Spectrum block shows incorrect dbm value
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Dear all,
In the attached model NCO generates a sine wave of 32MHz and spectrum is set to RBW of 1MHz, 50Ohm resistance. We can see in the Spectrum, that the peak value in dbm is 6.9823, which corresponds to a peak-to-peak voltage of 1.413V. However, in the scope block we can see, that the signal amplitude is 2V peak-to-peak, which corresponds to 10dbm (dbm = 10*log10(1000*Vrms^2/R)). Why so?
The model is attached.
Thank you!
4 个评论
Sergei
2025-6-26
Mathieu NOE
2025-6-30
funny
the difference you mention is exactly 3 dB (as if you swap Vpeak vs a Vrms value = sqrt(2) factor for a sine wave)
Mathieu NOE
2025-7-1
hello
"funny" was maybe not the most appropriate word, I was just suggesting that there is somewhere a mistake as somewhere we use Vrms instead of Vpeak and that's why we see a 3 dB difference
but as I can't open your slx file (because I run R2020a release) I can't say for sure where the confusion may come from
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David Goodmanson
2025-7-18
编辑：David Goodmanson
2025-7-18
Hi Sergei,
The plot shows peaks at both positive and negative frequencies. Each of those equal size peaks contains half of the power, so each peak will be down by a factor of 2, and
10dBm - 10*log10(2) = 6.9897dBm
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