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January 29, 2010
about 5 years ago
the answer to this depends, in part, on whether or not you want to keep the original range still available. Replacing C8 (220nF) with a larger cap would help achieve the lower frequency but you might have a gap between the lower and upper frequency range if you make C8 too big. You could make C9 (2.2nF) larger as well but would be sacrificing the upper range.
An alternative would be to replace the freq switch with an off board rotary one that allows 3 or more selections instead of the two currently shown. Personally, I’d go with 5 positions (2 low, 2 existing, and 1 high) using caps such as 2200uF, 22uF, 220nF, 2.2nF, and 220pF. Note, the caps are multiples of 100 in values except for the upper one (since the existing board goes to about 240kHz and 1MHz is the stated limit). The high end might be a bit touchy with a smaller cap.
You can also use larger potentiometers to lower the frequency but this will make the frequency more difficult to adjust with precision. To get 0.01Hz, I doubt you could pick a potentiometer with a practical resistance value (would be several Meg I think) so best to stick with modifying the caps.
Just built this kit. Been using Radioshack 273-0332 15V 1000mA adapter as a power supply. Make sure you get the “L” plug to use with the power jack if you do the same.
As built per kit: low freq range was measured at 15Hz to 4.793kHz. high freq range was 996.8Hz to 240.8kHz. Upper end of upper frequency range was very difficult to control with precision. I suspect supplied potentiometers used audio taper instead of linear taper based on in-circuit measurements. Nevertheless, the generator appears to function well.
Might be better to use linear pots or place an additional 1K pot in series with P2 & P3 to provide extra-fine frequency adjustment on high-freq end though that would necessesitate some off-board components. I plan to re-mount this board inside an enclosure and move some of the components off-board anyway.
Optional resistors appear to be parallel with P2 and P3, thus would raise lower frequency limit but not raise upper frequency limit. Better to adjust C8 bigger and C9 smaller if trying to extend frequency range I’d think.
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