Linearity testing of multi-level transmitters for communication systems

A method of testing multilevel transmitters is described which avoids the use of echo-cancellers but still uses a test signal which closely resembles random data with a wide spectral content, little autocorrelation, large excursions of running digital sum, and a rich selection of transmitted symbols of transitions. The transmit signal is based on a chirp coded to use only the allowable transmit symbols in a nose shaping coder. The chirp is modified to exhibit a spectral cavity, kept deep during the coding process by arranging a coincident noise minimum in the coder. Transmitter imperfections can be observed as they fill the cavity with distortion products. The method is assessed against a theoretical model of transmitter distortion, and measurements of an ISDN (integrated services digital network) basic rate 2B1Q transmitter are shown