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In recent years, with the development of hybrid domain oscilloscope technology, the oscilloscope not only realizes the function of traditional oscilloscope, but also the function of frequency domain and modulation domain. So it is necessary to realize the function of real-time digital down-conversion (DDC) in digital domain signal processing. Real-time DDC technology is the basis of extending the function of oscilloscope to frequency domain and modulation domain. The value-added application of oscilloscope can be realized, and the application field of oscilloscope can be greatly expanded. According to the characteristics of high-speed signal sampling, this paper presents a real-time DDC technology architecture, which consists of digital orthogonal mixing, FIR1-FIR3 filter and HB1-HB10 filter. For 20GSa/s sampled data stream, the maximum support is 1.25GSa/s I/Q data stream output and the minimum 305 kSa/s I/Q data stream output, which can satisfy most application scenarios. Then, the digital orthogonal mixing, FIR1 filter, FIR2 filter, FIR3 filter and HB filter are designed and analyzed in detail, and the implementation architecture is given. For FIR and HB filters, the optimal filter order and its amplitude-frequency response curve are also given. For digital orthogonal mixer and FIR1-FIR3 filter, signal processing is realized by means of multi-channel parallel processing because its digital rate exceeds the normal working time range of the FPGA. Finally, the EVM performance of DDC at 13 I/Q rates is evaluated by vector signal analysis software. The EVM performance of carrier frequencies of 1.5 GHz and 3 GHz is evaluated respectively. Through the evaluation, the EVM value is mostly concentrated below 0.5%, which can meet the needs of application.