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I am using a spectrum analyser to measure the 248kHz buck switching frequency and its harmonics in my board. I have set the frequency range from 200kHz and 1MHz and I am using a near field H-probe.

When I give peak search on the spectrum analyser, I get a peak at the 248kHz frequency with an amplitude of 40dB. 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.) 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.

Edit :

Please find the attached image from this AppNote from Keysight

enter image description here

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

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  • \$\begingroup\$ "dBmV" means the reference level is 1 mV (rms, I believe). \$\endgroup\$
    – The Photon
    Nov 13, 2019 at 3:10
  • \$\begingroup\$ Can you share a screen shot showing where you're seeing "dB" and where "dBmV"? \$\endgroup\$
    – The Photon
    Nov 13, 2019 at 3:11
  • \$\begingroup\$ the Reference Level may depend on attenuators you select to protect the input diode mixer of the Spectrum Analyzer. In older SA, you had to find that setting, and perform add/subtract. On computerized SA, the display will show the Reference Level in dBm (0 dBm across 50 ohms is 0.223 volts rms, or 0.632 volts peakpeak) \$\endgroup\$ Nov 13, 2019 at 3:47
  • \$\begingroup\$ The SA does not have a screen capture option. \$\endgroup\$
    – user220456
    Nov 13, 2019 at 4:04
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    \$\begingroup\$ the very top left reads "17 dBm", which is 50 milliWatts. \$\endgroup\$ Nov 13, 2019 at 5:21

3 Answers 3

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The reference level is an absolute power value (in dBm) that just belongs to the top of the screen.

If you change the reference level, the screen is simply vertically scrolled, but the measured curves stay the same.

All values shown in dB are displayed in relation/relative to the reference level.

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Reference level on a spectrum analyzer is the power level wherein all the inputs are compared for plotting on the screen. For example, if you are to plot a satellite downlink, reference level of around -65dBm may be appropriate. Wrong selection would mean that the plot will not be visible on the SA screen.

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R&S teaches for the signal of interest to be with 10dBm of the reference as shown on the picture

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