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Author Topic: Need some help with Si4362 (GFSK vs GMSK)  (Read 3896 times)

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Offline chicken

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Need some help with Si4362 (GFSK vs GMSK)
« on: August 20, 2013, 01:16:17 PM »
I will start out with a warning: I've no clue about RF.   :P

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.aspx

I 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)
Quote
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".

 :o 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.
« Last Edit: August 20, 2013, 01:19:25 PM by chicken »
 

Offline chicken

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Re: Need some help with Si4362 (GFSK vs GMSK)
« Reply #1 on: August 20, 2013, 01:23:51 PM »
PS: The help text in the configuration tool says
Quote
In the case of 2(G)FSK, deviation refers to the peak deviation (i.e., difference between the channel center frequency and the +/- transmitted frequencies).
In the case of 4(G)FSK, deviation refers to the INNER deviation (i.e., the difference between the channel center frequency and the nearest transmitted frequency level).
When using the conventional 4(G)FSK frequency map (-3dev, -1dev, +1dev, +3dev), this would be the difference between the channel center frequency and the +/-1dev levels.
Note:  some test equipment manufacturers define 4(G)FSK deviation using inner deviation, while others use outer deviation (i.e., center channel to +/-3dev levels).
:-//

 

Offline w2aew

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Re: Need some help with Si4362 (GFSK vs GMSK)
« Reply #2 on: August 20, 2013, 02:42:42 PM »
FSK is frequency shift keying. This means that the carrier frequency is shifted in response to the data being sent. The 'number' tells your how many different frequency states are used. 2FSK means thre are two frequencies used. For example, a '0' would be f1, and a '1' would be f2. Often, for 2FSK, the 2 is omitted. For 4FSK, there are four frequencies. Thus '00' would be f1, '01' would be f2, '10' would be f3 and '11' f4.  In other words, two bits per symbol.

The 'G' refers to the fact that the baseband digital data is filtered with a Gaussian filter prior to going to the frequency modulator.  This smooths the transitions from one symbol to the next, thus reducing the occupied frequency of the modulated signal.

MSK is a special case of FSK, where the difference between f1 and f2 is one half the symbol rate. This guarantees that the modulated RF signal is generated without abrupt phase discontinuities, further reducing the occupied BW.

So, if you set the peak-peak deviation to one half the symbol rate, and you apply a Gaussian filter to the baseband digital data stream ahead of the frequency modulator, you have GMSK. It sounds like this is all done in the chip. You just have to check if the 'Deviation' shown in the dialog refers to Peak Deviation (fc-f1) or Peak-Peak Deviation (f1-f2). The former is the more common definition of deviation, so you'll have to adjust the numbers accordingly.
« Last Edit: August 20, 2013, 02:44:28 PM by w2aew »
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Offline chicken

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Re: Need some help with Si4362 (GFSK vs GMSK)
« Reply #3 on: August 20, 2013, 03:06:48 PM »
Thank you for the detailed and comprehensible explanation! (love your videos for the same reason!)

So, depending on the definition of deviation, for 9600 bps it's either 4.8 or 2.4 kHz?

And with the help text saying "In the case of 2(G)FSK, deviation refers to the peak deviation (i.e., difference between the channel center frequency and the +/- transmitted frequencies)", the latter being the more likely choice.
 


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