You must compensate for the RBW of the analyzer yourself. The analyzer itself cannot do this because it cannot distinguish between signal and noise. CW signals do not scale with the RBW, but wideband signals and noise do.

-Ken

When you reduce the resolution bandwidth (RWM) of a spectrum analyzer, the noise floor tends to drop but the signal levels tend to stay the same. To see why, consider a pure tone. It has a particular power and zero bandwidth. Say it has 1mW power and you are measuring it with a 1kHz RBW. The spectrum analyzer will indicate that it is a 1mW signal. Now if you reduce the RBW to 100Hz, it still will report that that you are observing a 1mW signal. The reason being that the signal itself has a bandwidth that is smaller than the smallest bandwidth of the analyzer, and so the analyzer always reports the same amplitude for the signal.

Now consider a white noise signal with a power density of 1μW/Hz. The analyzer will report this as a 1mW signal when using a 1kHz RBW, but only a 100μW signal when using a 100Hz RBW. Thus, when comparing spectrum analyzer results measured with different RBW, you must compensate for the change. And how you compensate depends on the type of signal you are measuring. Generally people normalize their results to a 1Hz RBW. To do so, you would divide the noise power levels by the RBW to get a power density. You would not do anything to signals with bandwidths smaller than the measurement RBW.

-Ken

Thanks Ken and Wong! I found the reason why my phase noise get degraded while increasing RBW. It's because I set a too high RBW. like 3MHz to measure the phase noise at 1MHz offset. To illustrate this, I did several measurements with different RBW.

Span(Hz)      RBW(Hz)       Signal Power(dBm)        Delta Noise Power (dB)        Phase Noise (dBc/Hz)         10*log(RBW) (dB)

100M            3M                 -14.5                          -1.2                                    -65.5                                  64.5
10M              300K             -14.5                           -38                                     -93                                    54.7
2M                100K             -14.5                           -59                                     -109                                  50
1M                30K               -14.5                           -65                                     -109                                  44.7

So we need to set the RBW to be less than say 100KHz in order to measure the phase noise at 1MHz offset ACCURATELY.

Thanks, It's a great website!

Edward W.