Sampling: Multiplication by Square Wave

[cepheid]

We are considering a system in which the input signal x(t) is multiplied by a periodic square wave s(t) in order to produce an output w(t). The input signal is band limited with |X(j\omega)| = 0 \ \ \textrm{for} \ \ |\omega| \geq \omega_M , where \omega_M is the bandwidth. We are supposed to find (given a certain width of the periodic square wave, e.g. T/3), the maximum value of T (in terms of \omega_M ) for which there is no aliasing among the replicas of X(j\omega) in W(j\omega) .

I do not know how to approach this problem. This is not simple impulse train sampling. It is not zero order hold sampling. In fact...what the hell is this? Multiplication by a square wave?!?!? Sorry, I don't know where to start.

[berkeman]

Multiplication by a square wave is a form of modulation. Start off multiplying by a sine wave instead, and work out the images with sum and difference math. Then consider what a square wave's spectra looks like....