Using Cursors To Make Measurements; Figure 3 31: Paired Dots Cursors On An Fft Magnitude Display - Tektronix 11801C User Manual

Fast Fourier Transforms
Using Cursors to
Make Measurements
3 72
You can use the cursors to make measurements of an FFT trace display.
Select the FFT trace and touch the Cursors icon to display the Cursors
major menu. See Cursors on page 3 29 for more information on using
cursors.
When making measurements on an FFT trace, recall that the odd numbered
points in the trace record are derived by interpolation. The even numbered
record points are the frequency lines; peaks in the FFT magnitude always
occur on the even numbered points. You can see the peaks more clearly by
using Pan/Zoom to horizontally magnify the FFT trace.
The automated frequency domain measurements are described in
Measurements, on page 3 135.
When you first display cursors on an FFT trace, paired dots cursors are
automatically selected, with one dot cursor placed at DC and the other at
F . You can use the paired dots cursors to make relative measurements of
max
the peaks of an FFT trace. See Figure 3 31.

Figure 3 31: Paired Dots Cursors on an FFT Magnitude Display

The DC value displayed with the FFT magnitude is twice the actual DC
amplitude. This value does not include any vertical offset of the sampling
heads. When making measurements near DC, use the rectangular window
function.
NOTE
Reference