| 摘要 |
Direct digital frequency synthesizer (DDFS) are used for generating an oscillator signal ( Ð (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 ( Ð (kf)) caused by phase truncation, amplitude and time quantization errors introducing jitter into the waveform of the output signal ( Ð (t)). Thereby, discrete P-bit phase difference values ( DELTA ϕs(nT)) serving as frequency control words (FCWs) are repeatedly accumulated to generate P'-bit samples of a digital sawtooth signal (ϕs(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 (ϕs(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 ( Ð (nT)) of the approximated sine wave ( Ð (t)) is destroyed and its spurious discrete noise power density spectrum ( Ð (kf)) caused by said error signal ( Ð (nT)) is transformed into a broadband continuous noise power density spectrum ( Ð (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). <IMAGE>
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