Research On Precision Time Base Module Technology Of Sampling Oscilloscope

Sampling oscilloscope adopts equivalent time sampling technology,which can be used in low-frequency real-time sampling rate and can be used to sample high-frequency periodic repetitive signals without distortion.It is used widely in the waveform testing of microwave signals,ultra-bandwidth pulse signals,and radar signals.Compared with real-time sampling oscilloscope,it has higher input bandwidth and equivalent sampling rate.And the cost of implementation is relatively lower.The precision time base module belongs to the front end of the synchronous trigger signal input of the sampling oscilloscope.Through the tuning of the high-speed external synchronization trigger signal,a low-frequency high-precision step delay sampling pulse signal is generated,control the sampler to sample the high frequency signal,which is a key module to achieve high equivalent sampling rate.This article designs the precision time base module of sampling oscilloscope according to the theory of sequential equivalent sampling to support 2.5GHz synchronous trigger signal input.The delay precision and delay range can be flexibly adjusted according to the time base and the number of sampling points.The research content of this article is mainly elaborated from the following aspects:Firstly,Study the realization method and principle of random equivalent sampling and sequential equivalent sampling of sampling oscilloscope.Perform feasibility analysis on existing methods for realizing precise time base functions.Face the shortcomings of traditional methods.Combined with the research objectives of the subject,we propose a new stepping delay pulse generation method "stepping phase shifting delay method".It has laid a theoretical foundation for the design of precision time base module.Secondly,Design a precision time base hardware system with "FPGA + DDS" as the core architecture.Select the key high-speed devices in the circuit according to the indexrequirements.Complete the hardware circuit design of input signal conditioning circuit,DDS chip and peripheral circuit,clock signal processing circuit,signal synchronization circuit,FPGA system,serial communication circuit and power supply circuit.Thirdly,the software's design of the precision time base module which uses FPGA as the core control chip adopts equal precision frequency measurement algorithm to complete the frequency measurement of external trigger signal,and completes the conversion calculation of delaying time and phasing shift offset word parameters.Adopting DDS chip AD9914 to achieve high-precision step-shift clock generation,through configuring Profile mode and programmable modulus mode,generates an accurate clock signal with adjustable frequency and phase.According to the designed step-by-phase phase shift word accumulation program,it uses the SPI communication protocol to configure the AD9914 register,completes the generation of high-precision step-shift clock signal and inputs signal synchronization circuit to achieve 40 KHz step delay pulse signal output.Finally,it verifies the functions of the designed precision time base module of the sampling oscilloscope,uses Modelsim software to perform simulation verification on the AD9914 multi-register driver and tests and analyzes the frequency spectrum of the DDS output clock signal.Besides,it uses an oscilloscope to test and verify the AD9914's programmable modulus mode and the generation of a step-shifted clock.And time-domain testing of the step delay pulses output by the signal synchronization circuit,which has completed the basic functions of the precision time base module.In theory,it can output step delay pulses with a resolution lower than 10 ps at the highest to achieve an equivalent sampling rate of 100 GSPS,the delay range of the system reaches 2ns~200ns.