OFDM measurement problem using spectrum analyzer

Dear all,

I use the spectrum analyzer to measure the transmitted signal from the product. The signal is an OFDM in UWB mode 1. If I set the RBW in the spectrum analyzer is 1MHz, I could read the power mask of the signal is -41dBm. It is meet the FCC specification that the power mask need to be -41dBm/MHz.

However, I try to set the RBW < 1MHz. I find that the power level is decreasing. Why is it happen? Could anyone tell me why or give me some informations of the working mechanism of the spectrum analyzer?

Thanks

wccheng

that is because for 1MHz RBW, the power level is for the whole 1MHz bandwidth signal power. When RBW is less than 1MHz, for example, 500kHz, the power level is for the 500kHz bandwidth. the whole bandwidth signal power is composed of wanted signal power plus noise power. If wanted signal power is much higher than the noise power, the change of the bandwidth will not affect signal power level too much. However, if wanted signal is not relatively very higher than the noise power, the change of the bandwidth will affect wanted signal power very much.