In this circuit, we use a HCMOS integrated circuit in order to build a voltage-controlled oscillator (VCO). Although this application seems quite unusual for an HCMOS IC, the circuit works perfect.
The VCO is based on 4 NAND gates (IC1). Actually, each NAND is used as a NOT gate, because one input in every NAND is connected to logic 1. The “logic 1” is derived from the VCO control signal, which is a “supply voltage” variable between 1.5 and 5V. Gates N1, N2 and N3 are cascaded, and assuming that there is a signal at pin 2 of gate N1, this signal propagates across the cascaded gates and reaches pin 8 of N3 after some delay. The delay depends on the "supply voltage" of IC1 which is used as the control signal. Pin 8 of gate N3 is connected to the input pin 2 of gate N1, and by this way a closed loop is formed. So, there is feedback which causes oscillations at a frequency where the feedback happens to be positive (the total phase difference produced by the loop, happens to be zero 0o or an integer multiple of 2π rad, which is equivalent to 0o).
Since the peak output voltage cannot be greater than the supply voltage, i.e. the “control” voltage, its level must be adapted to those at the remainder of the circuit which normally will be 5V. This is ensured by inverter N5, which is powered by a genuine 5V supply. Actually, due to the feedback resistor R1, the inverter is arranged as a linear amplifier. It is therefore, sufficiently sensitive to amplify positive signals between 1.5 and 5V adequately.
The characteristic of the VCO is reasonably linear from almost 2 to almost 28MHz. After some modifications, the HCMOS VCO may work also at lower frequencies, if the total number of identical gates is extended by an even number. That, will increase the total delay time, so that the frequency is lowered.