Fundamental Modulation Limits for Minimum Switching Frequency Inband-Error-Free High-Efficiency Power Amplifiers
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.
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ABSTRACT: Digital pulsewidth modulators are used to transform nonconstant amplitude signals into pulsed signals, such that the information lying in the signal amplitude is encoded in the widths of pulses. Because of the inherent aliasing distortion in digital pulsewidth-modulated signals, additional signal processing steps are required to make pulsewidth modulation (PWM) suitable for applications like digital audio amplification or burst-mode radio-frequency transmitters. These processing steps, however, entail an undesirable increase in computational effort. This brief presents a multiplierless implementation of a digital aliasing-free pulsewidth modulator using lookup tables, adders, and arithmetic shifts only. Mathematical equations of asymmetric double-edge PWM are given, as well as a modified aliasing-free version of this PWM technique that directly integrates the distortion-avoiding signal processing steps into the pulsewidth modulator. Based on these equations, a multiplierless implementation of the aliasing-free PWM (AF-PWM) is developed. Simulation results obtained with a Simulink fixed-point model show that the proposed modulator implementation provides a feasible solution for realizing AF-PWM with low computational effort.Circuits and Systems II: Express Briefs, IEEE Transactions on 09/2013; 60(9):592 - 596. DOI:10.1109/TCSII.2013.2268431 · 1.19 Impact Factor
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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.Circuits and Systems I: Regular Papers, IEEE Transactions on 06/2013; 60(2):415–427. DOI:10.1109/TCSI.2012.2215776 · 2.30 Impact Factor
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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; 01/2011