Fast Fourier Transforms
TDS 420A, TDS 430A, TDS 460A & TDS 510A User Manual
3–33
10. Press Function
(main) ➞ Paired (side).
11. Place the vertical bar of each paired cursor to the points along the horizontal
axis you are interested in.
12. Read the amplitude between the X of the two paired cursors from the
top-most : readout. Read the amplitude of the short horizontal bar of the
selected (solid) cursor relative to either 1 V
RMS
(0 dB), ground (0 volts), or
zero phase level
(0 degrees or 0 radians) from the @: readout. Read the
frequency between the long horizontal bars of both paired cursors from the
bottom : readout.
You can also use automated measurements to measure FFT math waveforms.
Use the same procedure as is found under Waveform Differentiation on
page 3–132.
Considerations for Using FFTs
There are several characteristics of FFTs that affect how they are displayed and
should be interpreted. Read the following topics to learn how to optimize the
oscilloscope setup for good display of your FFT waveforms.
The following topics discuss the relation of the source waveform to the record
length, frequency resolution, and frequency range of the FFT frequency domain
record. (The FFT frequency domain waveform is the FFT math waveform that
you display.)
FFTs May Not Use All of the Waveform Record. The FFT math waveform is a
display of the magnitude or phase data from the FFT frequency domain record.
This frequency domain record is derived from the FFT time domain record,
which is derived from the waveform record. All three records are described
below.
Waveform Record — the complete waveform record acquired from an input
channel. The waveform record is not a DSP Math waveform.
FFT Time Domain Record — that part of the waveform record that is input to the
FFT. This time domain record waveform becomes the FFT math waveform. Its
record length depends on the length of the waveform record defined above.
FFT Frequency Domain Record — the FFT math waveform after digital signal
processing converts the FFT time domain record into a frequency domain record.
Automated Measurements
of an FFT
The FFT Frequency
Domain Record
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