[OZAPRS] MFJ1278B for APRS

[Darryl Smith]

One of the impacts of DSP with FM relates to how it is done. If you use an
SSB receiver, and send the SSB signal into an FM decoder you will get far
better results.

Firstly this would make it 'Coherent' demodulation, improving the
performance instantly by probably 1.5 to 3dB. The exact theoretical
improvement depends on the curves from the BER Vs Eb/No chart (Bit error
Vs
Energy per Bit). I just cannot remember the exact numbers off the top of
my
head.

Then because we have a frequency domain incoming signal rather than a time
domain signal, we do not need to convert back to frequency domain for
demodulation. This will improve things too. Depending on the DSP
algorithm,
narrow band noise sources can also be automatically removed leading to
significantly improved results. If straight FM demodulation is used, and
the
interfering signal is higher than the signal required for the FM capture
effect, then no data will be decoded. I would imagine that leaving out the
FM demodulator would give you another 1.5 to 3db. This adds up to
somewhere
between 3 and 6 dB

Now, I have no idea if you can send an SSB (or IF) signal into the
MFJ1278B
TNC. If not, there are units that you can send it into, such as the
FlexRadio SDR-1

Darryl

-----Original Message-----
From: ozaprs-bounces at aprs.net.au [mailto:ozaprs-bounces at aprs.net.au] On
Behalf Of Scott & Clare Evans
Sent: Friday, 7 September 2007 12:26 PM
To: 'VK / ZL APRS Users'
Subject: Re: [OZAPRS] MFJ1278B for APRS


G'day Gavin...

I wouldn't think that the DSP would do too much for a FM signal, however
you
can only try and see what happens! I have a DSP board in my PK-232MBX and
it
works great on HF but I haven't really noticed any improvement for VHF. I
did however use this TNC last year for a while on ISS and results were
good
but so was the signal I was working with!

73 de VK7HSE Scott Evans


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