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Interpolation filter sin(x)/x turned on, but interpolated data not read

Question asked by george001 on Mar 30, 2015
Latest reply on Apr 1, 2015 by vcherian
Hi All,

I have a problem on an infiniium 54180 (yes a pretty old scope) that I was hoping to resolve. The scope has bandwidth of 500MHz and a sampling frequency of 1GS/s I'm new to using these scopes so I apologize if this is a simple question. However, I tried the programmer's guide but could not resolve the issue. My problem as clear in the title, is that I cannot read back the interpolated data from the scope, although I turn the interpolation filter on.  Here's the MATLAB code that I am using (adapted from the code on file exchange in matlab central). visaObj is just the scope.
     

     fprintf(visaObj,'*RST; :AUTOSCALE'); 
     fprintf(visaObj,':STOP');
     fprintf(visaObj,':WAVEFORM:SOURCE CHAN1');
     fprintf(visaObj,':ACQUIRE:COUNT 8');
     fprintf(visaObj,':ACQUIRE:COMPLETE 100');
      fprintf(visaObj,':ACQUIRE:MODE INTERPOLATE');
     fprintf(visaObj,:ACQUIRE:POINTS 1024);
     % Now tell the instrument to digitize channel1
     fprintf(visaObj,':DIGITIZE CHAN1');
     % Wait till complete
     operationComplete = str2double(query(visaObj,'*OPC?'));
     while ~operationComplete
        operationComplete = str2double(query(visaObj,'*OPC?'));
     end
     fprintf(visaObj,':WAVEFORM:FORMAT WORD');
     fprintf(visaObj,':WAVEFORM:BYTEORDER LSBFirst');
     preambleBlock = query(visaObj,':WAVEFORM:PREAMBLE?');
     fprintf(visaObj,':WAV:DATA?');
     waveform.RawData = binblockread(visaObj,'int16'); fread(visaObj,1);
     %fprintf(visaObj,':CHANNEL1:DISPLAY ON');
     % process data
     % store all this information into a waveform structure for later use
     preambleBlock = regexp(preambleBlock,',','split');
     waveform.Points = str2double(preambleBlock{3});
     waveform.XOrigin = str2double(preambleBlock{6});    % in seconds
     waveform.XReference = str2double(preambleBlock{7});
     waveform.YIncrement = str2double(preambleBlock{8}); % V
     waveform.YOrigin = str2double(preambleBlock{9});
     waveform.YReference = str2double(preambleBlock{10});
     waveform.VoltsPerDiv = (maxVal * waveform.YIncrement / 8);      % V
     waveform.Offset = ((maxVal/2 - waveform.YReference) * waveform.YIncrement + waveform.YOrigin);         % V
     waveform.SecPerDiv = waveform.Points * waveform.XIncrement/10 ; % seconds
     waveform.Delay = ((waveform.Points/2 - waveform.XReference) * waveform.XIncrement + waveform.XOrigin); % seconds
     % Generate X & Y Data
     waveform.XData = (waveform.XIncrement.*(1:length(waveform.RawData))) - waveform.XIncrement;
     waveform.YData = (waveform.YIncrement.*(waveform.RawData - waveform.YReference)) + waveform.YOrigin; 
    

   I display a 200MHz sine wave signal on the scope and it looks good because of the interpolation filter. Now, if I plot the data I get back from the scope, the sine wave looks bumpy similar to what it looks like on the scope with the filter turned off. (Only 5 samples per sine wave cycle, so not looking so good). 

Does anybody have insight as to why this is happening ? I would also appreciate it if any errors are pointed out in the code. Thanks a lot !  

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