scanning spectrum analyzers that measure power spectral densities use an electronically tunable filter. here, the actual filter center frequency is fixed. various portions of the signal spectrum are swept into the range of the filter by frequency translation that result when the input signal is multiplied by a sinusoidal signal whose frequency id slowly changing. with reference to figure 2.17, this sinusoidal waveform is generated by applying a ramp voltage to a voltage controlled oscillator. the sinusoidal voltage multiplies x(t) which shifts the spectrum past the bandpass filter the ramp effect is to move the signal spectrum past the bandpass filter the ramp voltage can also be used to sweep the beam of an oscilloscope, and the output of the integrator which is proportional to the pas of x(t) can be used to deflect the beam vertically.