As an electronic signal passes through a filter, the amplitude of the signal is reduced by an amount depending on the frequency of the signal. The effectiveness of an electronic filter is specified using its amplitude response, the ratio of the output voltage amplitude to the input voltage amplitude at various frequencies. You will find several names for this ratio in the technical literature: transfer function, amplitude spectrum, and amplitude response. The phase spectrum or phase response tells how the phase of the output signal varies with frequency. The two functions together, amplitude and phase, describe the overall

frequency response-the output response of the circuit to input signals of various frequencies.