THD meter circuit needed

[terry michaels]

Hi list members:

Recently I have been working on a very low distortion triangle to sine
converter, and since I don't own a distortion analyzer, I designed and
built a notch filter to remove the sine fundamental and leave only the
harmonics.  The filter is centered at about 159 Hz, with a notch depth of
at least 80 dB.  To use this filter, set your VCO to 159 Hz, apply the sine
output from the VCO to the input of the filter, and observe the filter
output with an oscilloscope or sensitive AC voltmeter.  I use a Tektronix
7603 scope with a 7A22 differential amplifier plug-in to monitor the
distortion.  This is obviously not as good as a true distortion meter, but
it seems to work well enough for optimizing the usual type of VCO with
triangle/sine conversion.  I can measure distortion to less than 0.02%, and
circuit noise doesn't seem to be noticeable.   Feel free to email me if
anyone has questions or comments.  

The Q adjustment pots seem to work best when set to about 2/3 of maximum,
the actual set point is not critical.

The filter schematic is available for download at
http://ourworld.compuserve.com/homepages/towersites/notch.gif

Terry Michaels