Shikha did too! Fred
Fred Marshall wrote:

   ...

> - An energy signal has zero average power. A power signal has infinite > average energy.
Huh? Power goes with the square of magnitude (into a resistive load). It is a positive number for all non-zero magnitudes. To have zero average power, a signal must be brief and averaged over all time, or everywhere zero. ... Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Fred Marshall wrote:


> A signal is an "energy signal" if, and only if, it has nonzero but finite > energy for all time 0<Ex<inf:
...
> A signal is a "power signal" if, and only if, it has finite but nonzero > power for all time 0 < Px < inf:
Hunh? again. A sinusoid fits neither of those verbal descriptions. ... Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
"Jerry Avins" <jya@ieee.org> wrote in message 
news:ieKdndZsCawKcf7fRVn-ug@rcn.net...
> Fred Marshall wrote: > > >> A signal is an "energy signal" if, and only if, it has nonzero but finite >> energy for all time 0<Ex<inf: > > ... > >> A signal is a "power signal" if, and only if, it has finite but nonzero >> power for all time 0 < Px < inf: > > Hunh? again. A sinusoid fits neither of those verbal descriptions. >
Jerry, First off, these were all "quotes" ...... and I was questioning the whole thing from the get go. Second, why doesn't a sinusoid have finite but nonzero power for all time? er.... if I know what that even means! Again, if power is energy per unit time then finite for all time (in chunks of time) seems OK to me. Fred Fred
Jerry Avins <jya@ieee.org> writes:

> Fred Marshall wrote: > > ... > > > - An energy signal has zero average power. A power signal has > > infinite average energy. > > > Huh? Power goes with the square of magnitude (into a resistive > load). It is a positive number for all non-zero magnitudes. To have > zero average power, a signal must be brief and averaged over all time, > or everywhere zero.
This is correct, Jerry, as I understand it. Any finite-temporal-extent signal is considered a finite-energy, zero-power signal because of the first reason you stated. -- Randy Yates Sony Ericsson Mobile Communications Research Triangle Park, NC, USA randy.yates@sonyericsson.com, 919-472-1124
Jerry Avins <jya@ieee.org> writes:

> Fred Marshall wrote: > > > > A signal is an "energy signal" if, and only if, it has nonzero but > > finite energy for all time 0<Ex<inf: > > > ... > > > A signal is a "power signal" if, and only if, it has finite but > > nonzero power for all time 0 < Px < inf: > > > Hunh? again. A sinusoid fits neither of those verbal descriptions.
Why don't you think a sinusoid has finite but non-zero power? When Fred stated "... for all time..." I'm assuming he means "when averaged over all time." -- Randy Yates Sony Ericsson Mobile Communications Research Triangle Park, NC, USA randy.yates@sonyericsson.com, 919-472-1124
Fred Marshall wrote:
> "Jerry Avins" <jya@ieee.org> wrote in message > news:ieKdndZsCawKcf7fRVn-ug@rcn.net... > >>Fred Marshall wrote: >> >> >> >>>A signal is an "energy signal" if, and only if, it has nonzero but finite >>>energy for all time 0<Ex<inf: >> >> ... >> >> >>>A signal is a "power signal" if, and only if, it has finite but nonzero >>>power for all time 0 < Px < inf: >> >>Hunh? again. A sinusoid fits neither of those verbal descriptions. >> > > > Jerry, > > First off, these were all "quotes" ...... and I was questioning the whole > thing from the get go. > > Second, why doesn't a sinusoid have finite but nonzero power for all time? > er.... if I know what that even means! Again, if power is energy per unit > time then finite for all time (in chunks of time) seems OK to me. > > Fred
Fred, I was questioning the quotes, not you. A sinusoid is sometimes positive, sometimes negative. At the instant of transition, it is zero, violating the condition "finite but nonzero power for all time". Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
On Wed, 20 Apr 2005 13:02:47 -0400, Jim Thomas <jthomas@bittware.com>
wrote:

>Jerry Avins wrote: >> I was reversing the direction of a three-phase motor, and I > >[snip] > >Motor-direction-reversal reminds me of this conversation between Dad and >one of his customers: > >Monroe: What are you doing? >Dad: I'm looking at the fan blades so I know which > way the motor wants to turn. >Monroe: Well I'm surprised you didn't know this, but > EVERY motor turns clockwise when viewed from > the front. You're wasting your time! >Dad: Monroe, that's not true. >Monroe: Sure it is! >Dad: What about reversible motors? >Monroe: Well, when the motor reverses, the other end > becomes the front! > >-- >Jim Thomas Principal Applications Engineer Bittware, Inc
Hi Jim, Ha ha ha. That made me laugh out loud. Thanks, I needed that. [-Rick-]
"Jerry Avins" <jya@ieee.org> wrote in message 
news:OuCdnb_fOamQ4PvfRVn-oA@rcn.net...
> Jim Thomas wrote: > > More power to those guys! (and I hope, no more power to me). I was > reversing the direction of a three-phase motor, and I thought I had killed > the power to the switch-box that controlled it. I had not. During the > course of my exertions, I got zapped with 440 volts from thumb to thumb. I > landed on the concrete floor about 12 feet from where I had been standing. > My ass and left shoulder were sore from the landing, and I had a bruise on > my chin where a knee had hit it, but no burns. > > Jerry > --
Jerry, You were undoubtedly lucky! When I was young and in ham radio I couldn't initially afford an electrically actuated T/R switch so I rigged up a DPDT open knife switch and fastened it to the table top. This was reasonably OK for CW because there was no transmitter power unless the key was down. Then I switched to AM. The normal procedure was: Switch would be in the receive position. Listen. Decide to transmit. Switch to transmit position. Turn on transmitter. Talk. Decide to listen. Turn off transmitter. Switch to receive position. Listen. Fine. Except a couple of times this happened (yes 2!): Turn on transmitter. Talk (with microphone in right hand) Decide to listen. [does not turn off transmitter as indicated above] Switch to receive postion with left hand. ZAP! Microphone flies across the table. RF burn on left index finger - which looks like a deep round hole about 1mm in dia.. Shoulders / upper body got shaken pretty good. Thank goodness the reaction got rid of the mic. I bought a real T/R switch! Fred
Jerry Avins wrote:
> I was reversing the direction of a three-phase motor, and I
[snip] Motor-direction-reversal reminds me of this conversation between Dad and one of his customers: Monroe: What are you doing? Dad: I'm looking at the fan blades so I know which way the motor wants to turn. Monroe: Well I'm surprised you didn't know this, but EVERY motor turns clockwise when viewed from the front. You're wasting your time! Dad: Monroe, that's not true. Monroe: Sure it is! Dad: What about reversible motors? Monroe: Well, when the motor reverses, the other end becomes the front! -- Jim Thomas Principal Applications Engineer Bittware, Inc jthomas@bittware.com http://www.bittware.com (603) 226-0404 x536 Sometimes experience is the only teacher that works - Mike Rosing
Jim Thomas wrote:

   ...

> My summer job during college was as an electrician's helper, my dad > being the electrician. Dad also did HVAC and appliances. It was a > small town, and there was another guy - Bob - who also did HVAC work. > Bob was a great guy, and he and Dad got along famously. > > Bob used the same meter as Jerry's uncle - leathery fingers. He would > use the index and middle fingers of the same hand, touching them to the > live wires at very nearly the same time. > > Dad and I were working at a house one day, and Bob and his son were > working at the house next door. We heard Bob yell in his distinctive > gravelly voice, "REX! GO GET ME THE @*#&$^ METER! I CAN'T TELL IF THIS > IS 110 OR 220!"
More power to those guys! (and I hope, no more power to me). I was reversing the direction of a three-phase motor, and I thought I had killed the power to the switch-box that controlled it. I had not. During the course of my exertions, I got zapped with 440 volts from thumb to thumb. I landed on the concrete floor about 12 feet from where I had been standing. My ass and left shoulder were sore from the landing, and I had a bruise on my chin where a knee had hit it, but no burns. Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Jerry Avins wrote:
> About wet fingers: I used to help my uncle moonlight wiring houses. He > must have had leather fingers. He would test a light socket by sticking > his finger into it. Sometimes he couldn't tell, so he would wet the > finger.
My summer job during college was as an electrician's helper, my dad being the electrician. Dad also did HVAC and appliances. It was a small town, and there was another guy - Bob - who also did HVAC work. Bob was a great guy, and he and Dad got along famously. Bob used the same meter as Jerry's uncle - leathery fingers. He would use the index and middle fingers of the same hand, touching them to the live wires at very nearly the same time. Dad and I were working at a house one day, and Bob and his son were working at the house next door. We heard Bob yell in his distinctive gravelly voice, "REX! GO GET ME THE @*#&$^ METER! I CAN'T TELL IF THIS IS 110 OR 220!" -- Jim Thomas Principal Applications Engineer Bittware, Inc jthomas@bittware.com http://www.bittware.com (603) 226-0404 x536 Sometimes experience is the only teacher that works - Mike Rosing
Good story, Jerry!  No sense of touch or sight, huh?  Wow, that must be tough!

"Jerry Avins" <jya@ieee.org> wrote in message
news:NMydna6qBYeGHfnfRVn-hQ@rcn.net...
> About wet fingers: I used to help my uncle moonlight wiring houses. He > must have had leather fingers. He would test a light socket by sticking > his finger into it. Sometimes he couldn't tell, so he would wet the > finger. Once, he asked me to test a socket for him because he wasn't > sure. I refused. He was odd in another respect, driving nails in light > so dim I couldn't see the nail. He claimed that he couldn't see the nail > in any light, so it didn't matter.
Fred Marshall wrote:

   ...

> Really, whenever have you considered a sinusoid to possibly be of zero > power? > I'll just wet my finger and touch the "mains" when the power is zero and > ....... > oops, I'm not fast enough. My unit of time was too long! :-) or =|:-o
Knowing what meaning must have been intended doesn't make the expression of that meaning accurate. What is fine in conversation -- even via usenet -- isn't rigorous enough for a textbook definition. As I wrote, my cavil was with the expression "all time", not with you. About wet fingers: I used to help my uncle moonlight wiring houses. He must have had leather fingers. He would test a light socket by sticking his finger into it. Sometimes he couldn't tell, so he would wet the finger. Once, he asked me to test a socket for him because he wasn't sure. I refused. He was odd in another respect, driving nails in light so dim I couldn't see the nail. He claimed that he couldn't see the nail in any light, so it didn't matter. Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
"Jerry Avins" <jya@ieee.org> wrote in message 
news:NO-dncpu_arwwfnfRVn-sw@rcn.net...
> Fred Marshall wrote: >> "Jerry Avins" <jya@ieee.org> wrote in message >> news:ieKdndZsCawKcf7fRVn-ug@rcn.net... >> >>>Fred Marshall wrote: >>> >>> >>> >>>>A signal is an "energy signal" if, and only if, it has nonzero but >>>>finite energy for all time 0<Ex<inf: >>> >>> ... >>> >>> >>>>A signal is a "power signal" if, and only if, it has finite but nonzero >>>>power for all time 0 < Px < inf: >>> >>>Hunh? again. A sinusoid fits neither of those verbal descriptions. >>> >> >> >> Jerry, >> >> First off, these were all "quotes" ...... and I was questioning the whole >> thing from the get go. >> >> Second, why doesn't a sinusoid have finite but nonzero power for all >> time? er.... if I know what that even means! Again, if power is energy >> per unit time then finite for all time (in chunks of time) seems OK to >> me. >> >> Fred > > Fred, > > I was questioning the quotes, not you. A sinusoid is sometimes positive, > sometimes negative. At the instant of transition, it is zero, violating > the condition "finite but nonzero power for all time". > > Jerry
Jerry, OK. Well, definitions are wonderful things aren't they? I took "nonzero power for all time" to mean the "average energy over a unit of time" for all time. So, as long as the unit of time isn't zero then the sinusoid's power isn't zero. Normally we use integer multiples of the period for the "unit of time" - as in rms. But, if the unit of time is long enough the "integer multiples" part becomes unimportant. Really, whenever have you considered a sinusoid to possibly be of zero power? I'll just wet my finger and touch the "mains" when the power is zero and ....... oops, I'm not fast enough. My unit of time was too long! :-) or =|:-o Fred
Randy Yates wrote:
> Jerry Avins <jya@ieee.org> writes: > > >>Fred Marshall wrote: >> >> >> >>>A signal is an "energy signal" if, and only if, it has nonzero but >>>finite energy for all time 0<Ex<inf: >> >> >> ... >> >> >>>A signal is a "power signal" if, and only if, it has finite but >>>nonzero power for all time 0 < Px < inf: >> >> >>Hunh? again. A sinusoid fits neither of those verbal descriptions. > > > Why don't you think a sinusoid has finite but non-zero power? When > Fred stated "... for all time..." I'm assuming he means "when averaged > over all time."
It may be what Fred meant, but the originator of the definition needs to be more careful or explain why not. Nitpickers of the world, unite! Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Fred Marshall wrote:
> "Jerry Avins" <jya@ieee.org> wrote in message > news:ieKdndZsCawKcf7fRVn-ug@rcn.net... > >>Fred Marshall wrote: >> >> >> >>>A signal is an "energy signal" if, and only if, it has nonzero but finite >>>energy for all time 0<Ex<inf: >> >> ... >> >> >>>A signal is a "power signal" if, and only if, it has finite but nonzero >>>power for all time 0 < Px < inf: >> >>Hunh? again. A sinusoid fits neither of those verbal descriptions. >> > > > Jerry, > > First off, these were all "quotes" ...... and I was questioning the whole > thing from the get go. > > Second, why doesn't a sinusoid have finite but nonzero power for all time? > er.... if I know what that even means! Again, if power is energy per unit > time then finite for all time (in chunks of time) seems OK to me. > > Fred
Fred, I was questioning the quotes, not you. A sinusoid is sometimes positive, sometimes negative. At the instant of transition, it is zero, violating the condition "finite but nonzero power for all time". Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯