IMPROVED DATA CARRIER DETECTOR (DCD) 
                 CHARACTERISTICS FOR 2211 DEMODULATOR
                         (VHF NBFM OPERATION)
                              20 JUL 88
 
This modification will make the DCD more reliable detecting a valid data
carrier and less prone to false on noise.  Correlated noise like voice
signals, or noise coming through a narrow IF filter in the radio (not a
problem for VHF NBFM radios) will still cause a small amount falsing but even
this will be greatly reduced.
 
With this simple modification installed, the 2211 based demodulator will be
easily capable of reliable operation with unsquelched audio.  This can be a
real advantage if the squelch circuit is very slow to respond or there is a
desense problem at the site which would require the squelch threshold to be
set very high.
To perform the modification:
 
1.  If you have a TNC-2 or clone (except MFJ-1270B and 1274), 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 at the supply voltages used in the TNC-2 and this
    drastically reduces the modem dynamic range.  This will not only improve
    DCD performance but will improve modem performance in general for a
    TNC-2.  This 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 negative 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 from a TNC-2.
    This problem does not exist in the TNC-1.  So if you are modifying a
    TNC-1, it is neither necessary nor recommended that you remove the MF-10
    circuit.
 
2.  Replace C35 and C42 with 0.01 microfarad caps (TNC-2s except 1270B and
    1274).
 
3.  Replace the 470 K resistor at R73 (pin 3 of 2211) with a 180 K resistor.
    Be careful not to damage the circuit board pads or traces.  R73 refers to
    part number for TNC-2s.  Use 2211 pin reference for TNC-1.
 
4.  Form a parallel network with a 180 K resistor and a 0.01 uF cap.  Solder
    one end of this network to PIN 3 of the 2211.  Solder the other end to
    PIN 6 of the 2211.
 
5.  If doing the modification to a TNC-1, Install a 4.7 K resistor from pin 6
    of the 2211 to pin 1 of the 2211 (This is the + 12 volt supply).
 
This completes the modification.
 
Please note that this modification is intended only for NBFM operation.
Although it will also improve DCD characteristics somewhat for HF packet
operation, there are some further modifications to the DCD circuit which are
required to make the circuit behave properly for HF packet.  These
modifications are not presented here.
 
I hope you find this modification useful.
 
73  DE  Eric Gustafson,  N7CL