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I have verified that for a pure carrier the power is 0 dBm. But when i see a qpsk/psk modulated signal the power reduces. How can i calculate how much the power has been reduced? Is there any mathematics behind this

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  • $\begingroup$ "when I see a modulated signal, the power reduces": That means you already know the way to calculate the power of the modulated signal. Is that right? $\endgroup$ Oct 8, 2022 at 8:47
  • $\begingroup$ No I just checked on the spectrum analyzer. I observed that for a pure carrier its 0dBm. but for modulated signal level was less about -6dBm. I am not sure about the calculation behind this? $\endgroup$
    – mark
    Oct 8, 2022 at 9:19
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    $\begingroup$ ohhh are you sure you're interpreting what you read on your SA correctly? Unless your resolution bandwidth is > modulated signal bandwidth, then reading -6dB doesn't mean the power has been reduced. $\endgroup$ Oct 8, 2022 at 9:24
  • $\begingroup$ The total power of the modulated signal was 0dBM but the max level was reduced by 6dBm? $\endgroup$
    – mark
    Oct 8, 2022 at 9:37
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    $\begingroup$ sounds about right! notice that if your display says 6dBm, that's a power, not a power density. You really need to understand that the SA shows you powers – in observation bandwidths. So, logically, modulating a carrier needs to distribute the same power on a wider bandwidth. The peak cannot stay the same, unless the observation bandwidth is at least as large as the modulation bandwidth. You might want to revisit what a spectrum analyzer really shows you! $\endgroup$ Oct 8, 2022 at 10:51

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