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Hybrid recursive active filters for duplexing in RF transmitter front‐ends
Author(s) -
Gottardo Giuseppe,
Donati Giovanni,
Musolff Christian,
Fischer Georg,
Felgentreff Tilman
Publication year - 2016
Publication title -
radio science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.371
H-Index - 84
eISSN - 1944-799X
pISSN - 0048-6604
DOI - 10.1002/2016rs005969
Subject(s) - duplexer , baseband , amplifier , electronic engineering , bandwidth (computing) , computer science , passband , transmitter , radio frequency , adjacent channel power ratio , adjacent channel , band pass filter , predistortion , control theory (sociology) , channel (broadcasting) , engineering , telecommunications , antenna (radio) , control (management) , artificial intelligence
Duplex filters in modern base transceiver stations shape the channel in order to perform common frequency division duplex operations. Usually, they are designed as cavity filters, which are expensive and have large dimensions. Thanks to the emerging digital technology and fast digital converters, it is possible to transfer the efforts of designing analog duplex filters into digital numeric algorithms applied to feedback structures, operating on power. This solution provides the shaping of the signal spectrum directly at the output of the radio frequency (RF) power amplifiers (PAs) relaxing the transmitter design especially in the duplexer and in the antenna sections. The design of a digital baseband feedback applied to the analog power RF amplifiers (hybrid filter) is presented and verified by measurements. A model to describe the hybrid system is investigated, and the relation between phase and resonance peaks of the resulting periodic band‐pass transfer function is described. The stability condition of the system is analyzed using Nyquist criterion. A solution involving a number of digital feedback and forward branches is investigated defining the parameters of the recursive structure. This solution allows the closed loop system to show a periodic band pass with up to 500 kHz bandwidth at the output of the RF amplifier. The band‐pass magnitude reaches up to 17 dB selectivity. The rejection of the PA noise in the out‐of‐band frequencies is verified by measurements. The filter is tested with a modulated LTE (Long Term Evolution) signal showing an ACPR (Adjacent Channel Power Ratio) enhancement of 10 dB of the transmitted signal.