Two NE566Vs form a low frequency FM generator

As shown in the figure, two NE566Vs are used to form a low-frequency FM generator. In the figure, ICl NE566V is used to generate the modulation signal, and IC2 works on the carrier signal. Select Ctl to determine the modulation frequency range, select Ct2 to determine the center frequency of the carrier, and Sl can be used to select square wave and triangle wave for output modulation to achieve square wave modulation and triangle wave frequency modulation.