
Measurements (Audio and Video) 109
Figure
4.10 Turning a filter into an oscillator. Feeding the output from the basic
filter to its input via an inverting amplifier, using a pair of diodes for amplitude con-
trol,
gives a good sinewave. Switching the THD meter's bandwidth gives a 'virtu-
ally pure' third harmonic
buffered before feeding to, for example, an A-to-D converter, since the pin
7 to pin 1 path is part of the filter, and capacitive loading of even as little
as 30 pF at pin 7 may affect the filter response. A passive RC post filter is
also recommended to suppress the 10 mV pp (typical) clock feed-through
hash. At the other end of the