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H. Ruotsalainen, H. Arthaber, G. Magerl:
"A New Quadrature PWM Modulator With Tunable Center Frequency for Digital RF Transmitters";
IEEE Transactions on Circuits and Systems II: Express Briefs, vol 59 (2012).

State-of-the-art quadrature-type modulators for all-digital radio-frequency (RF) transmitters operate optimally at an RF center frequency (f_{c}) that is a fraction of the modulator switching rate. This is a major limitation if the modulators of this type are to be used in multichannel transmitter systems. In this brief, a novel quadrature modulator based on pulsewidth modulation with controllable in-band distortion is presented, which permits continuous adjustment of f_{c} in digital domain. In addition, distortion generation related to digital upconversion of pulsed baseband signals is studied, and suitable compensation methods are given. The performance of the proposed modulator is compared with those of previously published encoding methods in terms of reconstructed RF signal quality and computational complexity. The evaluation results suggest that significant improvements in signal band distortion can be expected with a computational load suitable for real-time reconfigurable hardware realizations.

http://dx.doi.org/10.1109/TCSII.2012.2222834