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[dsp] Analog front end design

scott at opentrac.org scott at opentrac.org
Tue Apr 26 04:37:53 UTC 2005


Good to know.  I read a paper on that modem (or a similar one - 'The new
G3RUH software modem for the DSP56002EVM', Etxebarria et al) but I don't
remember seeing anything on the specs of the processor.  I should be able to
get by with less work since I'm only going for 9600 baud and I won't need to
worry about the multirate filtering and such.  Yeah, higher rates would be
fun, but in this country at least we've still got thousands of hams running
TNC-2s, and many of those don't even run KISS mode.  An affordable 9600 baud
TNC would be a big step up.  =]

I *think* I'm going to go ahead and use a dual-channel 16-bit CODEC for the
full version of the TNC, rather than the on-board ADC.  It might save me a
couple of external op amps, should reduce CPU interrupt overhead thanks to
PDC driven data transfers, and with the sigma-delta converters and
decimation filters should reduce the hardware filtering requirements.  If I
was building it only for 1200 baud I'd go with a filter designed for Bell
202, but I want to keep it as open as possible for future expansion.  If
someone wants to write a POCSAG decoder, or WEFAX, or SSTV, or whatever, I
don't want the hardware getting in the way.

I'm also thinking about an analog switch matrix.  It might be needed to
switch inputs between the 1200 and 9600 baud pins on the mini-DIN
connectors, and I figure it should be possible to set it up to cross-connect
the two channels and use the device as a repeater controller.  I really need
to do some more research on that, though.  But it's got plenty of storage
for digital voice playback...

On the subject of the dual-port design, I think I can do that for less than
twice the CPU load of a single channel.  Once a set of coefficients have
been loaded into registers, it'll take less overhead to run the same
operations on two sets of samples at the same time.  Guess I'll find out
when I start writing the code.

Scott

-----Original Message-----
From: doug at dougbraun.com [mailto:doug at dougbraun.com]
Sent: Monday, April 25, 2005 9:04 PM
To: TAPR DSP Mailing List; willmore at optonline.net; scott at opentrac.org
Cc: dsp at lists.tapr.org
Subject: RE: [dsp] Analog front end design


The venerable Motorola 56002 EVM  has a great G3RUH modem, and
it does only 13 MIPS with a 27 MHz clock.

And I think 10 bits of ADC resolution is plenty.  Its SNR is probably
better than you ever get from an actual received FM signal.

So go for it...

Doug Braun
NA1DB
www.dougbraun.com






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