The Bell System Technical Journal volume IX no. 1 January 1940, includes H Nyquist and K W Pfleger, Effect of the Quadrature Component in Single Sideband Transmission.
BSTJ, American Telephone and Telegraph Co, NY 1940; vol IX no. 1, January 1940; whole issue, blue paper wraps, pp 1 - 160 inclusive; photographs, illustrations, references.