A new modulator for digital RF power amplifiers utilizing a wavetable approach

Abstract

This paper presents for the first time a wavetable-based coding scheme to generate a high-speed binary input signal for digital RF power amplifiers. The approach maximizes the utilization of the time domain handling capabilities of the pulse forming circuit. Key features are a greatly improved output spectrum purity in comparison with common digital modulators, the ability to adjust the modulator to any given pulse forming hardware and a built-in signal correction option that comes without additional computational cost. To give a first impression on the modulators behavior and its possibilities to adapt to different hardware constraints, simulations are carried out for different parameter variations and for different baseband bandwidths. Furthermore, the proposed concept is emulated with an arbitrary waveform generator to gather additional measurement data. For a 5 MHz wideband code division multiple access (WCDMA) signal (6.5 dB peak-to-average power ratio) at 900 MHz, an error vector magnitude (EVM) of 0.26% and better than >58 dB adjacent channel leakage ratio (ACLR) were recorded. To the authors’ knowledge, these are the best values achieved so far for a single-bit coding scheme without digital predistortion that still maintains maximum power coding efficiency.

Highly compact GaN-based all-digital transmitter chain including SPDT T/Rx switch for massive MIMO applications

“…The QAM signals are generated from random complex samples, which are processed by polar clipping and a raised-root-cosine filter to give the desired PAPR while closely resembling the statistical properties of WCDMA signals. For converting the BB signal into a binary pulse stream, the Matlab model of the modulator as described in the section “Modulator” and [8] was used. The output of a fast AWG (Keysight M8195A) was directly connected to the module described in the section “Realization”, providing ≈700 mV pp to the MMIC's first amplifier stage.…”

“…For all measurements the parameter correction feature of the modulator (see. [8]) was enabled in order to correct for amplifier distortions resulting from the selected dead time (between high- and low-side transistor switching) and amplifier imperfections. When comparing the results with and without the T/R switch, the slightly lower performance for measurements with the T/R switch in place as well as the slightly higher than expected average output power are a result of uncertainties in the parameter estimation process of this correction (see the section “Non-linearity correction”).…”

“…The digital PA modulator used in this setup is described in detail in [8]. Figure 3 provides a block diagram.…”

Concept for Continuously Tunable Output Filters for Digital Transmitter Architectures

“…The spectrum reveals harmonic power peaks at f n = (2n − 1)f c and a SQNR lower than 20 dB. [24]. The lower graphs present the multi-bandstop lowpass filter response for a operating frequency range from 1 GHz to 3 GHz.…”

“…The targeted filter concept is based on the combination of a multi-bandstop and a lowpass filter for the suppression of harmonic power peaks. By using a suitable modulation technique with high signal-to-quantization-noise ratio (SQNR) [24], suppression of the quantization-noise becomes obsolete.…”