4. SSPA linearity

4.1 Linearity requirements in telecommunications

In telecommunications, a digital signal carries a succession of symbols, each characterized by a precise position in a two-dimensional phase space, defined by amplitude and phase (known as PSK, APSK or QAM modulation). Any deviation from this theoretical position (noted EVM for Error Vector Magnitude) induces a degradation of the signal-to-noise ratio (C/I) and ultimately of the link error rate (BER). To limit the amplitude of these deviations, the gain of an amplifier and the phase shift between input and output must be independent of the level of the input signal and its exact frequency (within the operating band): these are the linearity constraints.

Phase is by nature modulated, and the amplitude (also known...