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I would like to visualize the RF signal of a 6 Channel Spektrum DX6i transmitter that operates in the 2.4 GHz band, in order to visualize changes in the signal when I move the control sticks.

The Tx converts the position of the control sticks into a Pulse-Position modulation (PPM) signal which is then modulated using Direct-Sequence Spread Spectrum (DSSS) and then radiated at 2.4 GHz.

Unfortunately, according to my teacher, the equipment that we have in our University labs don't have the required performance to analyze 2.4 GHz signals, and all the inputs that I got from him were discouraging, because the oscilloscopes that can operate in such bands are very expensive.

This is an incomplete list with some equipment that is available in our labs:

  • Digital Oscilloscopes:

    • 100MHz - 1Ga/s GwINSTEC GDS-2102
    • 100MHz - 1Ga/s Agilent DSO3102A
  • Analog Oscilloscopes:

    • 40MHz HAMEG HM-404 / HM-407
  • Spectrum Analyzers:

    • 100kHz - 3GHz Agilent N1996A
    • 9kHZ - 4GHz Anritsu MS2034A
    • 9kHz - 2.6GHz ADVANTEST R3361A
    • 10kHz - 3.5GHz ADVANTEST R4131B
  • Network Analyzers:

    • S-Parameter 30kHz - 6GHz Agilent 8753ES

The reason for posting this list is to give readers some reference. I am curious about Spectrum Analyzers and Network Analyzers because they can operate at 2.4 GHz so I have included them just in case they could be useful.

I'm not giving in yet and I am exploring other alternatives such as software-defined oscilloscopes.

But before going any further I would like to ask:

Is it possible to visualize a 2.4 GHz digital signal without an oscilloscope?


I read this question and found it useful.

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  • \$\begingroup\$ If you look at the baseband spectrum , that will tell you there is not much noticeable change in spectrum with PPM \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 28 '16 at 19:38
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    \$\begingroup\$ The picture you have is not of 72 MHz RF! This is picture of baseband signal. \$\endgroup\$ – AndrejaKo Oct 28 '16 at 19:38
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    \$\begingroup\$ Also, it's been a while since I've modern remote controllers, but they usually have an RF chip and a microcontroller that sends instructions to RF chip. You should be able to very easily probe the communication between the chips with the scopes you have available, since the communications will be at relatively low frequency. \$\endgroup\$ – AndrejaKo Oct 28 '16 at 19:43
  • \$\begingroup\$ A software defined radio receiver is probably the easiest way to analyze over-the-air modulation without something like a vector signal analyzer... does your university have a USRP kicking around? \$\endgroup\$ – Peter Oct 28 '16 at 20:29
  • \$\begingroup\$ A real cheap-ass way would be to use an RTL-SDR ($20) and a block downconverter so you can shift the signal in-band of the SDR: rtl-sdr.com/… \$\endgroup\$ – Peter Oct 28 '16 at 20:30
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According to the data sheet, the N1996a spectrum analyser has a 'zero span' mode, where you can use it as a fixed tuned receiver, and the trace sweeps in time rather than frequency, just like an oscilloscope. See page 101 of the manual N1996-90028.pdf

Set the resolution bandwidth to be wide enough to follow your signal.

If you tune it to the centre of your RF signal, then you demodulate the AM.

If you off-tune it, so that you are 20dB or 30dB down the side of the filter, then you can crudely demodulate FM. As the frequency varies, the response moves up and down the slope of the filter.

With both AM and FM demodulation possibilities, you ought to be able to get some response, whatever the original signal's modulation.

I notice the R4131D has zero span, as well as video and IF outputs, so the analyser can be used as a fixed tuned receiver, and you can display the IF or video on an oscilloscope.

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  • \$\begingroup\$ @TonyStewart.EEsince'75 a downvote for using existing lab gear to have a look??? I am astonished. You get the IF out of the Advantest for a better look on a scope. Are you really saying the output is pure 100% level, and that a using either a wide or narrow channel filter with on or off-channel tuning will give you no demodulation of anything the transmitter is sending. I had a PCM transmitter which just sent digits, so the PPM was not present as such on the RF. Comment yes, downvote, between engineers, what's the world coming to? \$\endgroup\$ – Neil_UK Oct 29 '16 at 4:24
  • \$\begingroup\$ it may be worth looking at ,to learn how to use the equipment. but the would look not much different than Wifi and certainly not be able to see anything remotely looking like PPM. It is a digital telemetry signal with all 6 channels or more. There probably is no IF with just direct conversion \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 29 '16 at 4:30
  • \$\begingroup\$ @TonyStewart.EEsince'75 Have you looked at a wideband spread spectrum signal with a narrow-tuned spectrum analyser? Your word 'probably' suggests not. I worked with 3GPP on 3G and 4G radio, developing test methods, and that's just the sort of test we did, funnily enough because the stuff for demodulating it was not available as test gear yet (problems of working on the bleeding edge) and the test houses needed to be able to qualify the gear with the steam-driven analogue spectrum analysers they already had. But I will agree, he won't see PPM. \$\endgroup\$ – Neil_UK Oct 29 '16 at 4:39
  • \$\begingroup\$ Yes I recently spent 2wks full time using Aaronia's 10GHz SA with resolution from 1kHz to 1Ghz at all digital spectrum from 900 Mhz to 5GHz in Atlanta and found it interesting but impossible to demodulate any intelligence from the spectrum other than knowing all frequency assignments in the area for every in car, mobile , WiFi, Bluetooth and other signals aaronia.com/Datasheets/Spectrum_Analyzer/… \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 29 '16 at 4:50
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There may be special modulation we don't know about, but you wont likely notice any significant change in modulated spectrum

After reading about the Receiver technology , your Pulse position servo information will be digitized and sent as DATA , therefore there is no change in your spectrum due your misleading video.

It is wideband DSSS !!! not narrow band like others.

i.e. Digital sbus over DSSS not Analog PPM ! http://www.dronetrest.com/t/sbus-or-ppm-which-is-better/1356

. . . .

Ignore the frequency used here just see the many harmonics that will get LPF filtered out. But this is what a crude PPM spectrum might look like before filtering., which has to do with your original video question on PPM (pulse position modulation + narrow FM) but has nothing to do with the transceiver Spektrum DX6i, which uses DSSS. (new digital technology)

enter image description here

http://www.falstad.com/fourier/ < Use mag/phase and Log view and draw any waveform or change any Fourier analysis to make arbitrary waveforms with N harmonics

Conclusion: Yes you see spectrum, No you cannot tell if it changes and how much without special setup and experience to decode spectrum.

I just checked and it uses spread spectrum, so it would look like a narrow spike or wide white noise with s 20MHz BW to the unaided eye on a 2.4GHz spectrum Analyzer. enter image description here

It does offer an advantage with dual path technology, a MUST HAVE for RC users with less chance of polarization blind spots otherwise known as Ricean Fading deadspots

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