tds2012_oscilloscope_guide.doc
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The
+
and
-
options will display the sum or difference of the two signals
on the display
. It is
important to note that the sum or difference will be calculated using the
visual
signals on the
display. This means that if one signal is displayed to a higher magnification level (i.e., different
VOLTS/DIV setting) than the other, the result will not be a true sum or difference. For most
situations it is best to set both waveforms to the same scale before using these options.
Table 2
FFT menu options
The
FFT
option stands for Fast Fourier Transform. This can be used to analyze/represent the
frequency components of a time-domain signal. To use this feature, you need to do the
following:
1.
Set up a source waveform in the time domain.
2.
Display an FFT spectrum
3.
Select an FFT window type
4.
Adjust the sample rate to display the fundamental frequency and harmonics without
aliasing
5.
Use the zoom and cursor controls to measure what you wish to measure
To set up the waveform in the time domain, use Autoset or the control knobs to center the
waveform vertically. Also make sure that the entire waveform is visible on the display.
Horizontally, the FFT will only consider the eight central segments of the display, so make sure
that what you want to analyze fits into this range.
The Sec/Div knob allows you to choose a resolution. Set it to show fewer cycles on the display
to show a larger frequency range and reduce aliasing. Set it to display more cycles to improve the
resolution.
The highest frequency that any real-time digitizing oscilloscope can measure without
errors is one-half the sample rate. This frequency is called the Nyquist frequency. Frequency
information above the Nyquist frequency is undersampled which causes FFT aliasing.
The math
function transforms the center 2048 points of the time-domain waveform into an FFT spectrum.
The resulting FFT spectrum contains 1024 points that go from DC (0 Hz) to the Nyquist
frequency. Once the time domain waveform is set up, push the Math Menu button and choose
the FFT option. Be sure to select the correct source channel. A sample FFT output is below:
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