engleski

Sigma-delta look-up table based half-sine pulse shaper

Due to the wide power spectrums of rectangular data streams, it is important for base-band signals to be heavily band limited before modulation. That can be achieved by pulse shaping of rectangular bits. The most common is half-sine shaping, which is used in Minimum Shift Keying (MSK) modulation. MSK modulator can be realized by direct injection of NRZ data into the frequency modulator using the modulation index of 0.5, or by replacement of the rectangular pulse-shapes with half-cycle sinusoidal pulses (half-sine) before modulation with I-Q modulator. The modulation of a high-frequency carrier by such shaped signals results with narrower power spectrum than rectangular shape. The most common solutions use PCM based look-up-table (LUT), which requires n-bit D/A converter. Our proposition is use of 1-bit Sigma Delta Modulation (SDM) LUT, which results with smaller ROM capacity, 1-bit wide output word, and 1-bit D/A converter realized as a simple first order low-pass RC filter [1]. Measured oscillograms and power spectrums are obtained for 10-bit pseudo-random sequence input signal for SDM LUT based half-sine shaper realized in FLEX AlteraTM PLD.

pulse shaping; sigma-delta; minimum shift keying; look-up table

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