Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

u(-t) = or not = to - u(t) ?

Status
Not open for further replies.

wcz

Member level 2
Joined
Jul 17, 2004
Messages
49
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,286
Activity points
582
according to the subject, could anyone show me a proof on that?
thanks.
 

Jayson

Full Member level 4
Joined
Oct 8, 2001
Messages
234
Helped
14
Reputation
28
Reaction score
5
Trophy points
1,298
Location
Brazil
Activity points
2,360
This isn't really a mathematic proof, but does explain.

u(-t) is always 0 for all positive values of t, and 1 for all negative values of t.
-u(t) is always -1 for all positive values of t, and 0 for all negative values of t.

- Jayson
 

freewilly30

Member level 3
Joined
Mar 5, 2005
Messages
59
Helped
2
Reputation
4
Reaction score
1
Trophy points
1,288
Activity points
1,741
this is not an even function to say that u(t)=u(-t)
revise the plots of those functions.....
you will find the answer yourself..
 

amraldo

Advanced Member level 4
Joined
Aug 29, 2004
Messages
1,185
Helped
145
Reputation
290
Reaction score
40
Trophy points
1,328
Location
Egypt
Activity points
5,880
in general:
f(-t) = -f(t) in case of odd functions only & u(t) is not an odd function.
 

suvendu

Full Member level 3
Joined
Oct 10, 2004
Messages
169
Helped
16
Reputation
32
Reaction score
3
Trophy points
1,298
Activity points
2,121
1>in case of u(-t),the independent variable is reversed but in case of -u(t),the function itself reversed.
2> unit step function is not an odd function.so we can not write u(-t)=-u(t)
3>if u plot these functions independtenly then u will see that u(-t) lies in second quadrant and -u(t) lies in fourth quadrant.
 

tantoun2004

Member level 1
Joined
Jan 23, 2005
Messages
41
Helped
2
Reputation
4
Reaction score
0
Trophy points
1,286
Activity points
454
Re: u(-t) = or not = to - u(t) ? the proof u need

Hi,
This relation is not correct.
The proof is very easy:
write u(t)=1 as t>0 ,0 as t<0
put -t instead of t you get:
u(-t)=1 as -t>0 i.e. t<0 and 0 as -t<0 i.e. t>0
So we get that:
u(-t)= 1 as t<0 and 0 as t>0
while -u(t)= -1 as t>0,and 0 as t<0
so it's evident that u(-t) is not equal to -u(t)
Regards,
 

sxg

Junior Member level 3
Joined
Apr 4, 2005
Messages
26
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
1,506
according the opinion of Singal and System,the y=u(-t) is roated the singal
with the axis of y,but the y=-u(t) is roated with axis of t.
 

the_jackal

Member level 4
Joined
Dec 15, 2004
Messages
71
Helped
1
Reputation
2
Reaction score
0
Trophy points
1,286
Activity points
726
Consider the four quadrants that we study with reference to trignometry.
For y(t) = u(t) ---> sketch is in the 1st quadrant
y(t) = -u(t) ----> sketch is in the 4th quadrant i.e. reflection of u(t) about the x-axis
y(t) = u(-t) ----> sketch is in the 2nd quadrant
y(t) = -u(-t) ----> sketch is in the 3rd quadrant.

Hope this visualization was helpful!
 

Highlander-SP

Member level 3
Joined
Sep 21, 2005
Messages
66
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,286
Activity points
1,908
it's a case of odd function, like u(t)=t:
for t=1
u(-t)= u(-(1)) = -1
-u(t)= -(1) = -1

for t= -1
u(-t)= u(-(-1)) = u(1) = 1
-u(t)= -u(-1) = -(-1) = 1

for t= 1/2
u(-t)= u(-(1/2)) = -1/2
-u(t)= -(1/2) = -1/2

and so on....
 

A.Anand Srinivasan

Advanced Member level 5
Joined
Oct 15, 2005
Messages
1,804
Helped
257
Reputation
514
Reaction score
39
Trophy points
1,328
Location
India
Activity points
10,680
they are not equal because u(-t) is on the left side and has a value 1 throughout but -u(t) is on the right side and a value of -1 throughout
if you mean to say in non signal terms then it is equal only for odd functions
 

afti_khan

Member level 2
Joined
Oct 29, 2005
Messages
53
Helped
2
Reputation
4
Reaction score
2
Trophy points
1,288
Activity points
1,697
in case of even function, it's symmetrical abt the y-axis and odd function is symmetric abt the origin
 

vivek_raj_verma

Junior Member level 2
Joined
Aug 28, 2005
Messages
22
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
1,482
Hi,
If u want a proof. I think the proof given by Tantoun2004 is correct. Otherwise there is no doubt that they r diff. as u can see them from their plot easily.

vicky
 

alizadeh_m

Junior Member level 2
Joined
Oct 29, 2005
Messages
24
Helped
1
Reputation
2
Reaction score
0
Trophy points
1,281
Activity points
1,475
u(-t) = - u(t) if u(t) is odd. But no one sided function can be odd!
 

hamidr_karami

Banned
Joined
Oct 17, 2005
Messages
77
Helped
4
Reputation
8
Reaction score
3
Trophy points
1,288
Activity points
0
hi

u(t) != u(-t)

beacuse u(t) is not an evev function => u(t)!=u(t)

u(t) = 1-u(-t)

bye[/img]
 

jawadasheikh

Junior Member level 3
Joined
Nov 10, 2005
Messages
28
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Location
pakistan
Activity points
1,493
it depends on the nature of the functon.in case of odd funstion u(-t)=-u(y) and vice versa.first you should check for even or odd nature.
 

Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top