After Calibration the amplitude was stable within some few percent (over the frequency range). The picture on the left shows the version with two antiparallel diodes (1N4148), whilst the one on the right side uses two 5.1 V Z-diodes (antiseries) to stabilise the amplitude. Please note the difference in amplitude, caused by the different diodes !





Both versions covered the calculated range of 300 Hz ... 3 kHz without any tuning or optimisation. From the graphs it is clearly visible, that harmonic distortion takes place. If it starts becoming visible , a THD of 1% is a good assumption (Klirrfaktor).

That's the place, where Bill Hewlett's idea of using a light bulb for amplitude stabilisation kicks in. In his Master Thesis (late 1930's, Stanford University) he proposed this idea for the first time. (We know what happened after :-)

VARIABLE FREQUENCY OSCILLATION GENERATOR, US PATENT 2268872

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