SDRs are nice to work with, but they are not without issues. Some of the problems with SDRs is poor shielding and RF overload. Users report all kinds of artifacts that aren't really there.
Depends on the SDR, some SDRs have more dynamic range than a top of the line traditional receiver. However, once you get the ADC clipping you have issues. Still, this is NOT a hard condition to detect, or correct.
Unfortunately, when you say “SDR” today many people automatically jump to the conclusion you are talking about one of the 8 bit RTL dongles, totally forgetting that there are much better SDRs out there. An 8 bit RTL SDR might have a dynamic range of only 45 to 50 dB, and an MDS of -110 dBm or better, meaning that without front end attenuation an honest S9 signal might be putting the SDR in clipping or overload. By the same token a good 16+ bit SDR might have an instantaneous dynamic range in excess of 95 dB (for example the Perseus is measured at 99 dB).
RTL dongles and associated upconverters are a mixed blessing. For under $100 they bring some capabilities that you could not touch for under $10000 just a few years ago. However, they ARE cheap devices, and they have several issues. There is a reason that “good” SDRs cost a lot more than these kinds of setups, unfortunately people forget that in general you do get what you pay for, and expecting a $100 receiver to perform on a level with a $1000 receiver is a pipe dream.
Some time ago I heard two hams talking from relatively close locations (200-300 miles). One ham was operating mobile and running about 50 watts. The other ham was using a SDR and proceeded to berate the other ham saying that his signal was 20KHz wide with spurs running up and down the band. Yet, I couldn't hear anything wrong with his signal and there were QSOs on either side of the mobile ham that had no interference at all. The ham with the SDR just needed to learn how to use a attenuator. Then he would have gotten a better idea of the other hams signal.
You are absolutely correct, if you have a big booming signal that looks dirty knock a little attenuation in the front end and see if the signal cleans up some. It might be that the signal is indeed dirty…or you might be getting some serious issues on your end. Learning the limitations and how to use your equipment is an important factor, everything is not a “turn it on and tells you everything, 100% correct” device.
But also people can be surprised by how wide a signal might be, I have seen the same issue with people the first time they use a station monitor or a spectrum analyzer. Some people do not understand that it is possible for a ham signal to be 20 kHz wide, even without major problems…if you are looking 80 or 90 dB down. And a signal that is 40 dB over S9 might be 100 dB above the noise floor, so you can easily look 80+ dB down. Bandwidth without a relative power measurement is sometimes meaningless.
T!
