Tikker

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A tikker (also spelled ticker) was an early device used in crystal radio receivers to hear continuous-wave (CW) radiotelegraphy signals. In the early 1900s, radio used radiotelegraphy, with transmitters turned on and off by a key to send Morse code. Spark transmitters produced damped waves that sounded like a buzz, while continuous-wave transmitters sent pure carrier waves that were quiet by themselves.

To hear CW, receivers had to create a tone during the carrier. The tikker, invented in 1908 by Valdemar Poulsen, did this with a vibrating switch between the detector and earphone that was opened repeatedly by an electromagnet. This interrupted the detector signal at an audio rate, producing a buzz whenever the carrier was present and making the Morse dots and dashes audible.

Around 1915, the tikker was replaced by the heterodyne receiver. The heterodyne method uses a local oscillator that mixes with the incoming carrier to produce a beat frequency in the audio range, which can be heard in the earphone. Reginald Fessenden had developed heterodyning earlier (1902). After vacuum-tube oscillators arrived in 1913 (Meissner), the heterodyne receiver became standard.

Today, the heterodyne method and beat-frequency oscillator (BFO) are still used in receivers to hear CW signals.