Nuts and Volts - May/June 2018

These oscillators produce a very pure low distortion sine wave. However, the frequency is fixed at the point where each RC section produces a 60 degree phase shift. That approximate frequency is: f = 1/2.6RC In the circuit of Figure 2, the frequency should be about 3. 85 kHz.

Colpitts Crystal Oscillator

Quartz crystals are often used to set the frequency of an oscillator because of their precise frequency of oscillation and stability. The equivalent circuit of a crystal is a series or parallel LC circuit. Figure 3 is a very popular sine wave oscillator of the Colpitts type, as identified by the two-capacitor feedback network. This is another widely used circuit because it’s easy to implement and very stable. Its useful frequency range is approximately 100 kHz to 40 MHz. The output is a sine wave with a slight distortion.

By the way, if you need a crystal oscillator with a sine wave out, you can usually buy a commercial circuit. They are widely available for almost any desired frequency. They are packaged in a metal can and are the size of a typical IC. The DC supply is usually five volts.

Square Wave and Filter

An interesting way to produce a sine wave is to select it with a filter. The idea is to generate a square wave first. As it turns out, it’s often easier to generate a square wave or rectangular wave than a sine wave. According to Fourier theory, the square wave is made up of a fundamental sine wave and an infinite number of odd harmonics.

For example, a 10 kHz square wave contains a 10 kHz sine wave, and sine waves at the 3rd, 5th, 7th, etc., harmonics of 30 kHz, 50 kHz, 70 kHz, and so on. The idea is to connect the square wave to a filter that selects the desired frequency.

Figure 4 shows one example. A CMOS 555 timer IC produces a 50% duty cycle square wave. Its output is sent to a low pass RC filter that filters out the harmonics, leaving only the fundamental sine wave. Some distortion is common as it’s difficult to completely eliminate the harmonics. A more selective LC filter can be used to improve sine wave quality. Keep in mind that you can also use a selective band pass filter to pick out one of the harmonic sine waves.

This circuit is designed for a frequency of 1,600 Hz.

Direct Digital Synthesis

An interesting way to produce a sine wave is to do it digitally. Refer to Figure 5. It begins with a read-only memory (ROM) that stores a series of binary values that represent values that follow the trigonometry equation for a sine wave. These values are then read out of the ROM

one at a time and applied to a digital-to-analog converter (DAC). A clock signal steps an address counter that then

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Figure 3. A popular crystal oscillator that works every time.

Figure 4. The CMOS version of the 555 is recommended, but you can make this work with a standard 555 by eliminating the 100K resistor. Figure 5. Direct digital synthesis.