# power of a modulated signal

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

• "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? Oct 8, 2022 at 8:47
• 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?
– mark
Oct 8, 2022 at 9:19
• 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. Oct 8, 2022 at 9:24
• The total power of the modulated signal was 0dBM but the max level was reduced by 6dBm?
– mark
Oct 8, 2022 at 9:37
• 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! Oct 8, 2022 at 10:51