Development Of Synchronous Timing Generator Based On DDS

In order to make the MRI system,mass spectrometer,spectroscopy and other equipment work normally and maintain high performance,it is necessary to provide a series of delay pulses to make the internal hardware units work together.Therefore,the research of synchronous timing generator is especially important.Based on the requirements of magnetic resonance imaging systems and analytical instruments such as spectroscopic instruments,this project has developed a DDS-based synchronous timing generator that can realize adjustable delay and pulse width resolution,output multi-channel adjustable frequency,delay,Timing signal for pulse width.The main work of this paper is as follows:(1)Based on the application requirements of the magnetic resonance system and the analytical instrument,the technical specifications of the synchronous timing generator are determined,and two working modes of the timing generating system are proposed.One is the internal trigger mode,which is combined with the frequency multiplier and DDS.The technical solution is designed with a frequency synthesizer to provide a variable frequency reference clock for the FPGA,which enables a single device to output multi-channel synchronous timing signals to meet the needs of complex devices and small applications;the other is the external trigger mode.Using the external input clock and trigger to obtain the global clock and pulse trigger signal,multiple timing generator cascades are realized,which meets the needs of largescale devices and large-scale engineering applications,and expands the application scenario.(2)The hardware design and software writing of the frequency synthesizing unit,the control unit,and the multi-channel waveform generating unit of the timing generator are completed.The frequency synthesizing unit uses the frequency doubling chain circuit to provide the system clock for the DDS chip,which not only satisfies the output frequency requirement,but also makes the output signal have higher frequency resolution,lower phase noise and jitter;the system control unit selects LPC1766 as the main control chip.To realize user interaction,FPGA communication,DDS control and other functions;multi-channel pulse generation unit is the function realization module of the instrument,based on FPGA to complete the acquisition,storage,verification and other operations of pulse parameters,output multi-channel timing pulses through delay counting signal.(3)Tested the functions and indicators of the synchronous timing generator.A single device can output 20 channel delay pulses with a delay range of 0~200ms and a resolution of at least 6.4ns.The internal trigger output multi-channel pulse jitter peak-to-peak value < 250 ps,RMS jitter value <20ps,rising edge <500ps,meeting engineering application requirements.In addition,the DDS frequency source is the core part of the instrument.The output sine wave signal frequency is 20~160MHz,the phase noise can reach-135 dBc/Hz@1k Hz,-145 dBc/Hz@10k Hz,and the narrowband SFDR is-85 dBc when the output frequency is 155.52 MHz.The 1GHz wideband SFDR is-60 dBc,and the spectrum analyzer measures signal period jitter <0.5ps,which satisfies the device specifications and provides a common frequency source for high-precision timing generators.