Spectrum of a Rectangular signal?

Question

I would be grateful if you can help me with i) ii) and iii) especially with ii and iii

Answer 1

Define $T=2$ms and write down the definition of the Fourier transform:

$$X(j\omega)=\int_{-\infty}^{\infty}x(t)e^{-j\omega t}dt=\int_0^{T}e^{-j\omega t}dt=-\frac{1}{j\omega}(e^{-j\omega T}-1)=\frac{1}{j\omega}e^{-j\omega T/2}(e^{j\omega T/2}-e^{-j\omega T/2})=e^{-j\omega T/2}T\frac{\sin(\omega T/2)}{\omega T/2}$$

The maximum value of $|X(j\omega)|$ is at $\omega=0$ and its value is $T$. The zeros of $X(j\omega)$ are are the zeros of $\sin(\omega T/2)$ apart from $\omega=0$, i.e. $\omega=2\pi k/T$, $k=\pm 1, \pm 2, \ldots$