Each measurement is a tab in the app. The controls are set across the top, RUN SWEEP or CAPTURE starts it, and results appear as a plot plus readout tiles. A crosshair reads values off the plot.
Some terms used throughout:
THD (total harmonic distortion): the energy at whole-number multiples of the test tone (2×, 3×, …), relative to the tone. A perfect device has none.
THD+N (THD plus noise): everything that is not the test tone, including hum and hiss.
SINAD (signal to noise and distortion): the same quantity as THD+N, written as a positive number in dB. 80 dB SINAD equals −80 dB THD+N.
These numbers differ on real equipment. A Heathkit oscillator used for testing has THD of 0.0122 % and THD+N of 0.1296 %, a factor of ten apart, because its noise dominates. That is why the app shows more than one.
One page per measurement
FFT spectrum: level against frequency, harmonics, THD and SINAD.
Multitone: many tones at once, for a fast response and distortion check.
Frequency response: gain against frequency, normalized to a thru reference.