Abstract: Three kinds of useful frequency sources are discussed, viz. The crystal oscillator-multiplying frequency sources; frequency sources in which both hybrid microwave integral circuit (HMIC) and phase-locked loop (PLL) are combined and the frequency sources where both monolithic-microwave integral circuit (MMIC)and digital PLL are combined. These sources possess many advatages: compactness. Light weight (a reduction to the order of 10 -- 100 times), highly reliability. Low cost & power consumption. Their application will have wonderful results. Besides, the paper suggests some proposals for the development of satellite-borne SAR frequency sources in China.