A ring oscillator is a device that generates periodic electrical signals that have a square waveform when it is powered. It’s made from semiconductor technology of digital logic gates. It’s made off the number of NOT gates that are connector in a chain manner and the last one connected to the first to make a ring. The output signal oscillates between 2 levels i.e., low and high. The output signal is fed back to the input of the first Not gate. The primary thing to remember is that the number of not gates has to be odd for the device to oscillate between high and low. The discussion below will discuss its construction, operation, and applications.
The odd number of NOT digital logic gates are connected serially or in a chain form. The last gate is connected to the first one so that the output is fed back to the input. It’s powered by a DC voltage of 5volts. The NOT gate is a digital logic otherwise known as an inverter. It’s made of a CMOS (complementary metal-oxide-semiconductor), which consists of complementary pair of p-type and n-type MOSFET (metal-oxide-semiconductor field-effect transistor). Capacitors may be connected between the output and ground to filter smoothen the output signal waveform. As an active device, the drain of PNP is supplied with DC voltage appropriate to it. The sources are connected together and form the output. The drain of NPN MOSFET is grounded. The not gate is one of the most common logic gates used to makes integrated circuits, microprocessors, and microcontrollers.
The ring oscillator works on the principle of capacitive delay switching of the transistor. There is a time delay from the time signal is applied to the gate terminal and the transistor turning on. This time delay is caused by capacitance that exists between the gate and the source. The parasitic capacitor is formed by an oxide layer (dielectric) between the gate and the source and the drain. The time period is needed to charge the parasitic capacitor after which it turns on the transistor. The more the not gates the more the delay time since they are connected in a chain manner. This off time generates a low signal output and the transistor ON time generates a high signal output.
The first NOT gate inverts signal say from high to low. Provided there is an odd number of NOT gates, the output will have to be inverted. The output of the last gate is fed as the input of the first one for another cycle and an oscillation result. The number of gates determines the delay time, this, in turn, determines the period of the output signal. The higher the gates the low the frequency and vice versa. Another key factor that determines frequency is the applied voltage magnitude. The high voltage causes fast switching as charging time is reduced. This makes frequency control be achieved by adjusting supply voltage. Voltage-controlled oscillators are of the ring type.
There are very many uses of this oscillator. They include;
- To make integrated chips (ICs) there are used in microprocessors, microcontrollers, and the computer world.
- In wireless communication- the high-frequency periodic waveform has used a carrier to transmit message signal wirelessly.
- Making voltage-controlled oscillators.
- To make jitters, a hardware random number generators.
- Digital electronics lab- to measure the effect of voltage and temperature on a chip.
The ring oscillator leverages the turn-on delay time of CMOS to generate the oscillating periodic signal. The key advantage of being able to control the frequency by adjusting voltage makes it a versatile device. Although it’s adversely affected by temperature changes and phase noise, the merits outweigh the demerits by far.