Design of a Fast-Sweeping C-X band Frequency Source Based on DDS and Frequency Multiplying Chain

This paper presents a C-X band frequency sweeping source with wide bandwidth, high frequency resolution, fast frequency hopping and low phase noise. In order to achieve a frequency hopping time within 10ns, ADI’s DDS IC AD9914 is adopted due to its high frequency reference clock. Meanwhile, it is capable of delivering an RF signal with a frequency higher than 1GHz, which is beneficial to minimize the frequency multiplication factor of the followed frequency multiplying chain. A comb spectrum generator circuit with a step recovery diode (SRD) is designed to produce a reference signal of 3400MHz for the DDS. By configuring DDS internal registers, a triangular wave frequency modulation signal with adjustable period and sweeping frequency is realized. An 8-time frequency multiplying chain is designed to extend the output frequency of DDS to 6GHz to 9GHz. To improve the flatness of the output power within 6GHz to 9GHz, an amplitude equalizing circuit is optimized utilizing lumped components. The measured results demonstrate that the frequency sweeping system has a power flatness better than ±1.1dB and a spurious suppression better than 40dBc in the range of 6GHz to 9GHz. The measured phase noise is generally better than -117dBc/Hz at 100KHz offset.