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need help in PWM and PPM theory

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suha

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Hello everyone...

i need to do some assignment about PWM and PPM theory..

my problem is i can not find any calculation, formula or equation that exactly describe the frequency modulation and sampling rate for PWM and PPM...

hope u can help me...
 
PWM and PPM are a constant frequency form of modulation, there is no frequency modulation. For the basics check Wikepedia for PWM and PPM.

The modulation sampling rate should be at least twice the highest modulation frequency. The higher the sampling rate, the easier it is to filter and recover the modulated signal.
 
Hello everyone...

i need to do some assignment about PWM and PPM theory..

my problem is i can not find any calculation, formula or equation that exactly describe the frequency modulation and sampling rate for PWM and PPM...

hope u can help me...

Hi there,


I assume you want the frequency domain representation as the time domain representation is rather simple.

For a pulse that starts out at t=0 and stays high until t=Tau at which time it goes low and is periodic with period T (in other words a PWM pulse train) the frequency domain representation would be:

F(s)=Vo/s*(1-e^(-s*Tau))/(1-e^(-s*T))

where Vo is the amplitude of the pulses.

The above is derived from the more general form:

F(s)=F1(s)/(1-e^(-s*T))

where

F1(s)=Laplace{(u(t)-u(t-T))*f(t)}

and that F1(s) is the transform of the first period of f(t).
 
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thank you everyone..

it this means that f(s) = 2 fmax ?

i'll try to understand the frequency domain representation that you are given as i'm not really good in Laplace transform ;(


is there any factor or component that will vary the output of PWM and PPM?

thank you so much for your help!

sorry because i'm not really understand PPM and PWM calculation and equation but i'll try it harder..
 
thank you everyone..

it this means that f(s) = 2 fmax ?

i'll try to understand the frequency domain representation that you are given as i'm not really good in Laplace transform ;(


is there any factor or component that will vary the output of PWM and PPM?

thank you so much for your help!

sorry because i'm not really understand PPM and PWM calculation and equation but i'll try it harder..


Hi again,

I am not really sure i understand your new questions, maybe you will have to restate them.
Maybe i did not understand your original question either because i am wondering why
you asked some of the new questions now. If the form was a surprise to you, then what
form of the equation did you expect to see here?

For example, to vary the PWM signal you would make Tau either shorter or longer (but
of course not longer than T). To vary PPM you would make T either shorter or longer
but not change Tau. I thought that was obvious as T and Tau specify the pulses.
 
hi suha, u can read the book"electronic communication systems" by kennedy or modern digital communication system by lathi. these contain detail information about pwm and ppm.
another thing, pwm and ppm are not belong to frequency modulation. in frequency modulation , frequency of carrier wave is varied with the amplitude of message signal. in pwm and ppm the frequency of carrier are kept constant.
and for effective modulation technique, sampling rate f(s)>= 2 fmax
 
thank you very much for your help.. :)

before this, im bit confusing about the frequency carrier and sampling rate for PWM and PPM..
i've searching in journal and Google about it but most of them only explaining frequency modulation PAM...

thank you so much for explaining about the frequency of carrier and the sampling rate..

:)
 
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