True RMS converter - requiring no special components

7-28 — elektor july/august 1982 A true RMS converter can be a very complex circuit requiring high tolerance components and precision calibration. It is fair to say that such a circuit would give a very high performance. The RMS converter here, however, consists entirely of readily available components and yet provides a very acceptable performance. The circuit diagram shows that the RMS converter really is an automatic gain control (AGC) amplifier circuit, which is constructed around 2 ICs, the well-known XR 13600 (Al, A2) and the XR 1458 (A3, A4). The circuit adjusts its gain so that the A.C. power of amplifier Al remains constant. This output level is monitored by the squaring amplifier formed by A2 and the average value is compared to a reference voltage with the aid of A3. The output of this amplifier provides the diodes of Al with bias current, via a 2 kS2 resistor and transistor T1, in order to attenuate the input signal. As mentioned before, the output power of Al is held...