Math
162
13.4.2
Parameter Display Area
The FFT parameters are displayed in the upper right of the spectrum waveform display area:
Figure 13.18
Parameter Display Area
•
FFT sample rate (Sa)
: FFT operation results present the first Nyquist zone (DC Sa/2) of the frequency spectrum.
Be aware that the FFT sample rate may be inconsistent with the sample rate in the time domain. Assuming the max
points is set to 2 Mpts:
–
When the number of points in the time domain, N, is less than 2 Mpts, the FFT takes the number which is an
integer power of 2 closest to N. In this case, the FFT sample rate = sample rate in the time domain.
–
When N is greater than 2 Mpts, FFT first decimates N by D and then takes the first 2 Mpts for the calculation.
In this case, the FFT sample rate = sample rate in time domain/D.
For example, in the case of sample rate in the time domain is 1 GSa/s and the number of samples is 10 Mpts,
the FFT first decimates the samples by 4, to 2.5 Mpts, and then takes the first 2 Mpts to calculate the spectrum.
In this example, FFT sample rate = 1 GSa/s ÷ 4 = 250 MSa/s.
•
FFT points (Curr)
: The current FFT points, which is an integer power of 2, up to 2 M points (2097152, to be
exact).
•
Frequency interval (f)
: The frequency interval between two adjacent points in the FFT sequence, which is
proportional to the frequency resolution.
•
Average count of FFT (Avg)
: Displayed only when the FFT mode is set to "Average", indicating the completed
average count.
13.4.3
Vertical
Touch
Vertical
in the math menu to recall the FFT vertical menu:
•
Touch
Scale
to set the vertical scale of FFT waveform by the universal knob
or the virtual keypad.
–
It can also be set by the scale knob shared by Decode, Digital, Math,
and Ref on the front panel.
–
The reference point for vertical scale scaling is the reference level.
•
Touch
Ref Level
to set the reference level of FFT waveform by the universal
knob or the virtual keypad.
–
It can also be set by the offset knob shared by Decode, Digital, Math,
and Ref on the front panel.
•
Set the unit (dBVrms, Vrms and dBm). When the unit is dBm, the
oscilloscope will automatically calculate the dBm value based on the Ext
Load value set in area.
Figure 13.19
Vertical Menu
