Spectrum chart
The spectrum chart is a frequency domain spectrum plot of a single channel. The frequency is displayed along the horizontal axis and the spectral power/amplitude is displayed along the vertical axis. The vertical axis can be configured in one of three modes described under Spectrum properties below.
The following options are available from the menu. Click the options menu button:
- Freeze: Select to pause the data display. Deselect to resume live data.
- Properties: Open the chart properties dialog box.

NOTE
Change default chart options, such as background color and grid display, in User preferences.
After entering information in the Spectrum properties dialog, click the OK button.
- Output type: Set the output type and scale for the Y axis.
- Amplitude (log10 Y axis): Spectral amplitude is shown on the vertical axis using a logarithmic scale.
- Power (dB from Peak): Attenuation from peak is shown on the vertical axis. The current peak amplitude is plotted as 0 and all other values are plotted in decibels below this peak value.
- Amplitude (linear Y axis): Spectral amplitude is shown on the vertical axis using a linear scale.
FFT Parameters
NOTE
The Hann window function is applied to the sample window when calculating the spectrum.
- FFT size: Set the size of the FFT window in number of samples.
Frequency Range (X Axis)
- Automatic: Select to automatically scale the horizontal axis. Zero Hz (representing the signal's DC bias) is placed at the left and the Nyquist frequency (half the sample rate) is placed at the right.
- Minimum frequency: If automatic scaling is not selected, specify the frequency value at the left edge of the horizontal axis.
- Maximum frequency: If automatic scaling is not selected, specify the frequency value at the right edge of the horizontal axis.
Output Range (Y Axis)
- Automatic: Select to automatically scale the vertical axis. The top and bottom values are set so that all spectral data is visible. The scaling may shift during a test as the spectrum changes.
- Minimum value: If automatic scaling is not selected, specify the value at the bottom edge of the vertical axis. For "Amplitude" output types, this value is in channel units. For the "Power" output type, this value is in decibels from peak (and values are always negative or zero).
- Maximum value: If automatic scaling is not selected, specify the value at the top edge of the vertical axis. For "Amplitude" output types, this value is in channel units. For the "Power" output type, this value is in decibels from peak (and values are always negative or zero).