A signal m(t) with bandwidth 500 Hz is first multiplied by a signal g(t) where
$$g\left( t \right) = \sum\limits_{k = - \infty }^\infty {{{\left( { - 1} \right)}^k}\delta \left( {t - 0.5 \times {{10}^{ - 4}}k} \right)} $$
The resulting signal is then passed through an ideal low pass filter with bandwidth 1 kHz. The output of the low pass filter would be