Spectrum characteristic analysis of DDS-based RF digital modulation

A conventional radio frequency (RF) modulator with phase locked loop (PLL) has disadvantages in modulated specification and complicated systems. With the fast development of direct digital synthesis (DDS), DDS can be incorporated into the RF modulator, which results in flexible changing parameters and adding digital filters. However, the DDS modulator also causes some problems such as low output frequency and worse spurs. This paper discusses the results of RF digital modulation combining DDS with PLL and analyzes its modulator character and output spectrum character for ideal conditions. As a result, the RF output character in the loop band width is decided by DDS, and out of the loop band width is decided by a voltage controlled oscillator (VCO). The spurs of the DDS output can be filtered out and thus a good modulated signal spectrum can be achieved. Experimental results are presented. By combination of DDS with PLL, an RF digital modulator with good performance and low cost can be obtained.