Start-up time is usually measured in microseconds (µS), but it is frequency dependent and controlled by the feedback loop. During this period there will be instabilities until the oscillations stabilize. Startup time is the period when an oscillator is first turned on. Changes in the phase angle will result in a change in the output frequency.
The exact frequency at which an oscillator will operate is dependent on the loop phase angle shifts within the oscillator circuit. In this type of crystal oscillator design, the filter consists of the crystal’s equivalent model and the external load capacitors. In Figure 1, we see a simplified feedback oscillator loop showing that a basic oscillator circuit consists of an amplification stage and a feedback network that acts as a filter that decides what the feedback loop gain should be.įigure 2 – Pierce Oscillator design using Processor with inverter and feedback resistor.įigure 2 shows a Pierce Oscillator design commonly used in digital processor designs. The gain in the feedback loop needs to be controlled to unity gain, otherwise the signal will be clipped and distorted.įigure 1 – Oscillator Feedback Loop Showing Conditions for Oscillation The signal is then amplified and looped back again causing the output signal to grow. Here a portion of the output voltage is feedback to the input with no net phase shift, so reinforcing the output signal. One way to accomplish this is by using a positive feedback loop. The principle behind the oscillator circuit is a stable steady state output signal. The process for making oscillator grade crystal blanks is time consuming, with many steps to ensure consistent high-quality product, but they offer vastly superior stability over RC oscillators. In the case of the quartz-based oscillator, the composition of the quartz material, and the angles that the crystal are cut makes this type of oscillator very precise and stable over a wide temperature range. They use mechanical vibration or tuned cavities to generate the clock signal. You may also find oscillators that use ceramic, SAW or MEMS based resonators to be the starting point for the operational frequency. The most prolific and best performing of these is quartz. Oscillators can have several different types of resonators associated with them. A square wave output signal has many uses but is extensively used to time the execution of instructions in a circuit or microprocessor. In practice, there will be some variation in duty cycle, so 45/55% or 60/40% may be more typical. An ideal square wave would have the minimum and maximum periods at equal duration, and this would have a 50/50% duty cycle. Square waves are a waveform that alternate from minimum to maximum amplitude at a steady frequency. In digital electronics we see square wave outputs. In RF communication, the oscillators sine wave output provides an accurate, low noise frequency reference to the transceiver. A sine wave is a continuous wave that represents a smooth periodic oscillation.
In analog applications such as RF radio transceivers that use a superheterodyne to receive and transmit signal chains, will typically find sinusoidal output waveforms. Other applications require a clock with very clean waveform and tight stability to produce high quality communications and data transfers.
Some applications require a basic clock to maintain operational intervals for processes. This signal can be in many forms depending on the application. International What is an Oscillator?Īn oscillator is an electronic circuit which generates a repetitive signal. Written by David Meaney, Vice President of Global Technical Sales and Marketing and Dean Clark, European Director of Operations at ECS Inc.