Key technique of an L band fast hopping frequency synthesizer

Direct digital synthesis (DDS) technology was adopted to implement a wideband low spurious free dynamic range (SFDR) L band fast hopping frequency synthesizer. The signal model was approximated, using a mathematical method. The DDS output signal was modeled as a kind of sine signal modulated by a small toothed signal. The relationship between the spectrum of the toothed signal and word frequency was investigated. Based on modulation theory, the relationships between the spurious spectrum position, spurious frequency, and word frequency were analyzed. Rules governing the relationships were proposed. The L band (1.225-1.425 GHz) fast hopping synthesizer was designed and developed by considering practical issues in applying these rules. The wideband SFDR of the synthesizer was improved at the cost of increasing frequency error within the limit. The tested wideband SFDR level of the filtered DDS output signal is better than -83 dBc. The SFDR level of the L band fast hopping frequency synthesizer is better than -70 dBc.