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Help: correct use of a spectrum analyzer (interpreting signal power measurements with various RBWs)

Hello,

I am trying to use a spectrum analyzer to ID various RF signals in an urban environment, and measure their power. I am having trouble interpreting some of my measurements, and I would greatly appreciate any help. Here's what happens:

1) I start out by sweeping the range from 0.5GHz to 6.5GHz, using large RBW and VBW (50MHz in my SA - it's a portable Spectran 6080 unit). The trace I see contains 3 very prominent peaks. They are centered at 1.8GHz (-22dBm), at 4.08GHz (-15dBm), and at 6.13GHz (1.1dBm!), and each is approximately 200MHz wide at its "base".

2) I "zoom in" on the largest of the 3 peaks (select a 6.13GHz center frequency, and a span of 200MHz), and cut the RBW down to 3MHz or

1MHz. Now, the large peak that used to be there has disappeared completely (why?). If I now set the spectrum analyzer to "pulsed mode" for detecting pulsed signals, then I see instead a series of roughly 10 peaks, however their magnitudes are much lower than the large peak I saw when measuring with the larger RBW, e.g., -22dBm instead of +1.1dBm before (why such a huge difference?). These smaller peaks do not stay at fixed frequencies but "move around" with each successive sweep(why?). Slower sweeps reveal many more peaks than fast sweeps do (why?).

Finally, out of the two magnitudes of the frequency peaks (the large values seen when measured with the 50MHz RBW, versus the smaller peaks when using 3MHz RBW), is either one supposed to be close to reality? I thought that the large dBm value (1.1dBm) obtained with the 50MHz RBW filter in place might correspond to the total power within the filter's bandwidth which then appears "broken up" over many peaks when using the narrower RBW, but that doesn't seem right because there aren't that many of the smaller -22dBm peaks to justify a total of

1.1dBm.

Thanks in advance to anyone who may be able to shed some light.

Dimitris

Reply to
varsakelis
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When you use large BW, the analyzer shows the total power density of the spectrum coprised in that BW, and doesn't matter the speed of frequency hopping of the incoming signal because all the pulsed signals fall inside that BW span. While when you use narrow BW you see only the heterodyned signal between the hopping speed of the incoming signal and the analyzer scan frequency. The slower is the scan speed the higher is the heterodyned signals you can see, but this doesn't mean you see them all. If you set the trace max hold function, and you leave the analyzer acquire for some seconds, you should have a better graphical representation of the signal spectrum distribution, but it could be incomplete anyway. To get its real distribution you should change various scan speeds leaving the max hold active (if your analyzer allows it). Have a great day. Massimo

Reply to
Max65

Thanks very much. The explanation makes sense, although I am still a bit puzzled both by the large power I see at around 6.13 GHz with

50MHz RBW (+1dBm - I don't know what could be transmitting at that frequency) and by the speed of the frequency hopping (it must be fast in order to justify the big (-22dBbm) reduction in peak power that I observe when reducing to RBW=3D3MHz).

I appreciate your help.

Dimitris

Reply to
varsakelis

It just means that the "hopped" signal is made of about 200 different carriers (not considering the analyzer precision and the amplitude distribution inside the two RBWs). More, since the dBm units aren't thought to express so precise measurements, the signal could much probably be made of 128 or 256 different carriers instead. I'm Italian and I don't know what kind of service could be placed on that band in your country. Have a great day.

Massimo

Reply to
Max65

Again about the +1dBm level, it should be either a signal directly visible by your antenna or a signal very close to you. Do you receive it moving the antenna in any direction?

Reply to
Max65

I receive it (only with "large" 50MHz RBW) no matter where the antenna is pointing (!?!), whereas if I cut down the RBW to, say 3MHz, it disappears. I tried spans varying from 20 to 200MHz, and various sweep rates (randing

I live in Greece - I believe that band is allocated to point-to-point microwave links, except that I always get the same measurement in various locations throughout the city (e.g., rooftops, office space, street-level - it's unlikely that they are all in the path of a point- to-point beam).

Dimitris

Reply to
varsakelis

Uhmm, it looks like a spurious self emitted signal of the SA. what does happen if you remove the antenna, or do you put it into a metal box? Massimo

Reply to
Max65

I tried removing the antenna - the readings went down by just 2 dBm compared to when the antenna was connected. I also put the SA in a metal toolbox (steel, I think) and there was no change in the readings compared to when the SA was outside. On the other hand, my cell phone still rings when inside the metal box, so some signal goes through, but I would have expected some drop in measured dBm. So you may be right about it being a self-emitted signal. I'm thinking I'll check with the manufacturer.

Dimitris

Reply to
varsakelis

Hi Dimitris, If your cell phone still receive the em field into the metal box, it may be made of not well conductive metal or paramagnetic metal too (inox steel for example). In that case if the thickness is not enough to completely shorten the magnetic or the electric component of the em field, it passes through the thin shield. Anyways, if you unplugged the antenna and the signal still there, it means that the signal comes unequivocally from the inside of the SA. My suggestion is to contact the manufacturer and explain the issue. Have a great day. Massimo

Reply to
Max65

Thanks very much for your help!

D.

Reply to
varsakelis

Unless i am mistaken 6.1 GHz is a major satellite downlink band. Actual transmit power on the order of 100 W through very high gain antennas (>30 dBi).

Reply to
JosephKK

That was in the C-band UPLINK frequency range. Everything goes up around 6 GHz and is offset by 2 GHz and comes back around 4 GHz.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
Reply to
Michael A. Terrell

I know that, we had it in Europe too, but it seems that Dimitris's SA has a spurious signal generated by itself, because it receives that signal everywhere at the same level, removing the antenna too! I take a look to his SA model specs, and discovered that it's a portable SA intended for EM "pollution" measurements. You should know that before this last generation of instruments, the people who made EM "pollution" measurements used for a long time simple broadband detectors connected to a characterized antenna. They had to specify the frequency where they made their measurements to correct the antenna gain, but since they didn't know the complete enviromental EM spectrum in that moment, they ignored for a long time that their measurements was wrong and could be in excess of 1000-2000% due to the received unknown signals. Narda Microwave produced those instruments and it was the first that changed the production to selective portable SA, and used the above consideration for advertize their new product. One friend of mine lead me one broadband detector one day to check it with my benchtop SA, and the results were demoralizing.

It's true, the time is changing and electronics teaches us that all can be made always smaller than yesterday, but when you deal with microwaves it's very important to take care of good shielding between the function blocks of your device, expecially when you device is a measuring instrument. Have agreat day. Massimo

Reply to
Max65

I've worked up to KU band on the production floor with dozens of benches full of operating test equipment. Our products were well designed in separate shielded modules and lots of semirigid cable. All it takes is a loose connector, a broken solder joint on a connector, or a loose shield and you'll have RF radiation or ingression problems.

If the problem is internal, he'll need a better instrument to troubleshoot his SA.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
Reply to
Michael A. Terrell

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