The thermionic valve oscillator generated continuous waves at a single frequency. It used the Triode – that is the one with three electrodes, Cathode, Grid and Anode. In practice, there is a fourth – the Heater. The cathode has to be heated - hence thermionic valve.
Up to that time, electrical sparks were used to generate a broad range of frequencies of what much later became known as radio communication. Digital data was added by interrupting the spark between coded long and short bursts.
Using positive feedback and the triode as the energy pump, the arrangement provides a sine-wave output at the resonant frequency of the parallel capacitor and inductor (coil) tuneable circuit. In the circuit at (a), the feedback is by magnetically coupling the inductor in the anode output circuit to the tuned circuit at the input. In the circuit at (b), positive feedback is via the capacitor Cb.
Several individuals discovered the circuit independently in about 1912, notably Reginald Fessenden*, Alexander Meissner, H.J. Round and Lee de Forrest** as well as others.
The arrangement at (a) is by Edwin Armstrong*** and at (b) by Edwin Colpitts. Both arrangements where the most widely used in the early years.
* Fessenden, is credited with the invention of amplitude modulation. He had the idea of multiplying a high frequency sine wave with lower frequency sine waves. It thus became possible to send voice messages and music over the radio signal.
** Lee de Forrest is credited with adding a grid between the anode and cathode of a thermionic diode resulting in the invention of the triode voltage amplifier. By modulating the voltage potential between cathode and grid, it was possible to modulate the current between cathode and anode and thus control the output voltage across a load.
***See a later post.
Bob Collins is our lead systems engineer mentor at Sentio Voci Ltd.
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