Spectrum Analysis
R&S
®
Scope Rider RTH
109User Manual 1326.1578.02 ─ 12
Aliasing
In FFT mode, no lowpass filter is applied. If aliasing occurs, try restricting the fre-
quency range of the input signal using the Channel Bandwidth setting.
Note that, as opposed to FFT mode, the Spectrum mode (option R&S RTH-B18) uses
an anti-aliasing filter.
The frequency axis is adjusted to the available spectrum and the selected X-Scaling
mode (linear or logarithmic). For linear scaling, the frequency axis starts at 0 Hz.
For logarithmic scaling, it starts at the frequency resolution (the distance between two
individual frequency values on the x-axis), which depends on the selected Frequency
Span and the Channel Bandwidth.
In logarithmic mode, not all possible amplitude values for a specific frequency may
occur. Each result is indicated by a bright, colored trace point.
The DC amplitude (at 0 Hz) is not displayed, as lg(0) is not defined.
The maximum frequency value depends on the following parameters and is adjusted
accordingly:
●
Sample Frequency (the sample rate with which FFT analysis is performed)
●
Channel Bandwidth
●
Installed bandwidth options
Amplitude range
The amplitudes determined for each frequency are displayed on the y-axis of the spec-
trum. The values are scaled in dBm (dBmW) based on 50 Ω impedance.
To change the displayed range, use the [RANGE] keys and the [POS] keys. The posi-
tion of the 0 dBm line is indicated by an "F" on the y-axis.
AUTOSET, vertical, and trigger settings
Vertical and trigger settings for the input signal must be applied in Scope mode, before
FFT mode is selected. The same applies to automatic adjustment of the instrument
settings to the input signal ([AUTOSET]).
6.1.3 Performing FFT Analysis
1. Before you can perform FFT analysis, adjust the waveforms by editing the vertical,
horizontal, trigger and acquisition settings.
For details, see:
● Chapter 3.2, "Vertical Setup", on page 40
● Chapter 3.3, "Horizontal Setup", on page 45
● Chapter 3.6, "Trigger", on page 50
FFT Mode
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