Electronics > RF, Microwave, Ham Radio

How to Find the Noise Floor with a Spectrum Analyzer

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Electro Fan:
With a spectrum analyzer, what is a good method for determining the local noise floor (in the lab area around the spectrum analyzer)?

If for example we wanted to determine the noise floor from 0 to 1 GHz would it be best to run a sweep using a full 1 GHz span, or would it be better to pick various frequency ranges with smaller spans?  The reason for this question is in part to find a practical way to determine the noise floor across the full range, but also it is driven by the consideration of antennas.  One antenna might be good for low frequencies (say a standard AM antenna that is designed for roughly 500kHz to 1.6MHz), another antenna might be designed for FM radio from about 88-108MHz, another might be better further up in VHF and/or UHF, etc.).  I'm primarily interested in HF, VHF, and the lower end of UHF.  It is as simple as using an appropriate antenna for each frequency range, adjust the span and center frequency accordingly, and take a look-see?  (Or is there a preferred method that doesn't use an antenna?)  Is there a better approach?

Another question, how would you recommend setting RBW and VBW?

Also, any wild guestimate on how much noise should be found in a standard home lab?  Does -85 to -90 dB for HF and -95 dB for VHF sound plausible?

Thanks

uncle_bob:
Hi

OK, backing up a bit. For a spectrum analyzer you do it fairly simply. You put a 50 ohm termination on the input and see what you get. The floor will be some number in dbm per square root Hz. This is also called "normalized to 1 Hz" on some analyzers. As you change bandwidth settings on the analyzer, the displayed number will change if you do not have a "normalize to bandwidth" button on the scope. If you don't have that button you will also need to calculate the actual noise bandwidth of the filters in the analyzer. That can be more or less exciting depending on the vintage of the analyzer.

Next up is video bandwidth. This is just an averaging process. Depending on how it is done in your specific analyzer, there may or may not be a correction for it. On older analyzers there is also a video peak to average correction (don't ask if you have a modern unit).

So let's assume you now have an analyzer that is calibrated and properly measuring noise in dbm / sqrt(Hz). You take a look and you might be getting -171 dbm. If so your analyzer has a 3 db noise figure front end. You might also get -131 dbm. In that case, your analyzer may still be in spec. It all depends on which one you bought and which options you got with it. Before we ever do anything with an antenna, there could be a 40 db delta....

Now we grab an antenna. At least it says it is an antenna. Consider that a half wave dipole at 1 MHz is 150 meters long. That thing in your hand is *not* quite the same thing, regardless of what it says it's lower frequency limit is. If you are reading low frequencies, you are using a probe rather than an antenna. The probe is likely either designed for E field or H field pickup. It has some magic calibration factor of "this field equals that dbm" buried in the spec sheet. It may have a 20 db amp with it, it could have a 60 db amp. There's another 40 db delta

We move up to a frequency (say 500 MHz) where we *can* build an antenna. It's either a monopole or a dipole. We read up on all the design details and have the directivity and gain all worked out. We hook it up to our -171 dbm analyzer and in the quite spots we get .... -171 dbm. Everyplace else you get discrete signals that are related to this or that. You can see the local TV stations. You can see all sorts of stuff. How well can you see them? It depends on your building construction and where you are in the building. I have seen people claim (and the FCC actually accept !!!) that they have 120 db buildings.

So what will you read in your experiment (after calibration) ... -171 to -91 with the 80 db fudge factors above. Does this matter? ... err ... do your people carry around cell phones? If so, the phones are likely checking in with this and that on a regular basis. When they do, you have at least +20 dbm on your analyzer. Do you have an in-plant pager system (RF not audio). Any bets what happens when it goes off? Same issue with in-plant cell repeaters. Then there's WiFi (outside your band).

Assuming you have tossed out all of everybody's RF gear on a permanent basis, what is reasonable? No matter how hard you try, the broadcast stations are going to be there. If you can hunt down every source in your lab and get them below what the broadcasts peak at. That's about as good as it gets. Good luck tracking down that final UPS that's spraying crud from DC to light ....it can be done ..  just keep at it.

Bob

Electro Fan:

--- Quote from: uncle_bob on February 14, 2016, 01:31:28 am ---Hi

OK, backing up a bit. For a spectrum analyzer you do it fairly simply. You put a 50 ohm termination on the input and see what you get. The floor will be some number in dbm per square root Hz. This is also called "normalized to 1 Hz" on some analyzers. As you change bandwidth settings on the analyzer, the displayed number will change if you do not have a "normalize to bandwidth" button on the scope. If you don't have that button you will also need to calculate the actual noise bandwidth of the filters in the analyzer. That can be more or less exciting depending on the vintage of the analyzer.

Next up is video bandwidth. This is just an averaging process. Depending on how it is done in your specific analyzer, there may or may not be a correction for it. On older analyzers there is also a video peak to average correction (don't ask if you have a modern unit).

So let's assume you now have an analyzer that is calibrated and properly measuring noise in dbm / sqrt(Hz). You take a look and you might be getting -171 dbm. If so your analyzer has a 3 db noise figure front end. You might also get -131 dbm. In that case, your analyzer may still be in spec. It all depends on which one you bought and which options you got with it. Before we ever do anything with an antenna, there could be a 40 db delta....

Now we grab an antenna. At least it says it is an antenna. Consider that a half wave dipole at 1 MHz is 150 meters long. That thing in your hand is *not* quite the same thing, regardless of what it says it's lower frequency limit is. If you are reading low frequencies, you are using a probe rather than an antenna. The probe is likely either designed for E field or H field pickup. It has some magic calibration factor of "this field equals that dbm" buried in the spec sheet. It may have a 20 db amp with it, it could have a 60 db amp. There's another 40 db delta

We move up to a frequency (say 500 MHz) where we *can* build an antenna. It's either a monopole or a dipole. We read up on all the design details and have the directivity and gain all worked out. We hook it up to our -171 dbm analyzer and in the quite spots we get .... -171 dbm. Everyplace else you get discrete signals that are related to this or that. You can see the local TV stations. You can see all sorts of stuff. How well can you see them? It depends on your building construction and where you are in the building. I have seen people claim (and the FCC actually accept !!!) that they have 120 db buildings.

So what will you read in your experiment (after calibration) ... -171 to -91 with the 80 db fudge factors above. Does this matter? ... err ... do your people carry around cell phones? If so, the phones are likely checking in with this and that on a regular basis. When they do, you have at least +20 dbm on your analyzer. Do you have an in-plant pager system (RF not audio). Any bets what happens when it goes off? Same issue with in-plant cell repeaters. Then there's WiFi (outside your band).

Assuming you have tossed out all of everybody's RF gear on a permanent basis, what is reasonable? No matter how hard you try, the broadcast stations are going to be there. If you can hunt down every source in your lab and get them below what the broadcasts peak at. That's about as good as it gets. Good luck tracking down that final UPS that's spraying crud from DC to light ....it can be done ..  just keep at it.

Bob

--- End quote ---

Ok then, no problem, I'll just get right on that process... sounds easy...

Just kidding, thanks for the detailed info.

I can't get to -171 dBm; best I can do according the specs on a HP8561E is ?145 dBm

I'm not expecting to find no noise or eliminate all noise.  Maybe I worded the heading wrong.  I'm just trying to see what my actual noise floor is given whatever noise is present - I don't need to see if HP was right about the -145 dBm

As for how accurate is the SA, fwiw when I put reasonably small signals into it from a couple signal generators the SA seems to line up with the sig gens to within about a dB (and I think most of the difference comes from the cable).

So all I'm trying to figure out is how much noise is in the vicinity.  The reason for this is that I'm starting to get interested in amateur radio and before I get too carried away with antennas and receiving and sending signals I'm curious to see what the baseline environment is all by itself.

A question back for you:  when you say that at "low" frequencies the "probe" (antenna) is likely designed for E field or H field, how "low" is the frequency threshold you are speaking of what are the typical reasons an antenna would be designed for one (E) or the other (H)?  (I was under the impression that an antenna creates both E fields and H fields.)

Thx again for the very thoughtful and informative reply.

T3sl4co1l:
FWIW, specs are usually designed for amplitude accuracy more than noise floor, so as long as you can attach a LNA ahead of the spec, and measure the noise floor of the amp and/or whatever it's attached to, you can perform accurate difference measurements.  Of course, dynamic range suffers by the same token (the LNA might have excess distortion or clipping for > ~0dBm signals, say), so keep that in mind too.

If you're doing an EMC sort of survey, you normally want RBW = 9kHz (if you don't have it, 10 is close enough) with quasi-peak detector, for the low frequency range (150kHz to 30MHz; which is normally measured as conduction along wires, not with an antenna).  Peak detector is okay; it will overestimate some, depending on the modulation waveform.  100kHz RBW and peak detection is used for the radiated range (30-1000MHz).  This would normally be done only to calibrate a shielded test room, with a generator outside and a detector inside.  But open air tests do need to subtract ambient sources from the measurement, so this would be an important step.

As far as what to expect, for HAM purposes -- you will be able to tell, qualitatively, with a very basic antenna (a hunk of wire poked into the BNC, perhaps?), what the major broadcast stations are.  You probably won't get a good idea of nearby switching noise and whatnot (in the < 100MHz range, say), for which you'll need a suitable antenna (conical dipole?) and enough sensitivity to detect it (really, what you're ultimately concerned about is, how low is the ambient noise floor, at levels where your receivers can detect it?).

Tim

uncle_bob:
Hi

Ok, -145 is not surprising. Often there is an optional external pre-amp that will get you another 20 db or so. As mentioned above that preamp does not help the dynamic range at all. You don't see a lot of them out there.

The 2 meter ham band is at 144 MHz. A half wave antenna there will be about a meter long. If you are playing around informally, that's about the biggest thing you want to wave around the lab. There are indeed setups that go lower and stay somewhat compact. To me, not worth the effort.

The E field and H field stuff is actually fairly simple. You either couple to things with a capacitor (E field) or with a coil (H field). In the good old days, we just wound little coils on the end of pieces of coax to probe things. It was called a often called a sniffer (coil or capacitor). For an E probe, put a clip (from a broken clip lead) on your scope probe. Wave it around the circuit. Instant capacitive coupling. Why use one over the other? Voltages create E fields, currents create H fields. Close to a circuit (near field) you can indeed have one without much of the other.

Oddly enough, the guy with all the neat toys ... Dave .. has a video on using E field and H field probes. He's also got a really nice set of probes and an amp. It's certainly on my list of things to steal if I ever drop in for a visit :) (let's see .. hop in the car ... start in Pennsylvania ... head for Australia ... hmmm ..).

In terms of chasing things down, I was not kidding about that stupid UPS. You will find things around your lab or house that are nasty RFI emitters. Some can be dealt with. Some can not be dealt with (for various reasons). Before you put up antennas, it's nice to know what is where. Normally, the RFI in your lab (or shack) is secondary to the area around the antennas. Modern shielded cable works pretty well. You will always have things like computers and the like making crud locally.

E fields do not go through houses very well at all. Most LF and VLF you can pick up is E field. A 1/4 wave vertical at 138 KHz is not practical even if you are a national government. As you go up in frequency, you start to get true E/M reception. An outdoor antenna will still massively out perform an indoor one. At least you can pick up something without a giant outdoor structure. By the time you get to VHF, antennas are getting pretty small and are a lot less difficult to deal with. Line of sight counts so elevation is a good thing.

One other hint ... check the title to your house / dwelling / cave / hut. Does it have a magic clause in it that reads "no antennas". If so be a bit careful. Sometimes those clauses get enforced. Yes this is empirical knowledge ...

Have fun !!!

Bob (the guy with the *very* small antennas)

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