0

I am reading research papers about LoRa, and I have come across the below figure in this one. The horizontal axis is clearly labeled frequency, but the vertical one isn't. I recognize the chirps in the middle, so I understand that the vertical axis is indeed time for that graph, but I still have no idea what the other, darker blue graph represents. Could you shed some light on this please?

LoRa spectrogram

Boba0514
  • 3
  • 2

2 Answers2

0

These are two different traces.

  • The blue one at the bottom is the raw signal with its received power (in dBm, left scale) vs. time. -90 dBm is 1 pW of received power.
  • The colored one is the spectrogram with time on the vertical axis and frequency on the horizontal axis.
asdfex
  • 2,669
  • 1
  • 13
  • 17
  • never saw amplitude over time being given in dB! – Marcus Müller Dec 30 '20 at 15:18
  • @MarcusMüller Sorry, that should read signal power in dBm in this case - but sure, dB can be used to measure the amplitude of a signal as well. – asdfex Dec 30 '20 at 15:20
  • of course it can, but honestly, I've never seen that be done as plot over time. The utility of such a plot would be pretty specific, so I'll venture a guess that you're not completely right about that. – Marcus Müller Dec 30 '20 at 15:21
  • Yeah, I was thinking about something like this, but if that's so, it is a terrible way to plot it here, since time is already used on the other axis, and the horizontal axis doesn't have a secondary labeling... – Boba0514 Dec 30 '20 at 15:26
  • @Boba0514 : The horizontal axis has two labels. "500 kHz span" and "100 ms/div". Although it's not clear what a division is as the marks are missing. As Marcus wrote - it's not a good graphic. – asdfex Dec 30 '20 at 15:29
  • Yeah, you're right. Thank you for the help. – Boba0514 Dec 30 '20 at 15:30
0

That's two diagrams in one:

  1. The background one, the turquoise zig-zag, is called a waterfall; the vertical axis is time, the horizontal is frequency, and power is coded into color.
  2. the line at the bottom is something else; maybe just a time trace; it's at the very least not a representation of the spectrum at the same bandwidth, because otherwise, the center would have to have higher amplitude (where the zigzagging happens)

Generally, if a figure is unclear, that's a fault in the publication you're reading, and since this curve isn't described any further, I'd recommend ignoring it.

Marcus Müller
  • 88,280
  • 5
  • 131
  • 237