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Modulation question

Started by sam November 18, 2003
Hi

we have an RF transmission that has some data modulated into it.  The
RF is transmitted at just under 1GHz.  My question is how can we find
out what the modulation scheme is?  we are not sure even if it is
analogue or digital, i.e. whether it is AM/FM or FSL/PSK/QAM etc.,
though suspect probably digital.  Can anyone suggest how to find out
what the scheme is?

thank you

sam
On 18 Nov 2003 09:26:23 -0800, samsspam@btinternet.com (sam) claimed:

>we have an RF transmission that has some data modulated into it. The >RF is transmitted at just under 1GHz. My question is how can we find >out what the modulation scheme is? we are not sure even if it is >analogue or digital, i.e. whether it is AM/FM or FSL/PSK/QAM etc., >though suspect probably digital. Can anyone suggest how to find out >what the scheme is?
Do you know the exact frequency? Bandwidth? Location? While it may be hard to determine the modulation-scheme without knowing anything about the transmitted data. Knowing the above parameters may further help the investigation... Go out there with a spectrum analyzer and a yagi-antenna, and start the research. Happy hunting :)
"sam" <samsspam@btinternet.com> wrote in message
news:ba2c2b89.0311180926.360ade82@posting.google.com...
> Hi > > we have an RF transmission that has some data modulated into it. The > RF is transmitted at just under 1GHz. My question is how can we find > out what the modulation scheme is? we are not sure even if it is > analogue or digital, i.e. whether it is AM/FM or FSL/PSK/QAM etc., > though suspect probably digital. Can anyone suggest how to find out > what the scheme is?
I've seen a research project that did something similar to what you want to do. Unfortunately, I saw a demo but not the details on how it was done - so all I can add is - "it can be done". I'm fairly sure that if the modulation scheme is not a known scheme, it's pretty hard to figure it out. A simple approach that comes to mind, is to apply demodulation of the various known schemes and look at the results. The result that doesn't look like crap (or the one that least looks like crap) is the most probable scheme. The 'result' is basically looking all the standard tools used to visualize demodulated data (eye diagram, I,Q vector diagram, symbol plot, etc). I'm not sure if what I said makes sense, but those are my first thoughts.
> > thank you > > sam
Hello Sam,

If this a signal that you are creating with a piece of alien equipment or is
this a signal that is causing interference? Find the exact frequency and
check with the FCC's spectrum allocation to see what this band allows? As
another suggested put a spec-an on it and further clarify the signal's
params. Let us know more about signal. Does it exist all of the time? Are
you near a military base? Also using a yagi, find the direction of the
signal, and then drive around in you car and triangulate it's source. If you
know where it is coming from, that will help a lot. I.e., a comm tower.

Clay




"sam" <samsspam@btinternet.com> wrote in message
news:ba2c2b89.0311180926.360ade82@posting.google.com...
> Hi > > we have an RF transmission that has some data modulated into it. The > RF is transmitted at just under 1GHz. My question is how can we find > out what the modulation scheme is? we are not sure even if it is > analogue or digital, i.e. whether it is AM/FM or FSL/PSK/QAM etc., > though suspect probably digital. Can anyone suggest how to find out > what the scheme is? > > thank you > > sam
On 18 Nov 2003 09:26:23 -0800, samsspam@btinternet.com (sam) wrote:

>Hi > >we have an RF transmission that has some data modulated into it. The >RF is transmitted at just under 1GHz. My question is how can we find >out what the modulation scheme is? we are not sure even if it is >analogue or digital, i.e. whether it is AM/FM or FSL/PSK/QAM etc., >though suspect probably digital. Can anyone suggest how to find out >what the scheme is?
With the right equipment this could be done in a few minutes. The bandwidth of the spectrum will give you some idea of the information transfer rate. Expect some kHz for audio and some MHz for video. If the spectrum has a flat top and doesn't seem to vary with the programme material, then it's probably digital. If the spectrum has a peak in the middle, and seems to change shape each second, then it's probably analog. If it's analog, some MHz wide, and the spectrum is rather asymmetric, it's probably a TV signal. (Also look for an FM sound carrier a few MHz away.) A good spectrum analyser will have AM and FM demodulation and an audio amp built in, so you can listen to the signal. Note that practical transmission systems have some AM<->PM conversion, so hearing a clear sound with the AM demod doesn't necessarily mean that it's an AM signal. If it's digital, you could use something like the Agilent 89600 series vector signal analysers to work it out. You'll need to tell it things like the centre frequency and the symbol rate, but it's simple to work them out from the spectrum. These analysers are a bit expensive, so you might like to "evaluate" one for the time that it takes to solve your problem. Regards, Allan.
samsspam@btinternet.com (sam) wrote in message news:<ba2c2b89.0311180926.360ade82@posting.google.com>...
> Hi > > we have an RF transmission that has some data modulated into it. The > RF is transmitted at just under 1GHz. My question is how can we find > out what the modulation scheme is? we are not sure even if it is > analogue or digital, i.e. whether it is AM/FM or FSL/PSK/QAM etc., > though suspect probably digital. Can anyone suggest how to find out > what the scheme is?
Hey sam, This will probably take a good bit of invention, but I can think of a couple of things to try: 1. Simply observe the signal on a spectrum analyzer for a period of time (10 minutes?). If you find that the signal "collapses" to a single spike (or some other simple state) at various periods of time, it might be analog. 2. Quadrature downconvert the signal, being careful to tune as close to the apparent RF center frequency as possible. Then perform an m-th power (i.e., y[n] = (x[n])^m) function on the signal and look at the spectrum of the result for various values of m such as 2, 4, 8, etc. If the spectrum collapses to a line at a particular value of m, then you may have m-ary PSK modulation, and the line frequence corresponds to the amount of RF offset (carrier offset) you have. In fact (if this is m-ary PSK) you could use this to frequency-lock the signal. 3. Quadrature downconvert the signal and do a "constellation histogram" in which you simply plot a few points in I/Q space. Try doing this while tuning the RF center frequency slowly. If the modulation is m-ary PSK or some type of QAM, may see the constellation "lock up" to a pattern if you get the center frequency close enough. This will also depend on the relationship of the symbol clock to your sample clock and the amount of transmit filtering being done. Just my $0.02. -Randy