Hi Tim,
Hi Tim,
It's for an ACARS (aircraft packet) decoder, which is 2400 baud MSK.
The planes use an AM transmitter, so it should be quite good with
respect to signal to noise ratio (compared to FM). It will only have
to receive.
My hope is to have a single decoder chip, on a board with a small
stand alone board containing a PIC (to extract the useful information
from the raw data of the modem output) and a non-volatile RAM chip to
store the extracted data for later retrieval.
Since the signals I'm interested in copying are 'dx' signals, the
ability to decode a slightly weak(er) signal is important. If I can
get good copy with a 6 db poorer s/n ratio, I'd like to use the better
method (if possible).
The overall purpose is to monitor the ACARS frequencies at 131 Mhz to
determine when/if the 2 meter band is open past it's normal range.
Many of the ACARS packets contain longitude and latitude data from the
planes GPS. Hence, when I start hearing planes more than 300 miles
away, I can start listening and transmitting on 2 meters.
I spent the last couple of hours on google, looking for single chip
psk modems-they don't exist. I did find a 400 pin super computer chip
that has been programmed to function as a PSK modem, but it's much
more than I need, both in terms of $$ and complexity. There are some
Motorola DSP EVM modules that have been programmed for stand alone
PSK, but they are horribly expensive, obsolete and draw way more
current than I want to deal with.
There were a few 'rumors' of PIC implementations being developed, but
I can't imagine a PIC with it's very limited A/D converters as being
very useful for weak signal work.
I actually have a DSP EVM here (the original one used for the DSP-10
transceivers, an Analog Devices EZ-Kit Lite DSP module). It's in the
box, has never been used. I can't program the beast-I'm software
challenged. More info on the DSP-10 is at:
http://www.proaxis.com/~boblark/dsp10.htm
There is an ACARS software decoder that needs to have a dedicated
computer and soundcard-I'd like to have something more 'stand alone'
though, hence my interest in rolling my own modem).
Regards,
Art