Welltech ePBX40 V.2.0 - Manual (Page 120)

Welltech Technology Co., Ltd. 120
Figure 1.
Step 2: select all of waveform then go to Analyze  Plot Spectrum.
(See figure 2)
Figure 2
Step 3: move the cursor to first peak. And you can see low frequency on red block.
(See figure 3)