Distortion Analysis for the Assessment of LTI and Non-LTI Transmitters

Abstract

Bounds on the frequency domain behavior of electrically small antennas are adapted to assess the time domain distortion or fidelity achievable by simple linear time-invariant (LTI) systems. Rigorous expressions for a TM01 spherical shell are used as a direct analog to the well known Chu limit on an antenna's bandwidth-efficiency product. These expressions are shown to agree with results obtained using narrowband single resonance approximations, allowing for the analysis of arbitrary small dipole-like structures through bounds on their single frequency Q-factor and efficiency. The resulting expressions are used as a basis for assessing the performance gains of electrically small non-LTI (e.g., direct antenna modulation) transmitters for which Q-factor and impedance bandwidth are not defined but which can be analyzed directly in the time domain via distortion.

An Energy-Synchronous Direct Antenna Modulation Method for Phase Shift Keying

“…Thus, the exponential transitions between symbol states are due to the slow charging and discharging of uniquely phased oscillations and are the time-domain manifestation of the bandlimited nature of electrically small antennas. Long transition times associated with very narrowband systems lead to high distortion [11], problematic intersymbol interference, and decreased noise tolerance. All of these factors decrease communication link performance.…”

“…Visual inspection of the time-domain waveforms, eye diagrams, and constellations in the previous section indicates the efficacy of DAM BPSK and QPSK in improving signal fidelity over a conventional transmitter. Here we quantify this improvement by comparison of the measured signals against an ideal PSK signal using distortion analysis [11], [16].…”