Note
1. The FFT of a waveform that has a DC component or offset can cause incorrect FFT
waveform magnitude values. To minimize the DC component, choose AC Coupling
on the source waveform.
2. To display FFT waveforms with a large dynamic range, use the dBVrms scale. The
dBVrms scale displays component magnitudes using a log scale.
3. The Nyquist frequency is the highest frequency that any real-time digitizing oscilloscope
can acquire without aliasing. This frequency is half that of the sample rate provided it
is within the analog bandwidth of the oscilloscope. Frequencies above the Nyquist
frequency will be under sampled, which causes aliasing.
5. Measuring an FFT Spectrum Using Cursors
You can take two measurements on FFT spectrums: magnitude (in dB) and
frequency (in Hz). Magnitude is referenced to 0 dB, where 0 dB equals 1 VRMS.
You can use the cursors to take measurements at any zoom factor. (Refer to
cursor measure2.11.2)
1. Measure FFT Amplitude
●
Input
a sine signal to channel 1, and press the AUTO button.
●
Press
the MATH button to enter the MATH menu.
●
Press
the Operation option button to select FFT.
●
Press
the Source option button to select Ch1.
●
Press
CH1 button to display CH1 menu.
●
Turn
the S/div knob to adjust sampling rate (at least double bigger than
frequency of input signal).
●
If
FFT display on full screen, press CH1 button again to remove channel
waveform display.
●
Press
the CURSOR button to enter Cursor menu.
●
Press
the Cursor Mode button to select Manual.
●
Press
the Type option button to select Voltage.
●
Press
the Source option button to select FFT.
●
Press
the CurA option button; turn the Universal knob to move Cursor A to
the highest point of the FFT waveform.
●
Press
the CurBoption button, turn the Universal knob to move Cursor B to
the lowest point of the FFT waveform.
●
The
amplitude (△T) displays on the top of the left screen.
2.Measure FFT Frequency
●
Press
the CURSOR button.
●
Press
the Cursor Mode button to select Manual.
●
Press
the Type option button to select Time.
●
Press
the Source option button to select FFT.
●
Press
the Cur1 option button, turn the Universal button to move Cursor 1
to the highest position of the FFT waveform.
●
The
value of Cur1 displaying on the top of the left screen is FFT highest
Frequency. This frequency should be the same as input signal frequency.
If you input a sine signal to channel 1, follow these steps:
22
Digital
S
torage
O
scilloscope
Digital Storage Oscilloscope
23
Use horizontal cursors to measure amplitude and vertical cursors to measure
frequency.
FFT
FFT
