The rectangular oscillator shown in the figure makes use of one of the four NAND ports of an integrated circuit 4093 and is characterized by being able to be modulated in frequency. A signal applied to the BF245 field effect transistor (FET) gate changes the resistance of the capacitor's charge and discharge circuit that determines the frequency of the signal produced. The circuit can generate signals ranging from a few hertz to close to 4 MHz with a 10 volts supply. The modulator signal can vary from -1 to +6 V in amplitude. The 10 k ohms resistor can be changed depending on the depth and the desired modulation range. The supply to the circuit can be done with voltages from 3 to 15 volts and the generated signal has a rectangular wave shape.
