Armstrong phase modulator

In 1933, Edwin H. Armstrong patented a method for generating frequency modulation of radio signals.^[1] The Armstrong method generates a double sideband suppressed carrier signal, phase shifts this signal, and then reinserts the carrier to produce a frequency modulated signal.

Frequency modulation generates high quality audio and greatly reduces the amount of noise on the channel when compared with amplitude modulation. Early broadcasters used amplitude modulation because it was easier to generate than frequency modulation and because the receivers were simpler to make. The electronics theory indicated that a frequency modulated signal would have infinite bandwidth; for an amplitude modulated signal, the bandwidth is approximately twice the highest modulating frequency.

Armstrong realized that while a frequency modulated signal would have an infinite bandwidth, only the first few sets of sidebands would be significant; the rest could be ignored.^[2] An amplitude modulated voice channel bandwidth would be approximately 6 kilohertz; a common frequency modulated voice channel bandwidth could be 15 kilohertz.