So many thanks about your help friends. It is so important really thanks!Use the noise marker, that is what it is there for.
The reason the SA provides the noise marker is to compensate for various errors which arise when simply viewing the noise floor on the screen (DANL):
1 - Noise is an RMS measurement. 859X series use envelope detector, not an RMS detector. This causes under response of about 1dB.
2 - 859X series performs averaging of the output of the log amp. For proper noise measurement averaging should occur before log amp (log of the average is not equal to average of the log).
This causes under response of about 1.5dB.
3 - 859X series uses resolution BW filter that is not a brickwall filter. Noise measurements are normalized to a 1Hz brickwall filter. This causes over response of about 0.5dB (brickwall filter captures 0.5dB less noise power than RBW filter of same 3dB bandwidth.)
Thus we expect a net ~2dB lower DANL vs. noise marker which is what you are seeing.
Your -113dBm reading (without noise marker) is probably because you have 10dB attenuation turned on. This is the SA default. Make sure attenuation is on manual (not auto). BTW you should be using 10dB attenuation as I mentioned in my previous post, particularly since you have a preamp.
So about my experiment i want to measure S/N .. about the noise ,activate noise marker on and what setting maybe i will make to be close to theory?
RBW=20KHZ ?is this good or larger or smaller ?my signals's bandwidth is 20khz
please check my attached