A Σ Δ fractional-N closed-loop modulator for DECT

Traditionally, modulators for DECT are implemented using open-loop modulation. However: open-loop modulators are too slow for 100% slot utilization. This causes the well known DECT blind-slot problem. One solution to this is to employ closed-loop modulation in stead. This paper presents a closed-loop modulator solution for DECI: The modulator b based on a combination of ΣΔ-techniques and use of a fractional-N PLL. The technique allows for digital phase and frequency modulation without use of mixers or D/A converters in the modulation path. Simulation results show a working modulator that is superior to state-of-the-art DECT transmitters using open-loop modulation. An internal reference frequency of 10 - 30 MHz gives an output PSD that meets DECT speci cation requirements on unwanted emissions due to modulation. The RMS phase error for a reference frequency of 31.104 MHz is as low as 1.06° and the peak phase error is 2.90°. The blind-slot problem is solved using the closed-loop modulator as the lock time is 1 - μs which, compared to the guard space requirements of below 50 μ, gives a large room for implementation imperfections.