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Hints for Midterm
Problem I.
Use the following facts:
- Plug in the formulae for a0, ak and bk in the equation for sK(t).
- cos(a)cos(b)+sin(a)sin(b)=cos(a−b).
- Integration and summation can be interchanged.
II. Properties of Fourier Series
Hints for various parts.
- P 2. The definition of periodicity: there exists a T such that s(t)=s(t+MT) for all M. The smallest such T is the period.
- P. 3. The proof is similar to that for the multiplicative property of Fourier Series. The multiplication property for periodic signals is as follows: Given two signals x(t) and y(t) with period T, and given that the Fourier Series coefficients of x(t) and y(t) are Xk and Yk respectively, the FS coefficients of z(t)=x(t)y(t) are ∑∞l=−∞Zk=XlYk−l. The proof is simple:
Zk=1T∫T0z(t)exp(−j2πktT)dt=1T∫T0x(t)y(t)exp(−j2πktT)dt
Writing x(t) in its Fourier Series form: x(t)=∑∞k=−∞Xkexp(j2πktT), we get
Zk=1T∫T0∞∑l=−∞Xlexp(j2πltT) y(t)exp(−j2πktT)dt
Since Xl is not a function of t and integration and summation can be interchanged, we can write
Zk=∞∑l=−∞Xl1T∫T0exp(j2πltT) y(t)exp(−j2πktT)dt=∞∑l=−∞Xl1T∫T0y(t)exp(−j2π(k−l)tT)dt=∞∑l=−∞XlYl−k
- Remember that if x(t) is even, x(t)=x(−t). If Xk is real, Xk=X∗k (The ∗ represents complex conjugation).
- Similarly, if x(t) is odd, x(t)=−x(−t). If Xk is imaginary, Xk=−X∗k
P. III. Examples of Fourier Series
The following properties will be useful:
- Time shift: if x(t)→Xk, x(t−τ)→Xke−j2πkτT