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Digitally Controlled Sine-Wave Generator

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Description

A similar version of this article appeared in the May 15, 2003 issue of EDN magazine.

The circuit of Figure 1 produces an accurate variable-frequency sine wave for use as a general-purpose reference signal. It includes an 8th-order elliptic, switched-capacitor lowpass filter (IC3) that is clocked with a 100kHz square wave generated by microcontroller IC2. (Any other convenient squarewave source is also acceptable.) The microcontroller is clocked by a 10MHz oscillator module. A voltage supervisor (IC1) ensures correct operation in the event of a power failure. IC3 sets the filter's cutoff frequency at 1/100 the clock frequency.

Figure 1. By removing harmonics from a square wave, this circuit generates an accurate and adjustable sine-wave output.

Figure 1. By removing harmonics from a square wave, this circuit generates an accurate and adjustable sine-wave output.

The 8th-order elliptic filter's sharp rolloff sharply reduces the harmonic amplitudes in a 1kHz square-wave input, thereby producing a near-perfect 1kHz sine wave at its output. Using divider-chain logic or a processor, you can then create a digitally adjustable sine-wave source by adjusting the clock and input frequencies while maintaining a ratio of 100:1 between them.

To prevent clipping at the positive and negative peaks, attenuate the input signal and superimpose it on a dc level of VCC/2. The result (for a 5V input) is a 2.25V peak-to-peak output.

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Designed, Built, Tested

Board pictured here has been fully assembled and tested. Not available for sale.

 

Maxim Devices (2)

Part Number Name Product Family Order Design kits and evaluation modules
MAX7400 8th-Order, Lowpass, Elliptic, Switched-Capacitor Filters Analog Filters Buy Now Not Available
MAX809 3-Pin Microprocessor Reset Circuits Voltage Supervisors Buy Now Not Available

Maxim Devices (2)

Part Number Product Family
Analog Filters
8th-Order, Lowpass, Elliptic, Switched-Capacitor Filters
Not Available
Voltage Supervisors
3-Pin Microprocessor Reset Circuits
Not Available

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