Viewing GFSK on a Spec Analyzer

I have a cheap 2.4 GHz rc transmitter that is using gfsk modulation and supposedly operates on 4 seperate channels. My question is from the attachments above which is which?

SCREN08 shows what I think are the Mark and Space frequencies seperated by 40 MHz and SCREN09 shows the Mark frequency (I think) with a much lower span. In this zoomed in shot you can see that even that is moving around in frequency is this the result of the seperate channels, or a poor quality transmitter, or that this is the actual Mark and Space frequency?

From what I have read about GFSK it changes frequency gradually (compared to FSK) which would lend the answer to the last theory. Any help would be appreciated, I am completely new to this. Thanks.

Regards,

Brian Nelson

I think I may have posted this question in the wrong section. I am sorry, I think it belongs in the RF section.

SCREN09 yellow looks like a typical FSK/GFSK spectrum. The differences between both are rather small. No FSK system would us a frequency separation of 40 MHz, so SCREN08 is either showing two separate channels or possibly frequency hopping.

I am guessing my center frequency is the Marker frequency from the picture. How would I tell what fd is from that in terms of Space = fc-fd and Mark = fc+fd? It is moving around so fast on the analyzer that I can't really tell. I'm thinking this is because there are 4 seperate channels with individual mark/space frequencies. Not sure though, it could be that seperate "channels" are simply different addresses in code once it is demodulated.

FSK frequency separation ("mark-space" difference) is usually in a few 10 to to few 100 kHz range. I think, it's around 100 kHz in SCREN09. You are right, that the multiple RC channels can be expected to use different codes rather than different frequencies. In so far, I don't know why there are two channels of about equal power. But I guess, they are not used at the same time? Does the transmitter possibly utilize frequency hopping? It would allow a higher radiated power in the FCC region.

Holy crap, how did you tell that it was 100 kHz just by looking at that image? I decreased the span on the analyzer and I think I got about the same answer. So that would put my space frequency at ~ 2.4109 and my mark around 2.4110? It is possible that the transmitter is frequency hopping though not advertised. Thanks for all you're help.

One thing to remember is that the spectrum of fm modulation changes dramatically depending on the data rate. So you want to view it with varying RBW settings, and pick the one that most looks like what you want!