Talk 1: Thermal Noise Limits in Radio Measurements
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- Опубліковано 2 сер 2024
- This talk explains the most fundamental limits on all radio receivers and measurement systems.
Have you ever wondered how a spectrum analyzer works, how to properly adjust all of the analyzer's parameters, or why a stair-step pattern initially appears on a spectrum analyzer screen when you turn it on? Do you know how to precisely calculate the analyzer's sensitivity in your head, merely by glancing at the screen display without any signal present? Are you uncertain about how much gain, and how low a noise figure, you ought to specify when you are ordering a low-noise amplifier (LNA) for a radio receiver? Do you want to know the difference between noise figure and noise factor? Do you wonder how to diagnose and solve radio interference problems?
If you have questions about how to make good radio spectrum measurements or how to diagnose interference problems, you will find the answers in the NTIA Seminar Series on Spectrum Measurement Theory and Techniques. In this series of talks, an NTIA engineer at the Institute for Telecommunication Sciences (ITS) laboratory in Boulder, CO, discusses the fundamentals of radio spectrum measurements. The speaker, Frank Sanders, who has nearly thirty years of experience in this field, recognizes that even for many engineers who routinely use spectrum analyzers, the fundamentals of how they work and how to use them may be a bit murky; even in university lab classes the instructors do not always understand these machines very well themselves.
Most of the talks, which are 80-100 minutes long, are divided into two parts. In the first portion of each video, Sanders explores a particular aspect of radio spectrum measurement technique or theory with a whiteboard lecture. In the second part, the lessons of the whiteboard discussion are implemented with actual measurement hardware and radio signals. A few of the talks, which for example involve large numbers of photographs of radar systems, are videos of his Microsoft Powerpoint presentations.
In this series, Sanders explains spectrum analyzer functionality in terms of convolution bandwidth and shows how, when convolution is understood along with the mechanics of analyzer design, spectrum analyzer operations and outputs become easy to understand and use. Other topics include (1) what you need to know to use spectrum analyzers to examine all types of radio signals, including mobile radios, radars, and digital data links; (2) the use of low noise amplifiers and how to specify the right gain and noise figure for your receiver and measurement applications; (3) how radar systems work, and how to understand and interpret the signals that you see coming from radars; (4) the ways that radio interference can occur; (5) a methodical approach for diagnosing and solving radio interference problems; (6) the math needed to convert spectrum analyzer measurements into field strengths of radio signals; and (7) the proper conversions for radiation hazard calculations. - Наука та технологія
An excellent series, well presented, thank you NTIA.
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.
Another good one. Thank you!
always enjoy this..
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.
I agree.
Small point, but in SI units, the degree symbol is not used with the kelvin symbol.
when the span is set to zero, the spectrum analyzer works like an oscilloscope at the center freq sampling rate?
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.
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.