Complex sentence: The beat frequency oscillator was invented in 1901 by Canadian engineer Reginald Fessenden, what he called the "heterodyne" receiver was the first application of the heterodyne principle.
Split into simpler sentences: The beat frequency oscillator was invented in 1901 by Canadian engineer Reginald Fessenden. What he called the "heterodyne" receiver was the first application of the heterodyne principle.