The best lecture I've found so far on Spectrum Analyzers! Looking forward to the entire series. This series needs more views! Thank you for putting this together.
I'm missing something toward the end of the lecture. The spectrum analyzer has about 20dB noise figure. It was also mentioned that commercial and consumer grade equipment, e.g., LMR and TV receivers has ~5dB NF. Why is it that this expensive piece of test instrumentation has so much worse NF than commodity electronics?
From Frank Sanders, via the ITS Publications Office: The short answer is, commercial receivers can use narrowband low noise amplifiers in their RF front ends, with low noise figures, because they only operate with very small dynamic ranges (maybe 20-30 dB) and across relatively small frequency ranges (compared to signal analyzers and spectrum analyzers). Spectrum analyzers and signal analyzers have to provide exceptionally wide frequency ranges and wide (about 60 dB) dynamic ranges. That forces design engineers to compromise on how low their noise figures can go, on those machines. Also, old-style spectrum analyzers use multiple mixer stages to attain a very wide available frequency range. Each mixer stage adds 5 dB or more of noise figure. Newer spectrum analyzers and signal analyzers attain lower noise figures, but still tend to run 10-15 dB noise figure in general. Even newer digital machines still use at least one mixer stage, to get to frequencies above ~ 3 GHz, and maybe another for frequencies above ~ 8 GHz. Again, that’s about 5 dB of additional noise figure penalty for each mixer stage. Now, for newer models of (digital) spectrum analyzers and signal analyzers, there is usually a built-in preamplifier that runs a lower noise figure, perhaps 8-10 dB noise figure depending on the model. BUT the machine’s user has to activate that preamplifier -manually-. So be aware that, as a user, you’ll have to look for, and find, that feature yourself. You might wonder why even the auxiliary preamps in the expensive test and measurement gear don’t have even lower noise figure, like maybe 2 dB or 5 dB? The reason is, the designers are trying to provide reasonably low noise figures while still providing fairly wide dynamic ranges (again, like, 60 dB; as opposed to TV or AM/FM receivers that only have perhaps 20-30 dB dynamic range, at the most). Preamps’ dynamic ranges inherently go -down- (decrease) and their noise figures go down to lower and lower values. The designers seem to think that the best trade-off between lower preamp noise figures (wanting them to be lower) and dynamic ranges (want them to be wider = larger = ~ 60 dB) is achieved when the auxiliary amplifier’s noise figure is around 10 dB. To do better than (lower than) 10 dB noise figure, one has to use their own, supplemental low noise amplifier on the analyzer’s RF signal input. But be warned: You need to know what you’re doing when you put your own low noise amplifier onto the analyzer’s input. You’ll have the same issues with dynamic range as anyone else would => The laws of physics aren’t on your side. And, you will also be potentially vulnerable to overloading your amplifier. Unlike with the analyzer’s built-in preamplifier, where you will probably get an automated warning on the machine’s screen display if you overload its amp, you are working without a net when you use your own amplifier-you won’t get any warning if it’s overloading.
Thank you. I'm a new cable guy. This is 100% what we do. Thank you. I always thought it was closer to Network Administration or Cisco. Nope. It's telecommunications engineering.
While it is well recognized convention, the shortened reference to P0 being 1 milliwatt as dBm, or dB(m) does present opportunity for confusion in formulae used for radio communications where distances or areas measured in meters are involved as these when reduced to dB form can also be referred to as dB(m) or dBm. Because power and distances are often both at play in radio communications, it's better to either keep the power values referred to as dBW or to use the properly specified reference to power referenced to 1 milliwatt as dB(mW) or dBmW.
