Fundamental Modulation Limits for Minimum Switching Frequency Inband-Error-Free High-Efficiency Power Amplifiers

Dept. of Electron. Eng., Tech. Univ. of Catalunya (UPC), Barcelona, Spain
Circuits and Systems I: Regular Papers, IEEE Transactions on (Impact Factor: 2.3). 11/2011; DOI: 10.1109/TCSI.2011.2123570
Source: IEEE Xplore

ABSTRACT This paper explores the modulation bandwidth limits for switching amplifiers, by analyzing the fundamental tracking capabilities of two-level switching signals. With this aim, this work synthesizes two-level switching signals by obtaining the distribution of switching events providing both minimum average switching frequency and inband-error-free encoding, targeting to minimize the amplifier switching losses when tracking a generic bandlimited signal. This analysis also provides a framework reference to characterize the deviation from such limit in modulations used in actual amplifiers.

  • [Show abstract] [Hide abstract]
    ABSTRACT: A discrete-time pulse width modulator (PWM) with zero baseband distortion for arbitrary band-limited modulating signals is developed in this paper. It is based on adjusting the duty-cycles of the PWM such that the samples of an ideal low-pass filtered version of the PWM signal coincide with the discrete-time samples of the modulating signal. Elaborating on previous approaches in the literature, it is shown that this problem can be stated as a multidimensional inverse function approach, and therefore it can be solved using iterative methods. Starting with the duty-cycle values of a uniform PWM, the successive iterations provide slight duty-cycle corrections that, in the limit, result in zero baseband distortion even for low carrier-to-modulating frequency ratios. Aiming at a practical, real-time implementation two new results are provided. First, explicit bounds on the improvement achievable after each duty-cycle correction are derived. Second, a block processing architecture suitable for real-time implementation is proposed, and the increase of distortion caused by its use is quantified. Several examples with typical band-limited signals demonstrate the performance of the algorithm.
    Circuits and Systems I: Regular Papers, IEEE Transactions on 10/2013; 60(10):2752-2762. · 2.30 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: This work deals with multi-level switching amplifiers, in the context of high-efficiency power amplification for signal tracking applications. In particular, this paper evaluates the reduction in the error signal's power due to multi-level power amplification (compared to conventional two-level amplifiers) and compares the performance of two multi-level pulse modulations: PWM and Asynchronous ΣΔ Modulation. First the intrinsic bandwidth limits of multi-level switching amplifiers are inferred, to clearly state the advantages and limitations of multi-level power amplification. From the existing analyses of Pulse Width Modulation already reported in the literature, PWM is herein extended to multiple levels based on an equivalent representation, which allows to derive a closed expression for the power spectrum of multi-level PWM in bandlimited signal tracking. The Asynchronous ΣΔ Modulation is extended to multiple levels and the resulting multi-level encoding algorithm is analyzed in both time and frequency domains. The performance of both modulations is characterized and compared at different operating frequencies and using different number of levels. The main outcomes of this in-depth characterization show that, if the switching frequency is high enough, the tracking error is independent of the modulation and the switching frequency, i.e., it only depends upon the number of levels, which points out the suitability of asynchronous modulations for relatively low switching frequencies (compared to the number of levels).
    Circuits and Systems I: Regular Papers, IEEE Transactions on 01/2013; 60(6):1621-1634. · 2.30 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Among other goals, multilevel burst-mode radio frequency (RF) transmitters have to achieve two main objectives: high efficiency, and good transmission signal quality. Both of these goals depend, to a large extent, on the driving signal modulator, which can, for example, be a digital pulse-width modulator. However, conventional digital pulse-width modulation (PWM) contains aliasing distortion that prevents achieving satisfying transmission signal quality. In this paper, an aliasing-free digital multilevel PWM method is introduced that allows for achieving good transmission signal quality, while at the same time offering the potential of reaching a coding efficiency of 100%. The mathematical equations for aliasing-free digital PWM are extended in a way that makes them suitable for the use in multilevel burst-mode RF transmitters. It is then shown how the aliasing-free multilevel PWM method can be used to achieve 100% coding efficiency. Simulations demonstrate the performance capabilities of the proposed PWM method.
    Proceedings of the 2013 IEEE International Symposium on Circuits and Systems (ISCAS 2013); 05/2013