The Riemann pump: A concurrent transmitter in GaN technology

Abstract

An original arbitrary waveform generator (AWG) architecture suited for software radio (SR) transmission is presented. A piecewise linear approximation of the wanted signal is generated thanks to a predefined set of slopes. The digitalto-analog (DA) conversion involved in this operation is based on a differential digital coding which drives a custom digital-toanalog converter (DAC), named here the Riemann Pump. This circuit is in charge of outputting the piecewise linear signal by integration of current steps into a capacitive load, potentially being the input impedance of a power amplifier. Simulations have been carried out on a first design, developed in a GaN technology, with a configuration that covers 1 GHz bandwidth with an oversampling ratio (OSR) of 4 and 3 input bits. The generation of concurrent modulated signals is demonstrated, with a rejection of 30 dBr over the whole band. The system exhibits promising performances as for the realization of a multi-standard concurrent radio frequency transmitter with moderate hardware complexity.

Low-Power Differential Input to Single-Ended Output GaN RF-DAC for RF-Signal Generation

“…The cointegration of a high-power RF-DAC with a power amplifier (PA) is called Riemann pump and was first introduced in [1]. This converter is controlled by a digital bitstream from a digital signal processor (DSP) in an open-loop configuration.…”

“…The linear voltage slopes exhibit a lower quantization error than the conventional voltage step approach, which makes this concept superior for a minimum resolution of 2 b for any given oversampling ratio (OSR) compared with the conventional PCM concept. The signal-to-noise ratio for an ideal N-bit pulse-code modulator with r = log 2 ( f sampling /(2 • f signal,max )) is SNR PCM = 6.02N + 3.01r + 1.76 (1) while the Riemann pump's SNR is given by SNR RP ≈ 6.02N +9.03r −7.78 + 10 log 1 − 1 2 N −1 + 1 2 2N .…”

“…For every doubling of the sampling frequency, which means an increase of 1 for r , the Riemann pump conversion improves its SNR by approximately 9 dB in contrast to the 3-dB improvement of the conventional approach. The basic principle of the underlying RF conversion and the derivation of these formulae is introduced in [1] and [2]. A comparison of experimental results based on this new concept is demonstrated in [3].…”