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6 March 2010
Author: Giorgos Lazaridis
High Frequency Function Generator with the Maxim MAX038

Worklog - The circuit (March 6 2009)

Another very important tool for an electronic techlab is a function generator. The one that i am preparing to use is based on the MAXIM MAX038. This chip is rather expensive, yet has many capabilities. It can provide 3 different types of waveforms, triangular, rectangular and sine wave, as well as it provides a TTL output with 50% duty cycle for synchronization reasons. The frequency can be adjusted from 0.1 Hz up to 20 MHz.

The circuit

The circuit is based on the circuit provided by Maxim. I've done some modifications of course to fit my needs. This is the schematic diagram:

The above circuit covers some of the most important features that a function generator should have. The frequency range can be selected by placing a different capacitor to the COSC input. A 12 positions rotary switch with 12 different capacitors is perfect for this reason. The output frequency can be selected from within the frequency range, by changing the R7 potentiometer. The R6 potentiometer is the fine adjustment for the frequency.

The DADJ input of the chip is the control for the duty cycle of the waveform. I have a switch to either disable it (50% fixed duty cycle), or enable the duty cycle adjustment. If enabled, the duty cycle can be adjusted by the R8 potentiometer.

The output waveform type can be selected from the "FUNCTION" switch. This is a 3-position rotary switch. The common of the switch is driven HIGH, while the two inputs of the chip, A0 and A1 are pulled LOW with 10K resistors. When both inputs are '0', the output is square wave. When A0 is '1', the output is triangular waveform, and when A1 is '1' and A0 is '0', the output is sine wave.

The output is buffered with an OP37Z OP-AMP. The peak to peak amplitude is controlled by the potentiometer R5. By default, the output waveform is symmetrical to the ground. A DC offset can be added to the output if the switch "OFFSET ENABLED" is closed. The DC offset can be controlled by the R22.