Function Generator: Resistance-Capacity Tuned Audio Oscillator with Lamp Amplitude Control (Hewlett, US 2,268,872, 1942)

The first Hewlett-Packard product: a test oscillator tuned by resistors and capacitors instead of coils. Two valve amplifiers feed their output back to their input through a series and a parallel R-C pair, which passes only one frequency in phase, so the circuit oscillates there. A second, negative feedback path through a small lamp holds the amplitude steady: as the swing grows the lamp warms, its resistance rises and the gain falls. One dial covers ten to one in frequency. Result: a sine-wave signal generator covering 20 to 20,000 cycles in three ranges, about 0.25 % distortion. Source: W. R. Hewlett, US 2,268,872 (1942). Unknown: valve types and component values. Picture: the patent drawing.