Direct digital frequency synthesizer for cellular wireless communication systems based on fast frequency-hopped spread spectrum technology

摘要

Direct digital frequency synthesizer (DDFS) are used for generating an oscillator signal ((t)) with an approximately sinusoidal waveshape and can advantageously be applied to wireless RF transceivers in a cellular communication system based on a fast frequency-hopped spread spectrum (FHSS) technology. The invention particularly refers to a method for reducing spurious harmonics, voltage and/or current glitches in the frequency synthesizer's noise power density spectrum ((kf)) caused by phase truncation, amplitude and time quantization errors introducing jitter into the waveform of the output signal ((t)). Thereby, discrete P-bit phase difference values (Δϕ_s(nT)) serving as frequency control words (FCWs) are repeatedly accumulated to generate P'-bit samples of a digital sawtooth signal (ϕ_s(nT)) with a predefined time period (N·T), which are then submitted to a phase-to-sine-amplitude conversion of the obtained discrete phase input values (ϕ_s(nT)) into a set of discrete sine amplitude sample values (s(nT)). After that, these sine amplitude sample values (s(nT)) are converted into an approximation of a quantized analog sine wave (a₁'(t)).;According to one embodiment of the underlying invention, spurious harmonics (DSCs) are spread over the entire signal spectrum by adding a P"-bit number word representation of a random number (r(nT)) to the obtained sine amplitude sample values (s(nT)). Thus, the periodical structure of glitches, phase truncation and quantization errors ((nT)) of the approximated sine wave ((t)) is destroyed and its spurious discrete noise power density spectrum ((kf)) caused by said error signal ((nT)) is transformed into a broadband continuous noise power density spectrum ((f)). Finally, the additive noise is compensated by subtracting a digital-to-analog converted representation of said random number (r(nT)) from the quantized analog sine wave approximation (a1'(t)) at the analog output port of the direct digital frequency synthesizing unit (DDFS).