DSB power

[yousef_ob]

hello, plz could anyone solve this for me:
in a DSB modulation if we assume that the modulating signal is a power signal, why this integration is zero, plz i want the mathemetical prove for the attached equation
thanx very much

View attachment 37798

 

[yousef_ob]

sure m(t) is the modualting power signal

 

[yousef_ob]

C'mon i have asked a question it wasnt a joke or a story. plz is there anyone could solve that for me??!!
???

 

[Yevgenip]

Can you explain what are the parameters and what are the functions? The equation is not very clear.

 

[yousef_ob]

hello,
in the attached equation m(t) is a power signal - not an energy signal. we know that DSB modulation is defined by the equation m(t) cos (2*pi * fc*t). so that the power of this signal is the integration of the sequared equation over the period - i.e power = integration over period of (m(t) cos(2*pi* fc*t)^2. we know that cos^2(wc*t)= .5(1+ cos(2*wc*t))
then multiply this by m^2(t) and integrate over the period, then the first term in 0.5 * the power of the mudulating signal (m(t)). and the second term is zero i am asking why it is zero. i gonna attatch the equation again

View attachment 37823

 

[yousef_ob]

Hello,
I thought that i give u the time to think in my quwstion. but to this moment i didnt get a solution to this problem.
i hope someone will help me.
bye

 

[audioguru2]

Hi Yousef,
I haven't looked at math like that for about 40 years but I'll try answering your question.
It looks like it is calculating average suppressed-carrier (?) DSB AM modulated power. I think the 2nd part of the equasion is zero because it is the minimum power and the 1st part is the peak power. ;D

 

[yousef_ob]

thanx a lot. nice try!!
i need a mathemetical solution.

 

[ECET0purdue]

been awhile for me too but dont you have to change the cos to sin when you do integration. thus you would get 0.

 

[yousef_ob]

i think that you didnt noticed that the cos is multiplied by another time variable function so you cant integrate that simple. anyway thanx abt ur try

 

[yousef_ob]

thanx i solved it

 

[prateeksikka]

even if u omit the signal integral of cosine wave i.e cos(2*w*t) over one period of T is zero.
thats why it is zero!
;D

 

[yousef_ob]

hello,
fom the maths point of view this is forbidden because m(t) as shown is a function of time not a constant
bye ;D

 

[prateeksikka]

hi yousef!
i didnt understand what did u say!
why cant we integrate a time function?
i guess theres no problem in it.

 

[ECET0purdue]

ummmm back to the math point of view....if i recall right was that not the if 1 person can build a house in 3weeks then 3000 people can build it in 3 secs....
dam math..

 

[prateeksikka]

i guess i cited same example for audioguru when we were discussing the difference between science (physics) and maths more precisely
scientist and engineer,
scientist says its possible to build while engineers say NO!
;D

 

[audioguru2]

There aren't any scientist animals around anymore. They were replaced by marketing folks who know nothing about anything. ;D

 

[prateeksikka]

Strange boss!
ultimately its the market to the end user!
So strange of you.audioguru!
could u please post an answer for my query in Circuits/general requests/Frequency selection
please!
thanks!