Fast Fourier Transforms
Reference
The FFT Frequency Domain Record
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 and displayed from the same channel or a reference memory. The
length of this
time domain
record is user-specified from the Horizontal menu.
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 wave-
form after it is transformed. 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 data from the FFT time domain record into a frequency
domain record.
Figure 3-24 compares the waveform record to the FFT time domain record.
Note the following relationships:
For waveform records ≤10 K points in length, the FFT uses all of the
waveform record as input.
For waveform records >10 K points, the first 10 K points of the waveform
record becomes the FFT time domain record.
Each FFT time domain record starts at the beginning of the acquired
waveform record.
The zero phase reference point for a phase FFT math waveform is in the
middle of the FFT time domain record regardless of the waveform record
length.