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How to use FFT Analysis on the MSO5000, DS7000 and MSO8000

Last Updated: Aug 30, 2019 04:34PM PDT
These instruments all come with an FFT analysis capability built in that can be accessed at anytime, in order to show you how to use the feature this demo below use the probe compensation port on the front of the instrument.  

Reset the oscilloscope and then connect an analog probe to channel 1 and then connect it to the probe compensation port.
 
  • Press “AUTO”
  • Press “Math”, then select your Math Channel then you will need to change the “Operator” to be FFT and then you will need to turn the “Operation” on.
  • Select “CH 1” as the source
  • Set your offset to be 20 dBV and your scale to be 10 dBV.
  • To change the instruments displayed span you will need to press “More” and then you can change the “Start” frequency and “End” frequency.  In the Image below, I used a “Start” frequency of 0 Hz and an “End” frequency of 50 kHz.
The FFT measurement has some advanced features built in, these include the ability so show the FFT in a frequency intensity display add labels to the signal along with easily change the signals span, scaling and display up to four different FFTs at once.  To demonstrate some of the advanced features you will need to use a waveform generator either the built-in generator or with an external waveform generator.
 
  •  Set up the waveform generator so that it is a producing a square wave with a frequency of 5 MHz and an amplitude of 5 Vpp.
  • Press “Auto” on the oscilloscope and then change the horizontal scaling so that it has a horizontal scaling of 20 us and then change the vertical scaling so that it has a vertical scale of 5 V and then move the signal so that it is on the upper part of the display.
  • Press “Math”, then select your Math Channel (Note that this instrument can have up to four different math operations operating at once and you can perform math operation of pervious math operations allowing you to perform some advance math functions), then you will need to change the “Operator” to be FFT and then you will need to turn the “Operation” on.
  • Select “CH 1” as the source.
  • Set your offset to be 20 dBV and your scale to be 20 dBV.
  • To change the instruments displayed span you will need to press “More” and then you can change the start frequency and end frequency.  In the Image below, I used a “Start” frequency of 0 Hz and an “End” frequency of 100 MHz.
  • To add a label to the FFT and display it in a frequency intensity display you will need to press “More”.
  • To add a label, you can either select a label from the built in “Library” or you can create you own custom label by pressing “Label” and then use the on screen keyboard to enter you desired label.  Once one has been selected turn the “Display” ON. 
To the FFT display the signal in a frequency intensity display mode press “Color” and turn it ON.  It will take some time for the FFT to propagate the display but after a sort period of time you will have an image similar to the one below. 
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