Frequency spectrum of the modulated signal g(t)

  • Thread starter raymond23
  • Start date
1. The problem statement, all variables and given/known data

Let the baseband signal be
s(t)=cos(2πfst+π3), where fs=5kHz. Radio carrier is
c(t)=sin(2πfct), where fc=100MHz. Using the amplitude modulation of g(t)=(1+s(t))c(t), what is the frequency spectrum of the modulated signal g(t)?
What are the amplitude and phase shift of each frequency component in g(t)?

2. Relevant equations

g(t)=(1+s(t))c(t)


3. The attempt at a solution

g(t)= (1+ cos(2πfst+π3)) sin(2πfct)
g(t)= sin(2πfct)+ cos(2πfst+π3) sin(2πfct)
g(t)=
 

Redbelly98

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g(t)= sin(2πfct)+ cos(2πfst+π3) sin(2πfct)
Looks good so far. The first sin(...) term gives you one of the frequency components.

You'll need to express "cos(2πfst+π3) sin(2πfct)" as the sum of distinct, single-frequency sin and/or cos terms. You can do that using these trig identities:

sin(x + y) = sin(x)·cos(y) + cos(x)·sin(y)
sin(x - y) = sin(x)·cos(y) - cos(x)·sin(y)

p.s. welcome to PF :smile:
 
I try this
g(t)= sin(2πfct)+ cos(2πfst+π3) sin(2πfct)
g(t)= sin(2πfct)+ (cos(2πfst)+cos(π3)) sin(2πfct)
g(t)= sin(2πfct)+ (cos(2πfst)sin(2πfct)+cos(π3)sin(2πfct))
but don't know how to continue
 

Redbelly98

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I try this
g(t)= sin(2πfct)+ cos(2πfst+π3) sin(2πfct)
g(t)= sin(2πfct)+ (cos(2πfst)+cos(π3)) sin(2πfct)
There's a problem there, because
cos(2πfst+π3) and cos(2πfst)+cos(π3)​
are not equivalent.

If you add the two equations in my earlier post, you'll have an expression for
sin(x) cos(y)​
which will be useful here.
 

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