Hi! I am not familiar with this graph. Can you please explain it to me in a simple manner?
1. What's with NEGATIVE db?
2. And i wish to determine the AMPLITUDE. I am confuse if the dB tells the amplitude of the graph.
Thank you in advance.

 

Hello,

The negative numbers on the dB scale indicate that the signals are below the reference level, wich normaly is set as 0 dB.

Bertus

 

What bertus said. The term db merely expresses a ratio. For example if we say the reference for 0 dbm (power) is 1 mW (.001 Watt) then it becomes a matter of a 30 db gain would be 1 Watt and a -30 db attenuation would be .000001 Watt. For power it would be calculated as db = 10Log^10* Plarge / Psmall. You can read the Wiki here. In your frequency analysis the spectrum analyzer is telling you that at 608 Hz the signal level is -15 db down from whatever the reference is setup as.Just remember db merely expresses a ratio.

Ron

 

i think i have found the answer . Thank you both sir Bertus and sir Ron.