The square wave (pulses) is coupled to the sine wave signal via a capacitor, resulting in a noisy output signal.
Because a capacitor's voltage is proportional to the *rate of voltage change*, combining one with the rising- and falling-edges of a square wave results in voltage spikes. These are superimposed on the sine wave - typically an undesired affect.
Capacitive coupling can be reduced by decreasing input (signal) resistance or the capacitance coupling the pulses to the signal.
