Reference
log scale, expressed in dB relative to 1 V
RMS
,where0dB=1V
RMS,
or
in source waveform units (such as amps for current measurements).
To display FFT waveforms with a small dynamic range, use the
Linear RMS scale. The Linear RMS scale lets you display and
directly compare components with similar magnitu de values.
Nyquist Freque ncy. To determ ine the Nyquist frequency, push the
Acquire men u button. This displays the current sample rate on the
bottom right area of the scre
en. Th e Nyqu ist frequency is one-half of
the sample rate. For example, if the sample rate is 25.0 MS/s, then the
Nyquist f requ ency is 12.5 MHz.
Zooming an F FT Display. Use the Zoom button , along with horizon tal
Position and Scale controls, to magnify FFT waveforms. When you
change the zoom factor, the FFT waveform is horizontally magnified
about the center v ertical graticule, and vertically magnified about the
math waveform marker. Zoom ing does not affect the actual time base
or trigger position setting s.
NOTE. FFT waveforms are calculated using the entire source w ave form
record. Zooming in on a region of either the source or FFT waveform will
not recalculate the FFT waveform for that region.
Measuring FFT Waveforms Using Cursors. You can use cursors to take
two measurements on FFT wavefo rms: magnitude (in dB or signal
source units
) and frequency (in Hz). dB magnitude is referenced to
0 dB, where 0 dB equals 1 V
RMS
. Use horizontal cursors (H Bars) to
measure magnitude and vertical cursors (V Bars) to measure f requency
Magnitude cursors Frequency cursors
TDS3000C Series O scilloscope User Manual 95
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