GMSK: Time for a stupid question :-)
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 ?
Please only a gentle smile, I realize I do look silly asking this :-) Henk
Henk, PA3GUO wrote:
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 ?
Please only a gentle smile, I realize I do look silly asking this :-) Henk
You're not the only one who doesn't know... I'm quite keen to find out more about using GMSK.
Gordon
Henk, AO-51 runs 9600 GMSK. Most people receive AO-51's GMSK digital on PSK Decoders. The early UoSats and their subsequent derivatives ran PSK. Understandably there is a slight degradation in performance copying GMSK on PSK decoders.
Connecting an oscilloscope to your TNC will identify the difference between GMSK and PSK. Put the TNC in test mode to verify the PSK waveform. Use AO-51 to compare one against the other.
The TNC3S has a GMSK selector, but I cannot remember whether it was for RX, TX or both. Its some time since I had the covers of or perused the manual.
So your PK6 should work "OK" on GMSK. 73, Colin VK5HI.
-----Original Message----- From: [email protected] [mailto:[email protected]]On Behalf Of Henk, PA3GUO Sent: Monday, 23 February 2009 08:15 To: [email protected] Subject: [amsat-bb] GMSK: Time for a stupid question :-)
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 ?
Please only a gentle smile, I realize I do look silly asking this :-) Henk
Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
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
Thank you to those that replied !
From the feedback I understand that;
- my TS2000X should be in FM mode, connected to the TNC via ACC2 (was indeed the case, thanks for confirming) - my external TNC, the PK-96, should be able to decode GMSK 9k6 (indeed I can decode AO51's downlink)
So...: I will need to team up with someone else in my footprint to see how the Cute digipeater (downlink) can be activated. At this point I have not even heared it, so it can be my ERP (antenna or RF pwr) or the way my PK-96 is generating GMSK 9k6 signals (vs. how Cute expects them).
Thanks so far, I'll certainly share it if I have more success ! Henk, PA3GUO
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 ?
Please only a gentle smile, I realize I do look silly asking this :-) Henk
Henk, I would suggest FM 2.4 KHz deviation. You will need a Gaussian filter after the 4800 baud digital to make the logic transitions raised Cosine. The FM modulation needs to be direct coupled not AC coupled. Never Had a TS 2000X still using a ICom 820 disaster!
GMSK provide for a modulation index of 0.5 making its performance a little worse than AM in performance. Works O K on strong signals, looses it on noise, multipath, or interference. There is no capture effect that is normally associated with FM and modulation indexes greater than 1 Good luck, Art, KC6UQH
-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Henk, PA3GUO Sent: Wednesday, February 25, 2009 12:21 AM To: [email protected] Cc: [email protected] Subject: [amsat-bb] Re: GMSK: Time for a stupid question :-)
Thank you to those that replied !
From the feedback I understand that;
- my TS2000X should be in FM mode, connected to the TNC via ACC2 (was indeed the case, thanks for confirming) - my external TNC, the PK-96, should be able to decode GMSK 9k6 (indeed I can decode AO51's downlink)
So...: I will need to team up with someone else in my footprint to see how the Cute digipeater (downlink) can be activated. At this point I have not even heared it, so it can be my ERP (antenna or RF pwr) or the way my PK-96 is generating GMSK 9k6 signals (vs. how Cute expects them).
Thanks so far, I'll certainly share it if I have more success ! Henk, PA3GUO
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 ?
Please only a gentle smile, I realize I do look silly asking this :-) Henk
_______________________________________________ Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
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On Thu, Feb 26, 2009 at 2:37 AM, Art McBride [email protected] wrote:
Henk, I would suggest FM 2.4 KHz deviation. You will need a Gaussian filter after the 4800 baud digital to make the logic transitions raised Cosine. The FM modulation needs to be direct coupled not AC coupled. Never Had a TS 2000X still using a ICom 820 disaster!
GMSK provide for a modulation index of 0.5 making its performance a little worse than AM in performance. Works O K on strong signals, looses it on noise, multipath, or interference. There is no capture effect that is normally associated with FM and modulation indexes greater than 1 Good luck, Art, KC6UQH
A follow-on question, if nobody minds.
I've recently bought a Delta-44 soundcard to do some higher definition SDR work. It looks like the softrock line of radios, for instance, will have a 2m version fairly soon. http://www.w9idx.com/node/42
What program is state-of-the-art when it comes to digital modes and SDR work? Or even when it comes to FM audio out, is there a 9600 baud software sound modem that equals or outperforms a dedicated modem?
participants (6)
-
Art McBride -
Bruce Robertson -
Colin Hurst -
Gordon JC Pearce -
Henk, PA3GUO -
Howard Long