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[aprssig] Suggestions for modem designs?

scott at opentrac.org scott at opentrac.org
Fri Oct 13 17:55:33 UTC 2006

> The tracker2 is open source; find it a source forge.  It uses a 2211  
> decoder.

It's open source, but watch out - it's still in development so I'm not exactly vigilant about making sure what's in CVS actually compiles and works properly.  And it's not really tied to the 2211... I've used it with an MX614 and TCM3105.  The standard circuit has a selectable equalization filter that lowers the 1200 hz tone to help compensate for twist.

> The WhereAVR is a zero crossing detector; meant for tracking and  
> light duty receiving.  http://www.knology.net/~gdion/whereavr.html

I wrote a delay line correlator for the T2 as an experiment.  I'm not sure it's even up to the WhereAVR's level of reliability yet, though.  But it'll run on a $2 chip with just a few passive components and a clock crystal.

> There are plans in the March/April 2005 QEX for a "modemless"  TNC  
> (zero crossing detector) based on a PIC.

Circuit Cellar had some designs a month or two ago.

> You've got several choices for the modem:  The MX614 you've already  
> found.  It does a decent job, requires its own crystal, and is  
> relatively expensive ($10 range).  There may be other chips 

I've got 'em for $9 at www.argentdata.com/catalog.  As far as I know, my store is the only one in the US where you can order a single one online with a credit card.  Some place in Italy has them for about €12.  The TCM3105 is gone.  The XR2211 is readily available and cheap, if all you need is RX.  CML sells some other Bell 202 modem ICs, but mostly 3 volts and not as simple to interface as the MX614.

If I get the correlator working well enough, I might reuse some Tracker2 code to make a simple KISS interface with a low-end HC08.  Don't have much spare time to work on it now, though.


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