# Spectrum Analyzer – Reference dB Level Explained

decibelspectrumspectrum analyzerswitching

I am using a Spectrum analyser.

Measuring the 248kHz buck switching frequency and its harmonics in my board.
I have set the frequency range from 200kHz and 1MHz. Using a near field H-Probe

When I give peak search on the spectrum analyser, I get a peak at the 248kHz freq with amplitude of 40dB and when I change the amplitude level in voltage, I get -14dBmV.

I also get other harmonics too.

I just want to understand two things :

1. I know the dB is just the ratio of two quantities (power or voltage). But when I get 40dB and -14dBmV, what is the reference level that is taken by the spectrum analyser which it compares with the output power? I assume the spectrum analyser takes a reference voltage or power level and compares it with my board output level and then only it provides the value of 40dB and -14dBmV. Am I correct?

2. What does the +ve and -ve dB value signify in electrical terms?

Please understand that I am just starting the learn the uses and the operation of spectrum analyser. Would be glad if someone could help with the answers in simpler terms.

Thank you

Edit :

Please find the attached image from this AppNote from Keysight

The images shows a reference level set at the top. How is this value chosen and on what basis?