dna@dsd.UUCP (02/28/84)
Posted: Fri Feb 17, 1984 7:55 PM PST Msg: HGIE-1720-9015
From: TCLARK
To: pacsat
Subj: TAPR TNC Modem Mods de KV7B
Tom,
For some reason my as-the-news-is-unfolding style confused a number of
people, you among them, I guess. The last item I sent, which is repeated below
in case it went thataway, is the canonical document. One possible item not
mentioned in it is that increasing the value of the cap. hanging off pin 8 of
the 2211 will probably improve your ber at low s/n, at the expense of any
operation at 1800 baud. (I realize that 1800 baud operation was not what we
were trying to optimize, but it was refreshing to have something go so right
that we were getting more than 90 percent on average length packets at 1800
baud. The increase in value of the cap. on pin 8 should be by about a
factor of 4/3, which cuts the detection bandwidth and hence ups the berper-
formance.)
_____________________________________________________________________
NEW MODEM DETAILS
Over the last couple days I've been keeping you up to date with
our discoveries about the TAPR modem. We've completed our tests, and
this note will describe in detail exactly what to do to convert
your modem. We've tested out the new design and it is MUCH better than
the old one. Whereas before FM noise at the level of light popcorn was
enough to kill communication, this is not true with the new modem. I have
no doubt the modifications below will make the modem operate properly on
OSCAR 10, and will probably improve matters vastly on HF too.
In addition to the modifications improving the 1200 baud design, we have
also come up with an optimized 300 baud design centered at 1700 Hz designed
for HF/OSCAR use. It should exhibit the same good noise immunity that
the 1200 Hz design shows.
On to the mods.
For Kit Boards
1. Change C20 to .0047 microfarad (on U34)
2. Change R46 to 47K (also on U34)
3. Look carefully at the trace just below U21, just above C18 and
R39. This trace connects U18, pin 7 to J5, pin 18. On the top
of the board, attach a 4.7K resistor between the plate-through
hole at the right end of this trace and the trace leading to pin 1
of U18. This will be the new +12 volt pullup on U18, pin 7.
4. Notice this trace passes under R76, R77, R79, and R78 on its way to
J5. Just to the right of these resistors AND THE PLATE-THROUGH HOLE
ADJACENT TO R78
break this trace carefully, exposing about 1/16 in. of bare board.
5. Attach a VN10KM (available at Radio Shack, etc.) as follows: The
drain should go through the plate-through hole. The source should
go to the ground strip at the bottom of R78. The gate should be
soldered to the trace coming from U18, pin 7, which was just cut.
6. Recalibrate the modulator center frequency to 1685 Hz by typing
cal:3/1094
while in calibrate mode.
This completes the modifications.
For Beta Boards
1. Change R25 to .0047 microfarad.
2. Change R14 to 47K.
3. Carefully lift the left end of R11 and re-attach it to the nearby
+12 volt power bus on the wire-wrap area.
4. Add a 4.7K pullup on the bottom of the board between U17, pin 27
and the nearby +5 volt trace attached to the .1 microfarad cap.
between U17 and U25.
5. On the top of the board, locate the trace coming from JP3 which goes
to a plate-through hole near the left end of J3. Turn the board
over and follow this trace until it ends at a plate-through
hole under U17. This is the trace connecting JP3 and pin 27 of
U17. Cut the trace near the plate-through hole under U17.
6. Attach a VN10KM under the board as follows: Attach the drain to the
plate-through hole. Attach the gate to the trace leading back to JP3.
Attach the source to pin 28 of U17 (the nearest ground).
7. Recalibrate the demodulator freq. to 1685, as above. You're finished.
300 Baud Modem Modification
For Kit Board
1. Change C21 from .0022 to .01 microfarad
2. Change R46 to 220K.
3. Set high tone to 1800 Hz (cal:2/1024)
4. Set low tone to 1600 Hz (cal:1/1152)
5. Set center frequency on the 2211 to 1700 Hz (cal:3).
6. Go for it.
For Beta Board
1. Change C11 from .0022 to .01 microfarad
2. Change R14 to 220K.
(Note: this can easily be done by making up a jumper plug
to fit over JP2 with 227K - 47K = 180K)
3. make modification to modulator to allow low tone to be set to
1600 Hz: change R28 to 24K.
4. As above.
Note: Due to differences between the kit and beta boards, the calibration
lights will not operate properly on the beta board. At the correct
frequency one light will probably be out or dim, and the other will
be on.
Performance: You should notice the modem requires much less
audio than before, perhaps to the bottom of your volume control range.
You should also find you're decoding weak signals much more reliably.
Finally, you may find you can operate at hbaud greater than 1200. (Check
for legality first!)
Credits:
This modification is primarily due to the diligence of Eric
Gustaffson, who made extensive measurements, and carefully analyzed his
results. We take our hat off to him!
73
Dan
MODEM NOTES
I've received several requests for alternative part numbers for the
replacement cap, resistor and mosfet. The cap can be a garden
variety ceramic, the resistor can be a 5 percent 1/4 w, and the mosfet can
be either the originally specified VN10KM (available in places from RS) or
an IRFDZ13 mosfet, which is RS part number 276-2073, and which
should be in stock everywhere.
Again, I'd like people to let me know how this modification
works out when they've put it in. Thanks
73
Dan