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The paper describes a new approach to synthesizing the Cs hyperfine frequency of 9.192 GHz that is designed to be sufficiently rugged for use in space, specifically for the Primary Atomic Reference Clock in Space (PARCS) planned for the International Space Station, as well as ground applications. This new approach requires no narrow band filters or frequency multiplication, and the primary source of cooling is conduction. Instead of frequency multiplication, it uses a custom regenerative divider stage followed by two commercial binary dividers and several mixing stages. A fractional frequency step of 2/spl times/10/sup -17/ is achieved by mixing the output of a 48-bit numerically controlled oscillator with the microwave signal. Preliminary tests on the new synthesizer design indicate an internal fractional frequency stability of 1/spl times/10/sup -15/ at 10 s and 1/spl times/10/sup -18/ at 1 d, dominated by the daily room temperature variations. The phase and amplitude noise are similar to our previous designs that used frequency multiplication and narrow band filters. The temperature coefficient is less than 0.2 ps/K.