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Author Topic: Is packet phase coherent?  (Read 1226 times)
KJ6MSG
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« on: November 26, 2012, 09:39:59 PM »

I've been digging around for days about packet radio, AX.25, and Bell 202 and I've found there is surprisingly very little information on the physical layer of packet radio. It took two days to stumble upon the fact that it uses NRZI encoding. I can not find concrete fact either way if packet is phase coherent or not. So, I beg the question of the horde... is the FSK modulation used in packet radio phase coherent?
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73,
KJ6MSG
@kj6msg
AC7ZN
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Posts: 42




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« Reply #1 on: November 27, 2012, 05:38:41 AM »

If you mean phase coherent to the mark or space carriers, it is not.  You can verify this by looking at schematics and computer programs that do the modulation.  They do so using a simple bit timing clock without regard to carrier phase (carrier phase information is usually not available when modulation and demodulation is through the voice channel of a transceiver).  Also, frequency transitions are often deliberately shaped or shaped by the inherent voice channel frequency response to reduce bandwidth, resulting in gradual transitions between the mark and space frequencies.

Does this answer your question?  Not sure I interpreted it correctly.

73,
Glenn AC7ZN


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G3RZP
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« Reply #2 on: November 27, 2012, 08:04:28 AM »

There's an article in the latest QEX (November/December) which might answer your question. I haven't read it yet, just noted that it seems to cover some of the ground.
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WA3SKN
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« Reply #3 on: November 27, 2012, 03:54:10 PM »

As I recall, the 1988 ARRL handbook had an excellent chapter 19 on digital... with a great history of going from Morse, RTTY, AMTOR, through packet.  And it addressed the different type analog Bell System modems too!  I do not think the latest handbook covers that subject as well as it used to.  FSK was used on HF, while AFSK was used on VHF/UHF.  I am not sure if that is still the case as I have been away from it for awhile now.  It covered serial ports and audio signal frequencies and levels... and impairments!
Now most have the computer directly connected via sound card to the radio, just load in software and it just might work!
73s.

-Mike.
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AA4PB
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« Reply #4 on: November 27, 2012, 04:14:45 PM »

Generally, phase-coherent means that the change from mark to space frequency occurs during the zero crossing of the modulation frequency in the case of AFSK. The packet spec doesn't require phase-coherent as it is dependent on the modulator. As a practical matter the modulation is generally done with a modulator chip that is indeed phase-coherent.
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KJ6MSG
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« Reply #5 on: November 28, 2012, 02:20:33 PM »

AC7ZN,

Thanks, that's what I was looking for. I'm doing some simulation in GNU Octave and wasn't sure if i should write the code to be phase coherent or not on the modulation.

The 2010 ARRL Handbook (the one I have) is not very detailed in FSK modulation and such. I find it completely glosses over a lot of it, though it does nice jobs in other areas of radio.
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KJ6MSG
@kj6msg
KQ6EA
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« Reply #6 on: November 28, 2012, 03:00:49 PM »

Another case where the older Handbooks are much better than the newer ones.

I have Handbooks from 1960, 65, 70, 80, 90, 2000, and 2010.

I seem to buy a new one every 10 years or so!

:-)

73, Jim
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