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Electrical Engineering
How does a spectrum analyzer measure Noise Spectral Density
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[QUOTE="sophiecentaur, post: 5427444, member: 199289"] Basically, Noise [I]Power[/I] is the most important thing in most systems, when you are trying to measure the performance of a communications channel. Noise is a random process and truly random noise can take any value at a given time (∞ in theory) but you can rely on noise power, because it is averaged over a longer period than a noise spike. Assessing channel noise with an old analogue Spectrum Analyser was very dodgy because you could only look at the 'grass' at the bottom of the trace and come up with some approximate estimate of a mean value. A noise measuring algorithm in a Spectrum Analyser will look at the signal volts which come out of the i.f. filter and do an MS calculation with a bunch of samples of the filtered signal to give the Power admitted through the filter over a time. If the i.f. filter has Δf noise bandwidth then the noise power density will be P[SUB]n[/SUB]/Δf Watts per Hz. That can be stated in terms of Volts (Power = V[SUP]2[/SUP]/R), rather than the Power as 'Watts per Hz' can be re-stated as 'Volts per √Hz' (square rooting the answer to give the RMS Voltage value). This isn't intuitive but it's a good system, once you get used to it. The resistance involved should be known if you want Watts from Volts and it would often be 50Ω in a comms system. The advantage of using Volts, rather than Watts is that the Impedance of the system doesn't matter and a power supply will have a low / unspecified impedance and the noise volts on the line will [I]tend[/I] to be independent of the load current. If your analyser gives you the noise per root Hz then yes, you can scale up the total noise for any bandwidth. (Square root the frequency ratio, of course, if you want RMS volts). A few 'private' calculations on the back of a fag packet could help you get familiar with this process of hopping between Power and Volts. [/QUOTE]
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How does a spectrum analyzer measure Noise Spectral Density
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