Bezpośrednia cyfrowa synteza częstotliwości realizowana dla potrzeb techniki radarowej

Bezpośrednia synteza cyfrowa częstotliwości (DDS) jest wykorzystywana w radarach niemal zawsze, jeśli wymagane są: duża rozdzielczość nastawy częstotliwości (poziom Hz i mniej) i duża prędkość przełączania (mniej niż 1 žs). Synteza taka zapewnia bardzo dobry poziom szumów fazowych, małą niestałości przełączania (jitter) oraz ciągłość fazową przełączania, jednak generuje bliskie harmoniczne. W artykule przedstawione zostało porównanie standardowych rozwiązań DDS na tle realizacji z użyciem FPGA.

Direct Digital Synthesis (DDS) is widely used in radar systems when high resolution frequency setting is the must (1 Hz or less). And, when modulation scheme of Continuous Wave (CW) radar requires fast changing in shape, or switching (1 žs or less) the frequency. DDS is less power consuming and more compact in hardware than direct analog synthesis. DDS preserves or even enhances noise level of the source clock, guarantees phase continuity during switching. The main disadvantage of DDS is the spurious level generated as the result of angle discrete integration in time and discrete voltage levels of the output periodic signal. Another disadvantage is the limited bandwidth of a few hundreds of MHz. In this paper a few standard DDS chips are presented. Examples are representative for high quality and high speed demands. Since FPGA technology covered a great piece of digital applications, it is also often used in radar technology. Apart from fast and parallel DSP applications, FPGA can generate arbitrary modulated CW signals. However, FPGA works with lower clocks than standard (or ASIC) chips, it further limits the bandwidth available. But because of the FPGA re configurability, some dedicated improvements for spurious reduction are available. This makes FPGA the attractive solution, making also system on chip integration possible. For these reasons, in this paper an example of FPGA implemented DDS is analyzed too, giving a reference in relation to the standard solutions.