RSA306B Spectrum analyzer to WIFI

[Marcel_21]

Hi everyone it's my first post. 

I have a RSA306B Spectrum analyzer to find weak Wifi signals.

When i calculate the power of a weak WIFI signal using marks and integrating this to calculate the total power of the signal, the minimum total power that i detect is -80 dBm with the spectrum display (avg detection, max hold on, RBW above 50 kHz, 40 MHz Span, omni antenna 5 dBi, ref lvl -30 dBm, preamp on), -81 dBm it's the same case that calculate the total power of the Noise floor on the 40 MHz Span. My conclusion is: The SA doesn't have the sensibility to detect weak signals of -85 dBm or -88 dBm or below. what do you think? a real time spectrum analyzer cannot detect weak WIFI signals of 40 MHz  ?. I need a LNA? Thank you all 

[Wallace Gasiewicz]

Welcome to the forum. I do not have your instrument, but Neg 80 dBm is a strong signal. On a Ham Radio S 9 is minus 73 dBm, which is a VERY strong signal.
  Your  Dynamic Range for your SA is:
 -160 dBm to +20 dBm
Set the Ref level lower like minus 50. 

 You should be able to see WiFi signals in your house, Move the antenna near the WiFi and see.

Edit  Also you may want to monitor the freq when you are downloading a large file over wifi.

 

[Marcel_21]

Thanks for answering Wallace, i need to find wifi acces points, in my cell phone with an app, i can detect acces point with signal of -88 dBm, but a i cannot view this signal with my spectrum analyzer.

[KE5FX]

Not with a resolution BW of 1 kHz, you can't.

You'll need to understand some more of the basic ideas behind spectrum analysis before you can make any useful measurements or inferences about spread-spectrum signals.  Now I'm wondering what to recommend -- I can't think of any introductory books that might be good for this.  Look through W2AEW's collection of videos on YouTube, maybe?  I believe he actually worked on this instrument at Tek, or in a related department.

[Marcel_21]

Do you think is a configuration problem? not sensibility? if i reduce the RBW  this implies a reduction in the noise lvl from spectrum view, also this implies a reduction in time resolution.

[KE5FX]

You'll want to use a wider BW.  The WiFi signal is several MHz wide, so there is very little energy in a 1 kHz bandwidth. 

Even though the WiFi signal looks like broadband noise, you could think of it as a narrowband signal that is sweeping up and down in frequency.  A wide filter is more likely to capture the signal no matter where it is at any given instant, while a narrow filter (or any given Fourier bin of the same width) will usually miss it completely.

Put another way: yes, you will lower your analyzer's noise floor when you reduce the bandwith, but the time resolution will work against you.  The fast-hopping signal will end up spread across multiple bins.  If you raise the RBW, the instrument noise floor will be higher, but there will be fewer places for the signal to "hide" at any given instant in time.

[G0HZU]

I haven't used an RSA306 for a few years now, but for stuff like this you should really try and use it within its real time bandwidth limit of about 40MHz.

To achieve the most fluid display within the 40MHz real time bandwidth, select DPX mode and limit the span to 40MHz and then set the resolution bandwidth to something like 100kHz. It is possible to use a lower RBW, but I would start with 100kHz.

I've got a couple of older RTSAs here and one of them is quite powerful. It has a 110MHz real time bandwidth and it can easily display WiFi signals. It does have an internal preamp but this isn't really necessary to use if the antenna is fairly close to a WiFi device. With the preamp on it can easily see traffic from WiFi across the house and possibly from nearby houses too.

[G0HZU]

I posted up a quick video showing my old Tek 6114A RTSA displaying some local WiFi traffic. I've selected a 100MHz span and 300kHz RBW and turned on the preamp and this is running enhanced DPX mode. I used a tiny dipole antenna cut for 2.4GHz and connected it to the RTSA via a short RF cable. Most of this WiFi (and other 2.4GHz) traffic is from other parts of the house and from nearby houses.

I rarely use this analyser (because I really don't like the user interface) but it is still quite a powerful instrument despite it being about 12 years old now. I don't know how the modern DPX capable Tek RTSAs compare. I have briefly used a Tek RSA503 and I had one of the early RSA306 models to evaluate a few years ago. Your RSA306B should be able to support DPX mode as well. My analyser has option 200 for enhanced DPX but I don't really know if this makes any visual difference here.



Sadly, the camcorder I used plus the youtube transfer have spoiled some of the fluidity of the DPX display. However, it still looks impressive even on youtube.

[Marcel_21]

Nice, Dpx spectrum is realy nice. Thanks a lot your answer.  Could you measure the 5GHz Band? I say , can you measure with your cell phone and an app like Wifi Analyzer a weak signal lower than -82 dBm an also  select it channel and measure the signal with your SA? I think that i need an antenna greather than 5 dBi, and a RBW greather than 100 kHz, because i can't measure this weak signals

[G0HZU]

I had a look up at 5GHz and it's much harder to spot any WiFi traffic. If I set the RTSA to the correct channel, my mobile phone does generate very brief signals here about 20MHz wide. However, they only appear for a split second. Sometimes they might appear in a sequence and then nothing for several seconds.
It's not worth making a video for this so I've posted up a screen grab below. You can see that the signal is quite transparent even with DPX, so this is a very brief event from my phone. I'd guess it appears on the DPX screen for less than 0.1 second. The span is 100MHz and the centre frequency is about 5.2GHz.

A WiFi analyser app on my phone shows plenty of 5GHz WiFi router signals at -80dBm up to about -65dBm. These are from nearby houses. However, my RTSA won't be able to see these signals. It might be able to see 'something' with a preamp, but the preamp option in my RTSA only works up to 3GHz.

The 5GHz band looks very different to the busy traffic on 2.4GHz here. I'm in the UK.

[G0HZU]

I moved and rotated the antenna and managed to see some fairly constant router activity about 10-20dB above the noise floor at 5.22GHz. This is at a peak waveform level of about -65dBm on the RTSA display with a 300kHz RBW. I think things would be a lot better with a decent preamp fitted.

[eb4fbz]

RSA306B minimum noise floor is -154dBm/Hz at 2.5GHz when ref level is at -50dBm. That’s equivalent to a NF of 20dB, so there is definitely room for improvement with an external LNA.

[Marcel_21]

I decided to buy an external 12 dB LNA.  On the other hand I saw another problem, an interference signal coming from the usb 3.0 input at 4.988 GHz :'c. I wrote to Tektronix support about the two problems.