HAM Radio 3.2/Ham Radio Version 3.2 (Chestnut CD-ROMs)(1993).ISO/packet/thenet08/dcdhf.doc

Text File | 1990-10-28 | 8KB | 183 lines

                         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.
73 Eric Gustafson, N7CL
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