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AD9914 phase synchronization in Linear Frequency Modulation application

Question asked by dsx058117@163.com on Sep 19, 2015
Latest reply on Oct 4, 2015 by dsx058117@163.com

Hello,everyone,recently I use AD9914 for Linear Frequency Modulation(LFM) in my project,my aim is to generate a stepped frequency waveform(57.5MHz-62.5MHz)the total points of each sweep is 291, the reference frequency of DDS is 3.5GHz,which is generated by a PLL that use an external clock of 80MHz, the FPGA use the same clock,that runs at the frequency of 80MHzthe digital ramp mode was selected and autoclear phase accumulator was set,my set is CFR1=0x00012008,  CFR2=0x000C0900, CFR4=0x01052120,DRG register is set by above mentioned, to enable autoclear phase accumulator,the profile pin was controled by an external pulse which name is FS_YT,DRCTL was controled by an external pulse which name is ASK(see follow pictures),the period of  FS_YT and ASK is both 40 microseconds,the width of  FS_YT is 1 microseconds,the width of ASK is 4 microseconds,ASK is delayed 5 microseconds.at this circumstance,I got the same starting phase at each frequency sweep.

    But when the external pulse(FS_YT and ASK) is not periodic, I can not get the same starting phase at each frequency sweep,what should  I do to get the same starting phase at each frequency sweep when the external pulse is not periodic?

Best regards!

Gareth.


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