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: 2.4). 11/2011; 58(10):2543 - 2555. DOI: 10.1109/TCSI.2011.2123570
Source: IEEE Xplore


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.

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    • "Note that for the PWM method described here, the amplitude signal a[n] has to fulfill certain constraints. First, the bandwidth of a[n] must be adequately smaller than the reference wave frequency f p = 1/T p [10]. Second, the amplitudes Copyright ©(c) 2013 IEEE. "
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    Circuits and Systems II: Express Briefs, IEEE Transactions on 09/2013; 60(9):592 - 596. DOI:10.1109/TCSII.2013.2268431 · 1.23 Impact Factor
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    • ", [21] and in the simulations in Sec. VI-A. "
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    ABSTRACT: Burst-mode operation of power amplifiers (PAs) is a promising concept towards higher power efficiency in radio frequency (RF) transmitters. Such transmitters use pulse-width modulation (PWM) to create the driving signal for the PA, and a reconstruction filter after amplification to obtain the transmission signal. However, conventional digital pulse-width modulated signals contain a large amount of distortion that cannot be removed by the reconstruction filter in a satisfactory manner. This paper introduces a method for digital PWM that is free of destructive aliasing distortion. First, a set of mathematical closed-form equations fully describing all baseband processing steps required in conventional PWM-based RF transmitters is developed. Analysis of the equations leads to the conclusion that destructive distortion in digital PWM systems originates from aliasing induced by the infinite bandwidth of pulsed signals that is entailed by the nonlinear operation of the pulse-width modulator. Based on this knowledge, a PWM method is developed that ensures that the generated signals are bandlimited and hence completely avoids destructive aliasing distortion. Simulations as well as measurements demonstrate the improvement that can be achieved with the proposed method compared to conventional methods. The results indicate that by using the proposed method it becomes feasible to implement filters that allow for obtaining satisfying transmission signal quality.
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    ABSTRACT: A mixed-signal continuous-time-processing standard CMOS implementation of an asynchronous sigma-delta modulator aimed to drive a switching amplifier operating as an on-chip wideband adaptive power supply is presented in this work. The paper first briefly discusses the fundamental limit tracking capabilities of a two-level switching signal to inband- error-free track a bandlimited signal with minimum average switching frequency. It is argued the adequacy of an adaptive asynchronous sigma-delta modulator (AA∑∆) to approximate such fundamental characteristics. The second part of the paper presents mixed-signal design details of the various subcircuits implementing a CMOS low-power digitally-programmable AA∑∆ modulator, with 7 MHz average switching frequency operation and 1000 µm x 640 µm area occupancy.
    International Symposium on Circuits and Systems (ISCAS 2011), May 15-19 2011, Rio de Janeiro, Brazil; 05/2011
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