BROADBAND TRANSMITTER FOR A SIGNAL CONSISTING OF A PLURALITY OF DIGITALLY MODULATED CARRIERS

摘要

It is described a broadband radio transmitter for a digital signal consisting of a plurality of not necessarily equispaced, digitally modulated carriers. The transmitter selects the second replica of the spectrum of the signal coming out from a digital-to-analogue converter DAC to increase the space of the useful signal from the continuous in comparison of the direct use of the base band spectrum would involve. The object of the invention is in fact to obtain a wider transition band and consequently a less stringent local oscillator residue and image filter at radio frequency. To eliminate or reduce the effect of the sin( pi fT)/( pi fT) function that envelopes the signal coming out from the DAC, the invention foresees the use of an equalization of the analogue type, or the adoption of a zero insertion technique (of the digital or analogue type) or in general a digital x/sinx equalizer. The broadband signal is converted at radio frequency and the image filter selects the lower side band to cancel the spectral inversion made by the DAC if an even order replica is used. The transmitter can be profitably employed in any broadband transmitter application and in any mobile system GSM 900 MHz, or DCS 1800 MHz or UMTS.