A digital delta-sigma modulator for controlling a multi-modulus divider in a fractional-N frequency synthesizer features cascaded delta-sigma modulator stages in a feed-forward circuit topology to extend the possible multi-modulus control output values between the range of 1 to 1. During the desulfurization process, the present invention provides for the progress of the desulfurization being monitored, using a change in a parameter relating to intervals, whereby the interval lasts from the beginning of an nth rich operating phase, until the measured value drops below a predetermined lambda threshold value downstream from the NOx storage catalyst. The PH information includes a first field including a plurality of indicators, each of the plurality of indicators indicating the presence of a second field indicating a PH level of a corresponding secondary cell.