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I am getting some odd results from my SA44B. I have it connected to a SAS-550 monopole antenna. I get good ambient measurements in that configuration, but when I have a noisy instrument under test I get strange readings (see attached example). I see some expected narrow band features but also some unexpected ones. When I investigate those features they come and go depending on the bandwidth setting and the center frequency. For example, spikes that appear near 680kHz will appear with a 600kHz center and 200khz range, but change character or go away if I move the center by 5 or 10kHz. I thought maybe I was saturating something so I tried manually controlling gain and even adding external attenuators but no change. Any ideas?
The SA44B definitely has some shortcomings when it comes to EMC testing. It relies on a software image rejection algorithm to get rid of both image responses and its own spurious responses. This algorithm tends to work better on sparse spectrum with narrow RBWs, where the chance of a spur and an image overlapping is low. When there are many signals closely spaced, especially at low frequency where there may be higher frequencies present mixing with harmonics of the LO, the chance of spurious signals overlapping on the primary and image sweep are high.
This is one of the reasons we strongly recommend the BB60C for EMC work. With hardware image rejection and specified spurious performance, you get reliable EMC sweeps.
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