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The equivalent circuit of a crystal is a series or parallel LC circuit.
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Quartz crystals are often used to set the frequency of an oscillator because of their precise frequency of oscillation and stability. In the circuit of Figure 2, the frequency should be about 3.85 kHz. However, the frequency is fixed at the point where each RC section produces a 60 degree phase shift. These oscillators produce a very pure low distortion sine wave. The pure output needs to be buffered with an op-amp follower if you are going to drive a load. A fixed frequency is a disadvantage, but for a single frequency is good. Figure 2 shows one popular variation.įIGURE 2. The amplifier can be a single transistor, single op-amp, or multiple op-amps. There are multiple variations of phase shifters, including a Twin-T RC network and cascaded RC high pass sections that produce either 45 or 60 degree shifts in each stage. Setting the gain of the amplifier to offset the RC network attenuation will produce oscillation. Phase-Shift OscillatorĪ popular way to make a sine wave oscillator is to use an RC network to produce a 180 degree phase shift to use in the feedback path of an inverting amplifier. Output amplitude depends on the power supply voltages. This circuit works and has a frequency of about 1,592 Hz. Other more elaborate schemes use an FET as a variable resistor to vary the gain. One working example is to make R2 390 ohms and R1 a type 327 bulb. If the output goes down, the current decreases, lowering the resistance and increasing the gain to keep the output constant. As the output goes up, the bulb current and resistance increases, and reduces the gain to compensate. Some compensation components are usually added to eliminate this problem.Ī simple solution is to replace R1 with a small incandescent bulb whose resistance changes with current. Its problem is that instabilities in the gain and phase can cause the circuit to go out of oscillation completely, or go into saturation producing a clipped sine wave or square wave. This circuit works great and produces a very clean low distortion sine wave. With a net closed loop gain of one, the circuit oscillates at a frequency determined by the values of the RC network: An op-amp is used to produce a gain of three that offsets the attenuation of the RC network. It uses an RC network that produces a zero degree phase shift from output back to the input, producing positive feedback that, in turn, produces oscillation. Frequency can be varied by using pots for R and using different values of C switched in. Wien Bridge OscillatorĪ popular low frequency (audio, and up to about 100 kHz or so) sine wave oscillator is the Wien bridge shown in Figure 1.įIGURE 1. This article presents some popular methods you should be familiar with.
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As it turns out, there are literally dozens of ways to generate a sine wave. Audio, radio, and power equipment usually generates or processes sine waves. Many electronic products use signals of the sine wave form. The sine wave is a naturally occurring signal shape in communications and other electronic applications.