Example 1: In this example, the function generators are producing two sinusoids at frequencies of 100 and 200 Hz. The sinusoids have the amplitudes of 2 and 1 V respectively. The output signal is displayed on channel 2, which hints the presence of two sinusoids. The output signal of a subtracting amplifier is given by

Substituting the resistor values, we find the following expression for the output signal:

Indeed, the amplitude spectrum displayed by the spectrum analyzer shows two peaks at 100 and 200 Hz of heights 0.83 and 0.5 V respectively. Because the spectrum analyzer can not display phase information, the minus sign in front of the first sinusoid is ignored.

Example 2: In this example, noise is added to a sinusoid. The spectrum analyzer shows the uniform noise spectrum and a single peak at 450 Hz corresponding to the sinusoid.