[amsat-bb] Re: GMSK: Time for a stupid question :-)

Howard Long howard at howardlong.com
Mon Feb 23 02:00:40 PST 2009

Hi Henk 

> While trying to work Cute-II, I started to think about GMSK ...
> (I have a PK96 external TNC hooked-up)
> My beautifull TS2000 has FM/AM/USB/LSB/CW modes.
> For GMSK, do I use FM or FSK ?

FM. FSK is for (relatively) narrow band RTTY type modes that traditionally
you'd have your radio set to USB or LSB. Your TNC modem will expect the
signal to have gone through a limiter/discriminator, so you need the
TS-2000's FM mode.

GMSK is FSK, but with two particular features.

Firstly the modulation index for GMSK is always 0.5, with state changes
occurring at zero crossings and phase continuity maintained. Secondly, after
the scrambler, the 1's and 0's go through a Gaussian filter and that's what
modulates the FM carrier: the -3dB point of that Gaussian filter is
determined by the BT.

The primary benefit of GMSK is that it reduces adjacent channel interference
at the expense of intersymbol interference. I doubt that;'s the reason it
was chosen for this mission though, much more klikely it was because you can
buy chips off the shelf to do GMSK. 

There's quite a good intro to GMSK here http://sss-mag.com/pdf/gmsk_tut.pdf

So yes, you should be able to demodulate GMSK with a standard 9k6 G3RUH
modem although theoretically it might not be optimal.

73, Howard G6LVB

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