I researched a number of spectrum analyzers and it seems that the lowest frequency that these operate at is 9 kHz.
Why is the lowest frequency supported by spectrum analyzers 9 kHz?
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4$\begingroup$ There are ones that go lower, Rhode & Schwarz has one specified as 2 Hz to 90 GHz. Probably not something you're going to buy for your home workbench. There are also audio frequency spectrum analyzers if you're only interested in that range. $\endgroup$– GodJihyoFeb 23, 2022 at 14:50
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$\begingroup$ If I recall correctly, some vendors also provide extended operation down to a few Hz as a (very) expensive option. We are talking of instruments in the 40/50k euro price range. $\endgroup$– Vladimir CraveroFeb 23, 2022 at 15:55
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2$\begingroup$ The 9 kHz limit might indirectly indicate the quality of its internal local oscillator...a lower limit indicates a higher-quality up-converter. Another indicator of "quality" is the resolution filter - how narrow a frequency span can you resolve? The jitter/phasenoise of an internal local oscillator can limit both lower frequency limit & resolution. $\endgroup$– glen_geekFeb 23, 2022 at 18:25
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$\begingroup$ Do you need a spectrum analyzer lower than 9 kHz? If so, why? What do you want to measure? $\endgroup$– Thomas DK3TUFeb 23, 2022 at 19:06
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$\begingroup$ @ThomasDK3TU,the measure target is around 400hz,is kind of EMP $\endgroup$– kittygirlFeb 24, 2022 at 3:16
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1 Answer
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Spectrum analysers are often specified down to 9 kHz because this is the lowest frequency of conducted or radiated emissions that is specified in the Electromagnetic Compatibility (EMC) standards.
These standards apply to most electronic devices. They regulate both the strength and frequency of intentional emissions like WiFi, but also the unintentional emissions, electronic noise radiated from all electronics, for example LED lightbulbs. Because unintentional emissions can happen at all frequencies, there is a limit line that specifies the maximum field strength that may be radiated, over a range of frequencies.
The standards all start with something like this:
Radiated emission limits; general requirements.
... the emissions from an intentional radiator shall not exceed the field strength levels specified in the following table
Several of the standards go down to 9 kHz, this is what drives the spectrum analyser design.
- FCC "Part 15" regulations for intentional radiators start at 9 kHz (summary)
- CISPR-11 and CISPR-22 specify radiated emissions from 9 kHz.
They're not free but you will find good summaries around the web, here's one. - MIL-STD-461 (freely available) specifies Radiated Emissions from 10 kHz to 18 GHz, (and conducted emissions from under 100 Hz!)
You will find that the spectrum analyser works below 9 kHz, often all the way down to DC, but it may not meet its specifications there.
There is good coordination between test equipment and regulatory standards. It goes in the expected direction, that equipment is designed to test things to the standard, but it also seems to go the other way, that the equipment manufacturers sometimes sit on the committees and drive the standards in their favour. [citation needed]
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2$\begingroup$ To the person who flagged the "Great research!" comment, thank you for participating in the running of the site. You're technically correct that the comment is no longer needed. However, many people wouldn't like it if moderators routinely deleted their comments that were no longer needed. We moderators try to tread lightly. Personally I only delete comments if they're clearly offensive. $\endgroup$ Feb 23, 2022 at 21:21
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$\begingroup$ "drive the standards in their favour" could be less cynically worded as "base the standards on things that are practical to test" $\endgroup$ Mar 21, 2022 at 14:08