400kHz function generator with ICL8038

Function generator with adjustable frequency from 0 Hz to over 400 kHz, adjustable amplitude, DC offset, duty, and of course the function selection – square, triangle, and sine. Generator based on good old ICL8038 integrated chip generator that gives pretty good shaped signals as for amateur purposes.
This circuit has been designed a little differently than ICL’s note or other similar circuits are suggesting. I tested a bunch of different configurations with different peripherals and chosen the best – so to get good wave shape at 400kHz. I got rid of some of the elements, I added my own solutions. The two ICL chips that I have can oscillate around up to 420-430kHz, and practically we can get good waveforms up to that frequency.
400kHz function generator with ICL8038The biggest problem of these circuits generators is strong distortion on sine wave output pin 2 at frequencies above 100kHz – so most of the solutions based on this chip has a limited frequency. I found a workaround by not putting the load on the square wave output from pin 9 (open collector that needs to be pulled up to the power supply). In some way the quality of other waveforms is dependent on the load on this pin. Pull-up resistor is activated only when switching output to the square waveform – with the same switch. I got rid of the recommended in the note 10M pot to “improve” the vawe forms. As it turned out its only making them worse, and add a characteristic spout on the top of the upper half of the sine.
Square output pin 9 with open collector, normally would never be able to give a good square wave at these frequencies. Rising edge depends only on the pull-up resistor and its growing very slowly. If we give too strong pull-up resistor, in turn, the trailing edge will be weak because the internal transistor is too heavily loaded.

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