Phil Hobbs wrote:


One simple method is to wire up a comparator to turn the internal
oscillator signal into a square wave.

** One needs a test that shows the instrument is doing the things a THD analyser is meant to.

1. Thoroughly remove the fundamental frequency.

2. Pass other frequencies up to 100kHz to the metering circuit.

Frequency sweeps from 20Hz to 100kHz will verify this - with the instrument set to notch a variety of spot frequencies in the audio band.

The notch should be sufficiently narrow at each test point that little attenuation exists at half and double the centre frequency and beyond up to a -3dB point at 100kHz.


.... Phil