Peter Singerl

Aliasing-Free Digital Pulse-Width Modulation for Burst-Mode RF Transmitters

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.

An RF Carrier Bursting System Using Partial Quantization Noise Cancellation

This paper introduces a method for bandpass cancellation of the quantization noise occurring in high efficiency, envelope pulsed transmitter architectures-or carrier bursting. An equivalent complex baseband model of the proposed system, including the -modulator and cancellation signal generation, is developed. Analysis of the baseband model is performed, leading to analytical expressions of the power amplifier drain efficiency, assuming the use of an ideal class B power amplifier. These expressions are further used to study the impact of key system parameters, i.e. the compensation signal variance and clipping probability, on the class B power amplifier drain efficiency and signal-to-noise ratio. The paper concludes with simulations followed by practical measurements in order to validate the functionality of the method and to evaluate the performance-trend predictions made by the theoretical framework in terms of efficiency and spectral purity.

An Efficient Scheme for Nonlinear Modeling and Predistortion in Mixed-Signal Systems

A novel identification and predistortion scheme of weakly nonlinear systems for mixed-signal devices, which takes into account practical implementation aspects, is presented. It is well known that for the identification of weakly nonlinear systems, despite the spectral regrowth, it suffices to sample the input-output (I/O) data of the system at the Nyquist rate of the input signal. Many applications such as linearization and mixed-signal simulations require system models at a higher sampling rate than Nyquist. Up to now, the construction of such high-rate models has been done by oversampling the corresponding I/O data. This leads to high computational complexity, ill posedness of the estimation, and high demand on the analog-to-digital-converter sampling rate for the implementation. This brief discusses an efficient way to obtain high-rate models and predistorters from low-rate models and shows the validity of the proposed scheme for a very-high-speed-digital-subscriber-line power amplifier, where an adjacent channel power supression of 20 dB is achieved

Coding efficiency for different switched-mode RF transmitter architectures

Conventional RF transmitters with power amplifiers in e.g. Class-AB operation provide only moderate electrical efficiencies for complex modulated signals. To increase the overall efficiency considerably, we can employ switched-mode power amplifiers (SMPA) with an appropriate baseband signal processing. Such efficient SMPAs generate a lot of spectral out-of-band components, which has to be attenuated by a bandpass filter before the signal is fed to the transmit antenna. Unfortunately, this impacts the electrical efficiency because these unwanted signal components are usually dissipated within the transmitter. A measure for the electrical efficiency degradation for complex modulated signals in a switched-mode operation is the coding efficiency. In this paper we develop different switched-mode transmitter architectures based on baseband PWM coding and analyze the coding efficiencies for different signal statistics.

Coding efficiency optimization for multilevel PWM based switched-mode RF transmitters

Switched-mode power amplifier (SMPA) techniques with pulse-width modulation (PWM) encoding are one of the most favored techniques for the highly efficient radio frequency (RF) transmitters. Unfortunately, SMPAs can only provide good efficiency for maximum output power and signals with low peak-to-average power ratios (PAPRs). With multilevel PWM based transmitters, a superior electrical efficiency can be provided for a large signal dynamic. In this paper, the coding efficiency, which is the upper bound of the electrical efficiency, is investigated for multilevel pulsed transmitter architectures. It is shown that the optimum coding efficiency can be obtained by changing the input threshold according to the probability density function (pdf) of the signal. Closed-form expressions for determining the optimum threshold as well as the maximum coding efficiency are derived for given signal statistics. Two examples are presented to demonstrate the derived equations.

The frequency spectrum of polar modulated PWM signals and the image problem

Pulse-width modulators (PWMs) are implemented in wireless transmitters to utilize highly efficient switched-mode power amplifiers (SMPAs) such as Class-D,E,F,J amplifiers. This paper gives the analytical expressions for the natural-sampling trailing-edge PWM (TE-NPWM) signals with input signals bounded by [0, 1]. Such PWM modulators can be incorporated in a polar architecture. Important relations of polar modulated PWM signals are analyzed in detail by exploiting the results of the analysis. The image problem is illustrated when a baseband PWM signal is modulated to the passband with the phase-modulated carrier. A properly designed lowpass filter can be used to reduce the image distortions, but increases the total design costs. It further introduces extra delay and has the possible drawback of reducing the overall efficiency. Based on the spectral analysis, two alternative approaches are proposed to show how the image distortions can be reduced without a lowpass filter. The performance of the presented approaches for the image reduction is demonstrated through numerical simulations.

Reduction of Aliasing Effects of RF PWM Modulated Signals by Cross Point Estimation

The trend in transmitter systems is to move the digital domain closer toward the antenna using digital modulators and drivers to reduce circuit complexity and to save power. One promising approach is the use of RF pulse width modulation (RF PWM). Unfortunately purely digital discrete time RF PWM suffers from aliasing problems which limit the achievable resolution. For a 40 MHz bandwidth signal at 2.6 GHz carrier frequency for example the achievable signal quality is limited to ~ 43 dBc. This paper describes the root cause of this effect, an error in the determination of the cross points, due to the sampled nature of the signal and proposes a method to compensate for it. It is shown that by interpolating the signal and estimating the cross points the signal quality can be significantly improved. The interpolation is simplified by interpolating the decomposed outphasing signals instead of the full signal. This has the advantage that a constant instead of a phase modulated reference function can be used. It is shown that by simple cross point estimation the signal quality can already be improved to 65 dBc. When either considering a second modulator or when using a delta sigma like noise shaping architecture the signal quality can be further enhanced to 75 dBc.

Efficiency Optimization for Burst-Mode Multilevel Radio Frequency Transmitters

The utilization of a burst-mode power amplifier (PA) together with pulse-width modulation (PWM) is a promising concept for achieving high efficiency in radio frequency (RF) transmitters. Nevertheless, such a transmitter architecture requires bandpass filtering to suppress side-band spectral components to retrieve the wanted signal, which reduces the transmit power and the transmitter efficiency. High efficiency can only be expected with the maximum transmit power and signals with low peak-to-average-power ratios (PAPRs). To boost efficiency for signals with high PAPRs and signals at variable transmit power levels, the burst-mode multilevel transmitter architecture has been widely discussed as a potential solution. This paper presents an efficiency optimization procedure of burst-mode multilevel transmitters for signals with high PAPRs and signals at variable transmit power levels. The impact of the threshold value on the transmitter efficiency is studied, where the optimum threshold value and the maximum transmitter efficiency can be obtained according to input magnitude statistics. In addition, the relation between the threshold value and the efficiency expression of burst-mode multilevel transmitters and those of Doherty PAs is investigated. It is shown that the obtained optimum threshold value, although originally designed for burst-mode transmitters, can also be applied to Doherty and multistage Doherty PAs to achieve maximum transmitter efficiency. Simulations are used to validate the efficiency improvement of the optimized burst-mode multilevel transmitters compared to two-level and non-optimized multilevel transmitters.

A fully digital delay-line based GHz-range multimode transmitter front-end in 65-nm CMOS

A fully digital up-converter for wireless transmission in the GHz range is presented. The system consists of a polar modulator which uses PWM for the amplitude modulator (AM). Phase modulation (PM) is implemented by shifting the carrier in time. Both the PWM and the PM are implemented using asynchronous delay lines which allow time resolutions down to 10 ps without the need for high-frequent clock signals. The system is designed to drive two class-E power amplifiers with a power combiner. It supports a continuous range of carrier frequencies starting at 946 MHz and limited upwards only by the desired resolution. The modulator has been implemented in 65-nm CMOS. Results show error vector magnitude (EVM) values between 1.24% (−38.1 dB) at 946 MHz and 3.98% (−28.0 dB) at 2.4 GHz for 64-QAM OFDM signals.

Chebyshev approximation of baseband Volterra series for wideband RF power amplifiers

The Volterra series represents a very powerful mathematical tool for system-level analysis to describe the input-output behavior of nonlinear devices such as RF power amplifiers, which generally incorporate memory effects. Starting from the most general complex baseband representation of a nonlinear RF power amplifier, an equivalent nonlinear system with lower complexity is developed. The complexity reduction is obtained by a bandwidth dependent approximation of the multi-dimensional Volterra kernels with orthogonal polynomials in the frequency-domain. The resulting nonlinear time-domain model consists of a cascade of complex linear pre-filters and a memoryless polynomial with complex parameters. If the input signal bandwidth becomes sufficiently small, we obtain a special well known system from the most general approximation which can be represented in the baseband region by purely static nonlinearities (AM/AM- and AM/PM-conversion). We show for a 3rd-order nonlinear system with typically smooth kernels that the number of free parameters can be reduced while maintaining the required modeling accuracy.