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Re: The 38k4 Cost Question



I'd like to echo AL7CR's and N8MH's recent comments with just one
minor nitpick:

> To copy 9k6 you needed an expensive modem.

If you're willing to homebrew, this is not necessarily true.

In fact, it gets substantially easier (for a newcomer) if the design
uses analog rather than digital bit shaping (filtering) (in my opinion).

I haven't thought about this in a while, but if I am not mistaken,
there is no reason why an analog modem cannot be taken for 9k6/38k4
asymmetric pacsat communications.  The REAL problem (as I see it) is
the TNC.

So... how about a simple, KISS-only modemless TNC based on a low-cost
PIC microcontroller?  I think TAPR has a 1200 bps "bit-banging" circuit
designed about a 16F84 PIC microcontroller.  I think an 16F88 might be
a better chip for this task.  It can do synchronous as well as asynchronous
serial streams at the data rates in question.  They can be purchased for
under $3.00.  (You can probably mine enough loose change in a busy parking
lot to buy one.)  Your groundstation software (or Linux kernel -- also free)
will handle the AX.25 stuff for you.  We just need someone smart to write
some code for it to make it work in this application.  Hmmmm... ;-)

If push came to shove, I think even an old scanner could be put into
service as a "front end" for a 38k4 downlink receiver.  (A rig with
10 or 20 MHz tuning range is overkill when you're really only interested
in a single downlink (+/- Doppler)).  An outboard IF strip and detector would
be needed, but you'd probably need that anyway.  Unfortunately, FM quadrature
detector chips have become a bit scarce in recent years (Motorola and National
Semiconductor no longer make them), but I think Phillips still does (if you
can find 'em).

An old crystal-controlled 2-meter FM rig can be "FSKed" for an
uplink transmitter.  They're not expensive, either.  Again, it's only
a single uplink frequency we're interested in.

So, let's see:

Cost of homebrew modem: low
Cost of homebrew TNC: almost zilch
Cost of uplink TX: next to nothing
Cost of the downlink RX: low
The thrill of getting everything working: priceless

What I've said is really nothing new.  People have been writing
articles and pointing out this stuff for years...

It's not state-of-the art, but then again, it doesn't have to be.


73, de John, KD2BD


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