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Function generator features variable frequency

03 Oct 2014  | Adolfo Mondragon

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The Exar XR-2206 function-generator IC can produce square, triangular, and sinusoidal signals with low distortion. Its output frequency is inversely proportional to the components in an RC network, according to the formula F=1/RC.

Use a potentiometer as the resistor component to provide a frequency variation similar to a logarithmic scale. To change this behaviour, the manufacturer's data sheet recommends connecting a resistor network to a variable external voltage source. The voltage should be stable and vary from 0 to almost 3V.

Instead of using an external voltage, the circuit described here uses an internal reference voltage of approximately 3V at Pin 7 of the XR-2206. With this internal reference, the circuit requires no voltage regulators—not even in the power supply. The circuit requires a power supply with only a 12V, 500-mA centre-tapped transformer, a bridge rectifier, and two filter capacitors (figure 1). You can define the frequency equations using figure 2 as a reference.

When VX is 0V, you determine the frequency using F=1/RC. The current trough, IR, equals 3/R, where 3 is the voltage reference in Pin 7. From this equation and resolving the reciprocal of R, you define the frequency as IR/3R=1/R, as a function of the current, F=IR/3C.

When VX>0V, you define the current as IR=(3−VX)/R. Replacing IR from the previous equation, you can define the frequency as a direct function of the voltage: F=(1/3RC)(3-VX).

Figure 1 shows the final circuit to generate the waveforms. The circuit's frequency ranges from 1Hz to 100kHz in five scales. The rotary switch lets you select the scale by switching in a set of capacitors.

About the author
Adolfo Mondragon is with Electrolux Products.

This article is a Design Idea selected for re-publication by the editors. It was first published on February 17, 2011 in

To download the PDF version of this article, click here.

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