Abstract: A means for obtaining an output signal which is the sum of the frequencies of two periodic input signals that may vary in amplitude and frequency over time. The apparatus, which provides means for realizing trigonometric functions of the form sin(?+?)=2 sin ? cos ??sin(???) or sin n?=2 sin(n?1)? cos ??sin(n?2)?, comprises three basic circuit elements including one or more analog multipliers, one or more envelope detectors, and a subtracter. A method is disclosed for generating a series of even and odd harmonics of a single continuously varying input signal using a plurality of cascaded harmonic generator circuits.