to transmissions on adjacent frequencies. The first transmitters able to produce continuous wave were the arc converter (Poulsen arc) transmitter, invented by Danish engineer Valdemar Poulsen in 1903, and the Alexanderson alternator, invented 1906-1912 by Reginald Fessenden and Ernst Alexanderson. These slowly replaced the spark transmitters in high power radiotelegraphy stations. However, the radio receivers used for damped wave could not receive continuous wave. Because the CW signal produced while the key was pressed was just an unmodulated carrier wave, it made no sound in a receiver's earphones. To receive a CW signal, some way had to be found to make the Morse code carrier wave pulses audible in a receiver. This problem was solved by Reginald Fessenden in 1901. In his "heterodyne" receiver, the incoming radiotelegraph signal is mixed in the receiver's detector crystal or vacuum tube with a constant sine wave generated by an electronic oscillator in the receiver called a beat frequency oscillator (BFO). The frequency of the oscillator is offset from the radio transmitter's frequency . In the detector the two frequencies subtract, and a beat frequency (heterodyne) at the difference between the two frequencies is produced: . If the BFO frequency is near enough to the radio station's frequency, the beat frequency is in the audio frequency range and can be heard in the receiver's earphones. During the "dots" and "dashes" of the signal, the beat tone is produced, while between them there is no carrier so no tone is produced. Thus the Morse code is audible as musical "beeps" in the earphones. The BFO was rare until the invention in 1913 of the first practical electronic oscillator, the vacuum tube feedback oscillator by Edwin Armstrong. After this time BFOs were a standard part of radiotelegraphy receivers. Each time the radio was tuned to a different station frequency, the BFO frequency had to be changed also, so the BFO oscillator had to be tunable. In later superheterodyne receivers from the 1930s on, the BFO signal was mixed with the constant intermediate frequency (IF) produced by the superheterodyne's detector. Therefore, the BFO could be a fixed frequency. Continuous-wave vacuum tube transmitters replaced the other types of transmitter with the availability of power tubes after World War I because they were cheap. CW became the standard method of transmitting radiotelegraphy by the 20s, damped wave spark transmitters were banned by 1930 and CW continues to be used