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TNC2MDM4.TXT
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1988-12-09
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APPENDIX D
MODIFICATIONS TO TNC-2 DATA CARRIER DETECTOR (DCD) CIRCUIT
These modifications are to allow TNC-2 modem operation with
a narrow IF filter on HF.
NOTE! For 1200 baud HF work on 10 meters, the normal 2.4
KHz SSB filter constitutes a "narrow" filter for
the demodulator.
There are three objectives to these modifications:
1. Provide threshold control for the DCD circuit. This
allows the operator to adjust the demodulator to
compensate for the DCD threshold shift which accompanies
bandwidth limiting the audio fed to the demodulator when
a narrow filter is used in the radio. This also allows
the operator to adjust the DCD threshold for operation
at 1200 baud on VHF FM to prevent excessive DCD falsing
when using unsquelched audio.
2. Add hysteresis to the Data Carrier Detector. This
reduces the DCD's sensitivity to noise. It does this
WITHOUT DEGRADING DCD ACQUISITION TIME for a valid data
carrier. It also prevents short glitches from appearing
at the DCD output on attack and release.
3. Add DCD hang time on release. This is to keep DCD valid
through a short multipath hit in the middle of a packet.
Without this, a TNC watching the DCD line waiting to
transmit will key up before the other station is
finished sending its packet. Currently, this happens
very frequently on HF packet channels. Again, this
benefit is gained WITHOUT DEGRADING DCD ACQUISITION TIME
for a valid packet.
The MFJ-1278 is currently the only commercially available
TNC I know of for HF packet use which has a modem that is
truly optimized for the mode in this respect. The
modifications given here will upgrade the modem in any TNC-2
or clone (including MFJ-1274) which has a 2211 demodulator
to the level of DCD performance of the 1278 modem.
Part numbers referred to are for the original TNC-2 as
produced (briefly) by TAPR. I have not reviewed schematics
of clones produced by all manufacturers so I cannot be sure
that these numbers will be correct in all cases. Since I
have seen information on the MFJ series, I can say that the
numbers are ok for a 1270 and 1274. If you are in doubt
about part number correspondence, obtain a schematic of the
original TNC-2 as produced by TAPR. Comparison between the
TAPR schematic and the schematic of your TNC should resolve
any differences. The TNC I used to verify this modification
was a 1274 which I use primarily on HF packet. Since the
physical layout of the modem area of the 1274 is different
that the layout of the original TNC-2 or "pure" unaltered
clones, I have avoided giving specific physical location
information in the modification procedure.
Any manufacturer of the TNC-2 can feel free to incorporate
this change into their hardware if they wish without
incurring any obligation to myself or TAPR. I am available
to answer any quetions in this area.
NOTE! Do NOT use TNC-2 firmware earlier than version 1.6
with this modification.
Firmware prior to V1.6 has no facility for
detection of a DCD fault condition and therefore
cannot warn you when the threshold control has been
improperly set.
STEP BY STEP MODIFICATION
1. If you have a TNC-2 or clone (except MFJ-1274 and
1270B), and have not already removed the MF-10 filter
and associated header parts, do so at this time. The
reason for this is that the operation of this filter
circuit is marginal in the TNC-2. This has the effect
of drastically reducing the modem dynamic range.
Removing this filter is a very easy modification.
Simply remove both the MF-10 and the header associated
with the MF-10. Then under the board solder a jumper
between pins 1 and 8 of the header socket. Removing
the MF-10 also unloads the -5 volt supply, improving
its regulation and reducing the noise generated by the
charge pump circuit.
There is ABSOLUTELY NO PERFORMANCE PENALTY for
removing this filter.
2. Replace C35 and C42 with 0.01 microfarad caps. This
step is not necessary or possible in a MFJ-1270B or
1274.
3. Remove the 470 K resistor at R73. Be careful not to
damage the circuit board pads or traces as they will be
needed later in the modification.
4. Remove CR13.
5. Replace R70 with a 47 K resistor.
6. Lift the cathode end of CR15 from the circuit board.
Install a 47 K resistor in series with CR15. Solder one
end of this resistor to the hole vacated by CR15's
cathode end. Solder the other end of this resistor to
the cathode end of CR15 above the circuit board.
7. Replace R74 with a 4.7 K resistor.
8. Form a parallel network consisting of a 180 K resistor
and a 0.01 microfarad cap. Make this network as compact
as possible as it will have to fit underneath the
circuit board. Solder this network in place under the
board. One end goes to pin 3 and the other to pin 6 of
the 2211 socket (U20).
9. Replace R38 with a 100 K resistor.
10. Above the circuit board, using leads as short as
possible, install a 470 microfarad, 10 volt rated
electrolytic capacitor between the -5 volt pad on the
tuning indicator connector (J3, pin 1) and ground.
Connect the positive lead to ground. Remember that
this cap will have to clear the cabinet so position
it as near the board as possible.
11. Under the circuit board, solder a 2.2 microfarad, 16
volt rated electrolytic capacitor from the junction of
R38 and R42 to the -5 volt supply. The negative end of
this cap goes to the -5 volt supply.
12. Under the circuit board, solder one end of a 22 ohm
resistor to the junction of R38 and R42. The other end
goes to pin 5 of the 2211 (U20).
13. On the front panel, Mount a miniature 100 K, linear
taper potentiometer (yes, there really IS room for
this). This will be the DCD threshold control. It will
be used to set the DCD trigger point to the proper
value.
14. Solder a wire from the wiper of the 100 K pot to the pad
vacated by R73 which connects to C45 and pin 3 of the
2211 (U20).
15. Solder one end of a 27 K resistor into the pad vacated
by R73 which connects to the -5 volt supply. Solder a
wire from the other end of this resistor to one of the 2
remaining leads from the 100 K pot. Use the lead that
is set to zero resistance when the shaft of the
potentiometer is turned fully CCW.
When operating a TNC with a DCD threshold control, set the
control so that the DCD LED on the front panel flashes
occasionally when there is no signal present. The "false
DCD" duty cycle should be approximately 20 percent.
When operating VHF FM with the radio squelched, the DCD will
not false. If you MUST operate with the radio squelched
(thus incurring the penalty of the additional delay time of
the squelch circuit), set the threshold fully clockwise as
described below.
The audio bandwidth of some VHF FM radios is so wide that
the DCD will not false regardless of the threshold control
setting. This will almost always be true when the audio is
obtained ahead of the radio's squelch controled stage before
deemphasis. For these radios simply turn the control fully
clockwise. This sets the DCD to maximum sensitivity. DCD
operation will not be impaired.
This completes the TNC-2 modem modification.