I found on Wikipedia that:

“In 1901, Reginald Fessenden demonstrated a direct-conversion heterodyne receiver or beat receiver as a method of making continuous wave radiotelegraphy signals audible. Fessenden's receiver did not see much application because of its local oscillator's stability problem.

A stable yet inexpensive local oscillator was not available until Lee de Forest invented the triode vacuum tube oscillator. In a 1905 patent, Fessenden stated that the frequency stability of his local oscillator was one part per thousand.”

How could they make sustained oscillation before they had the triode?

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    \$\begingroup\$ High frequency alternators were one way. \$\endgroup\$
    – Kartman
    Jan 23 at 12:43
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    \$\begingroup\$ The Poulsen Arc was another way (possibly a bit later than 1901) \$\endgroup\$ Jan 23 at 16:20
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    \$\begingroup\$ Really old automation systems actually use mechanical clocks to drive digital logic (relay logic). A pendulum clock has the pendulum make a full swing every 1 second. But you can put electrical contacts on both sides of the pendulum thus giving you a clock every 0.5 seconds = 2Hz. The clock could also be configured to generate pulses every minute, every 5 minutes, every hour, every 12 hours, every day etc. The mechanical clock is used to both synchronize processes to time (eg, time-lock a bank vault) and also to drive the logic behind the automation. \$\endgroup\$
    – slebetman
    Jan 24 at 2:43
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    \$\begingroup\$ @slebetman That's still a commonly used trick in electronics, clock doubling. It's what gave the name Double Data Rate to DDR RAM and is more or less how a CPU multiplier works. Those two systems are very important in modern computing electronics. \$\endgroup\$
    – Mast
    Jan 24 at 6:30
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    \$\begingroup\$ For a while car radios generated a high voltage for vacuum tubes from the low voltage DC battery by using a vibrator, which had a vibrating contact like an old doorbell, which could then feed the generated AC into a step up transformer. An early "switched mode power supply". \$\endgroup\$
    – Ian Bland
    Jan 24 at 11:47

5 Answers 5


Three-terminal devices aren't the only way to produce amplification, and therefore sustained oscillation. It's actually possible to build free-running oscillators (and even amplifiers) using only two-terminal devices as long as one of these devices has negative differential resistance - that means it has a region in its characteristic operating curves where it conducts less current as the voltage across it rises, or a region where the voltage drops as more current flows through it. A tunnel diode exhibits this characteristic, for example, but a simple gas discharge in the form of a neon lamp or even a spark in air also has negative differential resistance.

You can build a free-running oscillator using such a device. Here's an example of an oscillator which uses a tunnel diode to continuously supply additional energy to an LC resonant circuit. You don't have to use a semiconductor device, though - you can also use a simple electric arc. These early oscillators operated in the exact same way as modern tunnel diode RF oscillators and produced continuous, undamped oscillations way before vacuum tubes were invented.

  • \$\begingroup\$ Do you thing they were aware of the fact they were using negative resistance at that time, of were they just filtering / resonating a source of noise ? \$\endgroup\$
    – Camion
    Jan 23 at 14:14
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    \$\begingroup\$ They were very much aware and even knew the oscillation criterion: -dV/dI>R. earlyradiohistory.us/1904pou.htm \$\endgroup\$ Jan 23 at 14:26
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    \$\begingroup\$ The Esaki-Diode (tunnel diode) was introduced in 1957 (more than 10 years after the transistor). \$\endgroup\$
    – LvW
    Jan 23 at 14:27
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    \$\begingroup\$ Such a circuit is also parasitic in LDO regulated circuits with inductive filters and decoupling capacitors . \$\endgroup\$
    – crasic
    Jan 23 at 23:47
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    \$\begingroup\$ Il also had this reaction at first, @LvW, but Jonathan S. introduced this to make link with the concept of negative resistance which is shared with shared with the electric arc. (I new about tunnel diodes and their use, but I didn't know electric arcs had this property too). \$\endgroup\$
    – Camion
    Jan 24 at 3:57

Fessenden applied for a patent in 1901 pertaining to a method of generating RF energy using a high speed alternator.

In 1902 he filed a patent for a heterodyne receiving system using alternators. This is likely to be the receiver design mentioned in your question.

  • \$\begingroup\$ From the patent, it looks like an LC circuit that is driven in a high frequency harmonic of a low frequency generator. The antenna is used as capacitor and the inductor is a simple coil. \$\endgroup\$
    – fishinear
    Jan 25 at 13:11

Relaxation oscillators are very simple devices.

There are lots of ways make them electronically, like a capacitor in parallel with a spark gap or neon lamp, and lots of ways to make them mechanically, like a Shishi-odoshi or Drinking bird.

They even occur naturally or accidentally in many contexts. It's what makes glasses squeak when you rub them, for example.

Store energy gradually until something breaks and then reforms, allowing energy to be stored again.

  • \$\begingroup\$ Marconi's first trans-Atlantic transmitter used a spark gap along with transformers and capacitors. I don't think there's any way to make a spark gap anywhere near clean enough to avoid cross-channel interference, but I wouldn't be surprised if things could be adjusted to get 50% of the energy at the right frequency (of course, modern standards require 99.99...% efficiency, with lots of nines) but back then the goal was simply to ensure that enough power was radiated at the proper frequency to ensure the signal could be received at the other end. \$\endgroup\$
    – supercat
    Jan 24 at 15:40

Well, know alternating current (AC) like in the power grid? It is oscillating! At 50 or 60 Hz depending on where you are.

And it has been oscillating for a very long time. The first AC generator was made in 1832, though it took a few decades for it to become popular. (Wikipedia)

To make AC, you start with a heavy rotating wheel. The mass keeps the frequency stable. To get the wheel rotating you can use a water wheel or a steam turbine or something.

You fix a magnet to the wheel and then wrap wires around its path. Electricity is induced in a an oscillatory manner.

  • 1
    \$\begingroup\$ Fessenden needed frequencies in the order of 2MHz. Not really doable with a mechanical rotating wheel directly. \$\endgroup\$
    – fishinear
    Jan 25 at 13:14

It wasn't until the 1920's that triodes became competitive with alternators. RF alternators used lots of poles, fast rotation, and sometimes incorporated frequency multiplication through nonlinear magnetics.


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