I have an 5350B with a LOT of electrical noise coming from the input of my 5350B counter. As in, sweeping signals approaching 0dBm! Something tells me this is not normal for these counters. I'll include a pic below. I noticed this when I was having issues with accurate readings on some frequencies, particularly with one SG. I have to really crank up the amplitude to get over the noise or it reads hundreds of MHz low at 4GHz. It's fine around 2GHz, but as you can see from the photo, the noise is much lower in that range also.
I'd be interested to hear others experience on this. Please note, I had the 8566B SA set at -10dBm, and a 10dBm pad in line. My pad was only rated for 4GHz, so it's likely the noise is happening at much higher frequencies, but it's not making it through the pad. So if anyone tries this I'd put some attenuation in line to be safe as the signals are strong.