I will start out with a warning: I've no clue about RF.
So I recently stepped into the deep end when I started a little project around the Si4362.
http://www.silabs.com/products/wireless/EZRadioPRO/Pages/Si4362.aspxI designed a small breakout board and the chip talks to me and can be initialized without complaints. But now I want to actually receive some data and am kind of stuck.
It's datasheet says it does GMSK, but the documentation makes no mention about it outside the feature rundown. Nor does the configuration tool (WDS) offer that option. OOK, 2(G)FSK and 4(G)FSK are all there is.
After some more digging I found this knowledge base entry for a closely related IC:
(edit: removed link as it's useless without session - searched for GMSK and it was the only hit)
Question
Can the Si443x family of chips operate with MSK / GMSK modulation?
Answer
Yes. The Si443x family of chips are capable of transmitting and receiving MSK and GMSK modulation.
MSK (minimum shift keying) and GMSK (Gaussian-filtered minimum shift keying) are special instances of continuous phase frequency shift keying (CPFSK). Specifically, MSK is obtained when the modulation index is exactly h = 0.5. Thus the frequency separation between the two transmitted frequencies (i.e. for positive deviation and negative deviation) is exactly equal to 1/2*Tb, the result is a 90 degree phase rotation between each transmitted symbol. This orthogonality between adjacent symbols provides a benefit under certain forms of demodulation. Additionally, the continuous-phase nature of the signal helps to suppress the spurious sidelobes of the transmitted signal, and thus the transmitted spectrum appears cleaner.
The ability to obtain a mod index of precisely h = 0.5 depends upon the ability to program exactly the required value of frequency deviation, and thus depends upon the frequency resolution of the PLL Synthesizer. On Si443x devices, the frequency resolution of the synthesizer is 312.5 Hz in High Band (480-960 MHz) and 156.25 Hz in Low Band (240-480 MHz). For larger values of deviation (e.g. 20 kHz to 50 kHz), this frequency resolution is sufficient to provide an excellent approximation to h = 0.5 For smaller values of deviation (e.g. 1 kHz to 5 kHz), this frequency resolution may be a bit coarse and the resulting modulation index may not be exactly h = 0.5. However, in most cases, it is "close enough".
No clue what this means, could just as well be Chinese (at least then I could use Google Translate)
The configuration tool, among many other things, offers the options modulation type (OOK, 2FSK, 2GFKS, 4FSK, 4GFSK), data rate (kbps) and deviation (kHz). See attached screen grab.
Any ideas what I should fill in to receive a GMSK encoded package at 9600 baud? 9.6 kbps data rate is easy, but is it 2GFSK or 4GFSK? And most importantly, how to determine the deviation in kHz?
Unfortunately I don't have a test signal in my modest lab and have to drive a few miles to receive a real-world signal. So I'd like to keep trial and error to as few options as possible. Any input is appreciated greatly.