How to set the RWB and VWB of spectrum analyzar ?

spectrum analyser rwb

How to set the RWB and VWB of spectrum analyzar I can get a correct result when i measure the phase noise?Any corelation between RWB and VWB?Anybody can help me?
Thank you in advance,

spectrum analyzer rwb

They are two very different things.

Lets start with the video bandwidth first. This is simply an analog smoothing function. If you have data on the display and it looks a little spikey, and you have some justification for smoothing the spikes out, then set the VBW to something smaller. Except for smoothing the spikes, changing the VBW does not change the phase noise reading off the face of the display.

Resolution bandwidth RBW is the most important button. Phase nioise is usually specificied as Single Sideband in dBc/Hz. To use a spectrum analyzer, center the signal on the screen. Decide on where you want to make a reading. Lets say you wish to know the SSB phase noise at 1 KHz offset from the carrier. You would set the RBW to something <10% of that 1 KHz offset (lets say 100 hz RBW), set to marker to peak search, use a Δ marker to move + 1 KHz, and read out the the marker drop in dBc. Lets say the marker dropped from the peak to -25 dBc at the +1 KHz offset. To calculate the phase noise you are almost there, but have to convert the reading you just took, with was in dBc/100Hz into the form dBc/Hz. 20 LOG (100) is 20 dB, so you are letting in 20 dB more noise in the 100 Hz bandwdith than if you had set the spectrum analyzer RBW to 1 Hz, so the SSB phase noise is -25 -20 = -45 dBc/Hz.

If you really want to get fancy, there are also small correction factors for your individual spectrum analyzer IF filter bandshape of a dB or two, read the manual.