Design Of Analog Front-end Of 16 GHz Bandwidth Digital Oscilloscope Based On Four Sub-band Sampling

With the continuous development of modern communication technology and radar technology,high-bandwidth and high-sampling rate oscilloscopes are required to capture and analyze high-speed signals.The bandwidth of the oscilloscope is mainly determined by the bandwidth of the analog front end and the ADC,and the domestic oscilloscope is limited by the ADC bandwidth to meet the needs of high-bandwidth oscilloscopes.Sample-and-hold circuit,ATI technology and subband sampling technology are common methods to improve the bandwidth of oscilloscopes,and the scheme using sample-andhold and ATI system technology solutions have high requirements for the accuracy of the clock system,and the circuit structure is relatively complex.Therefore,the thesis uses subband sampling technology to expand the analog bandwidth of the oscilloscope,and then realize a digital oscilloscope with a bandwidth of 16 GHz.The main research contents of the thesis are as follows:First,according to the demand analysis of oscilloscope bandwidth,sampling rate and gain,the design scheme of 16 GHz digital oscilloscope analog front end based on subband sampling was formulated,and the analog front end with 16 GHz bandwidth was divided into four subbands,of which the first subband bandwidth range was DC～6 GHz.The second subband bandwidth range is 6～10 GHz;The third subband bandwidth range is10～15 GHz;The fourth subband bandwidth range is 15～16 GHz.Second,design and implementation of analog front-end circuits.The first subband realizes the impedance matching of the input signal by designing an impedance conversion circuit with high and low frequency separation;The addition and subtraction operation amplification circuit is used to realize the fine controllable adjustment of DC bias.The combination of cascaded fixed-gain amplification circuit and programcontrolled attenuator realizes the variable gain conditioning function of analog signal,and can complete the single-ended to differential conversion of input signal.The second to fourth subbands are realized by mixing frequency amplification circuit,which mixes with local oscillator signals of 10.5 GHz,9.5 GHz and 14.5 GHz,respectively,and moves the input signal spectrum to the bandwidth range of 500 MHz～5.5 GHz to complete the acquisition.The gain compensation circuit is used to optimize the flatness of the frequency response in the output bandwidth.On the basis of ensuring the bandwidth of the analog front-end,the cascaded low-noise amplification circuit combined with the digital control attenuator realizes the flexible adjustment function of subband gain;The ADC driver circuit is designed to achieve single-ended to differential conversion of the signal without changing the output gain.Third,the design of shielding chambers.Aided by CST simulation software,a rectangular resonant cavity is used to suppress crosstalk between local oscillator signals and spurious signals in the output spectrum.Based on the transmission line theory,the ground coplanar waveguide transmission line is used to suppress the radiation of the local oscillator signal.The PCB gold plating process and gradient trace method are used to optimize the amplitude-frequency curve at the impedance discontinuity in the signal transmission path.Through the test,the analog channel meets the controllable range of 16 GHz input bandwidth,50 Ω input impedance and-18～22 dB gain,and the spurious signals in each subband are less than-40 dBc;The gain controllable range of the first subband is-9～22dB;The gain controllable range of the second subband is 3～33 dB;The gain controllable range of the third subband is-3～30 dB;The gain controllable range of the fourth subband is-0.5～30 dB,which meets the design requirements.