Penguin Mint Function Generator

This little project arose from the need for a small, solid-state, relatively disposable audio-frequency sine-wave generator. The Exar XR-2206 Monolithic Function Generator was a shoe-in choice, particularly since I had a number of them lying around.

It took some searching for a suitable enclosure before the realization hit that an Altoids tin was the perfect size. About two seconds later a Penguin Caffeinated Peppermints tin was substituted, and the project was underway. (Be advised that those links lead to some thoroughly obnoxious site design.)

The circuit is copied from Figure 12 of the Exar datasheet , "Circuit for Sine Wave Generation with Minimum Harmonic Distortion". A 7812 voltage regulator was added to permit operation with a variety of 'wall wart' transformers. The board layout was designed in Cadsoft Eagle to fit on a small protoboard, Radio Shack part # 276-148. I prefer to do a full board layout in advance for perfboard projects; it usually speeds construction, and the final product is neater and more reliable. If the project ends up being useful enough to copy, a PCB run is trivial. For layouts destined for perfboard, it's a good idea to arrange component pins on a .1" grid.

This circuit works well, and does seem useful enough to copy, so here's the board layout:

This layout does not include the power indicator (an LED and 1K resistor in series on the regulated 12V side), and depicts DIP headers for connection to the outboard components. I opted to solder the connections directly for greater reliability, and also because there was no room for the headers--the box ended up being a very tight fit.

A 1-meg pot for timing resistor R, combined with a value of .1uF for timing capacitor C, resulted in a frequency range of 14.7Hz to 14.28KHz. A 25K pot was found to allow maximum amplitude before clipping occurred. Peak to peak output is 11V (non-adjustable) for the square-wave function; 7V for triangle; and 4V for sine, flat across the spectrum with minor distortion of the square wave above 10KHz.

Frequency adjustment is very non-linear in the upper range, making precise tuning difficult. A log-taper pot may compensate for this to some degree. I plan to add a multi-turn trimpot in series for the time being. Note that scale linearity was not of particular concern since the XR2206 is not a reliable frequency standard to begin with. The generator was intended for use in conjunction with external measurement equipment, hence the lack of any front-panel markings.

For power I used a generic "wall wart" style 12VDC transformer. An actual 12V supply would not be sufficient for proper operation of the 7812 regulator, but this transformer measured around 18V even under load. Cheap wall warts are notorious for their poor regulation.

Contact: reboots at g-cipher.net
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