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Fast Fourier Transforms
TDS 620A, 640A & 644A User Manual
3Ć45
Record Length
Most often, you will want to use a short record length because more of the
FFT waveform can be seen on screen and long record lengths can slow
oscilloscope response. However, long record lengths lower the noise relative
to the signal and increase the frequency resolution for the FFT. More imporĆ
tant, they might be needed to capture the waveform feature you want to
include in the FFT.
To speed up oscilloscope response when using long record lengths, you
can save your source waveform in a reference memory and perform an FFT
on the saved waveform. That way the DSP will compute the FFT based on
saved, static data and will only update if you save a new waveform.
Acquisition Mode
Selecting the right acquisition mode can produce less noisy FFTs.
Set up in Sample or Normal Mode Ċ Use sample mode until you have set
up and turned on your FFT. Sample mode can acquire repetitive and nonreĆ
petitive waveforms and does not affect the frequency response of the source
waveform.
Hi Res and Average Reduce Noise Ċ After the FFT is set up and disĆ
played, it might be useful to turn on Hi Res mode, on TDS models so
equipped, to reduce the effect of noise in the signal. Hi Res operates on
both repetitive and nonrepetitive waveforms; however, it does affect the
frequency response of the source waveform.
If the pulse is repetitive, Average mode may be used to reduce noise in the
signal at a cost of slower display response. Average operates on repetitive
waveforms only, and averaging does affect the frequency response of the
source waveform.
Peak Detect (on TDS models so equipped) and Envelope mode can add
significant distortion to the FFT results and are not recommended for use
with FFTs.
Zoom and Interpolation
Once you have your waveform displayed optimally, you may magnify (or
reduce) it vertically and horizontally to inspect any feature you desire. Just
be sure the FFT waveform is the selected waveform. (Press MORE, then
select the FFT waveform in the More main menu. Then use the Vertical and
Horizontal SCALE knobs to adjust the math waveform size.)
If you wish to see the zoom factor (2X, 5X, etc.) you need to turn Zoom on:
press ZOOM ➞ On (side). The vertical and horizontal zoom factors appear
on screen.
Whether Zoom is on or off, you can press Reset Zoom Factors (side) to
return the zoomed FFT waveform to no magnification.