Sine wave generator using square wave input - delivers integral of sum of fundamental square wave and its first, third and seventh harmonics

The circuit comprises an operational amplifier (OP) with its non-inverting input to ground and a capacitor (6) feeding back from its output (11) to its inserting input. The amplifier thus functions as an integrator with an associated input resistance network. The network comprises three resistors (RO,R1,R2) connected in parallel to an input voltage (Ue). Two of the resistors (R1,R2) are selectively switched in and out of circuit by switches (S1,S2). The input signal (Ue) is a square-wave voltage of frequency, fo. One switch (S1) is switched as a function of 2fo and 8fo, namely 2fo EX-OR 8fo. The other switch (S2) is controlled by 4fo EX-OR 2fo. The resistors are weighted in ratios 4:2:1. Thus, the output is an approximation to a sine wave, made up of varying angle ramps, which are a function of the integral of the input applied to the non-inverting input (10) of the operational amplifier.