Design and Optimization of Microwave Frequency Synthesizer Based on DDS

This paper introduces a new frequency synthesizer scheme –Nested-mixer DDS driving PLL frequency synthesizer. It is a dual loop topology. The AUX PLL uses high comparison frequency. The main PLL comparison frequency is high and changed continuously. It overcomes the shortcoming of long switching time which happens in the traditional DDS+PLL. Taking some optimized measures helps to further improve the synthesizer performance. Oversampling is used to reduce DDS output spurious level. Optimizing the circuit layout and structure, using 6 layers PCB, making control and data signal lines as a separate layer, and sandwiching it between analog ground and digital ground plane, minimize the external noise impact on the frequency synthesizer. Finally, a high performance microwave frequency synthesizer with high resolution has been achieved. It has been applied successfully to a military transmitter and a measuring receiver.