Noise Floor Change

Bill, All the answers here are correct. In the ham community, the term "noise floor" is tossed around a lot without an explanation for what it really means. When you hook an antenna to your radio, there is broadband noise from the atmosphere, etc. You hear this noise in your receiver on sideband, but the noise level goes down when you switch to CW mode. This is because the filters in your radio switch from, say, 3kHz, down to let's say 500Hz. By filtering out 5/6 (83%) of the noise with the narrower filter, the amount of noise -- the power level of the noise -- is reduced. For receivers, the noise level is all sent right out your speaker and to your ears. On CW, you notice the reduction of noise as a loss of noise at certain frequencies because your brain can measure signals by frequency. When switching to CW, you hear all of the high frequency noise go away. Because your ear (brain) are used to filtering out signals, you do not perceive anything other than this -- the CW signal may seem to have about the same noise on it. Your brain is doing the equivalent of an FFT on the audio. The panadapter simply measures the signal in a given bandwidth and draws what it hears. If you look at any given pixel, it represents a certain amount of bandwidth. We call this the "bin size" of the FFT that is used to produce the display. If you cut the bin size into two pieces, the amount of noise in each piece goes down by half (3dB). In PowerSDR, the bin size is generally fixed for any given setup and does not change when you zoom. This is why the resolution gets worse as you zoom in on PowerSDR -- you begin to show one bin with multiple pixels. But for SmartSDR, we knew we wanted to have a larger range of zoom and this method was no longer acceptable. So we vary the bin size across a 1000:1 range. So the noise in each bin also varies. 1000:1 is a change of 30dB so from min zoom to max zoom, the noise in a bin will lower by 30dB and you see this change in the panadapter as you zoom in and out. When people talk about noise floor in ham radio they are generally talking about the noise level with a 500Hz bandwidth. When the panadapter is zoomed in to the max level, the bin size today is about 5.8Hz. This is a 19dB difference in noise from where a ham would say the noise floor is to what you can see on the panadapter. This means that the panadapter can see 19dB below what most hams would call the noise floor. Your ear and brain are also able to hear below the noise floor in 500Hz because of how they work. But there are limits to how well you can hear. If you've ever worked JT65 or another long-term integrating mode, you have noticed that your computer can copy signals that you cannot hear. So if you ask another ham "where is your noise floor on 80 meters" and he says "S5," what has he told you? Well with most hams, you don't know because you don't know the answer to these questions: 1. What bandwidth are you using to measure the signal? 2. Is your S-meter calibrated? An S5 signal corresponds to -97dBm. And if he's getting this on sideband set to, say 2.8kHz bandwidth then the actual noise floor in 500Hz would be -97 - 10*log(2800/500) = -104dBm. There's nothing magical about 500Hz, it just happens to be the convention for measuring noise in the ham radio world. In SmartSDR, if you set the passband filter to 500Hz, the S-meter in the slice will show you the 500Hz noise floor. If you start at maximum zoom and begin zooming out, you can see a point where the noise reading of the panadapter equals this number. What do you think this point is? ... if you've been following along, you will realize that this is the point where the FFT bin size is 500Hz. To get a rough idea if this is right, you could measure the width of your panadapter window and divide the amount of frequency displayed by this number. It should be in the 250-1000Hz range. The answer will not be exact because we do not continuously vary the FFT bin size -- we adjust it in steps and don't tell you where the steps are or what size they are. We do what's right for what you are viewing. I know that was a long-winded answer, but I hope it provides some insight into noise and how it changes what you see and hear. Steve