On the relation of RF amplifier S-parameter curvature to vector modulation constellation and ACPR LTE FDD signal transmission

Modulation constellation, EVM and ACPR as parameters of broadband transmission quality of RFFE (RF Front End) amplifier have been investigated against the S-parameter characteristics of RF transistor of RF Amplifier for broadband front-end. LTE FDD 64-QAM modulated signals with 20MHz bandwidth at 1800MHz and 2400MHz band have been used to observe the 64-vector point modulation constellations and their EVMs, thereof. Vector points in modulation constellation of 64-QAM LTE FDD signals at the output of RF amplifier exhibit enlargement to small circles with irregular radius at frequency of 1.8GHz and 2.4GHz, which show signal distortion with EVM increase from 0.366%rms and 0.322%rms at the input to 1.5249%rms and 2.23%rms at the output of RFFE amplifier of 1.8GHz and 2.4GHz, respectively. The bandwidth and ACPR (adjacent channel power rejection) of the LTE signals have been observed at frequency of 1.8GHz and 2.4GHz, where the ACPR at the input of RF Amplifier before signal amplified -54dBc compared to ACPR of -33dBc and -34dBc at the output RFFE amplifier at 1.8GHz and 2.4GHz, respectively. Eventhough S₂₂ shows of encircling the origin behavior at 2.4GHz compared to S₂₂ of 1.8GHz, the curvature of S₁₁ tends to shift away from the origin at 2.4GHz resulted lower S/N at the output and more significant effect of noise to signal distortion as shown by larger circles of modulation constellation and higher EVM.