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(C) 31 JUL 88

Eric Gustafson,  N7CL
2018 S. Avenida Planeta
Tucson, AZ 85710



          DPLL  Derived Data Carrier Detect (DCD)
          For Filter Based and Single Chip Modems


                        INTRODUCTION

If  you  have  a TNC which uses either the  AMD7910  or  the 
TCM3105 single chip modem, or a TNC which uses a modem based 
on audio filters like the PK-232, you can vastly improve the 
DCD performance of your modem for packet radio use.

These  single chip modems were originally designed for  land 
line  use.   The designers,  who had no idea that the  chips 
might  one  day  be applied to a  radio  system,  made  some 
assumptions  about  the incoming signal that simply  do  not 
apply  to  the radio environment.   The data carrier  detect 
function  for them was not nearly so critical a function  of 
the  modem as it is for us on a busy packet  radio  channel.  
For the intended purpose of these chips,  there was expected 
to  be  only 2 stations involved on any 1 channel at 1  time 
and  these stations were connected by a nice  quiet  twisted 
pair.   Under  these circumstances,  the Carrier Detect (CD) 
function  built into these chips is entirely  adequate.   In 
the  packet radio environment,  the built in CD function  is 
next to useless.

Since  I  can't make the same defense for the  designers  of 
filter  based modems specifically intended for packet  radio 
from  the  beginning,  I won't try to speculate  about  what 
drove their design decisions.

The  circuit  presented here will allow your TNC to be  used 
with  unsquelched audio thus avoiding the unnecessary  delay 
of the squelch circuit found in typical VHF FM radios.  This 
circuit  also  provides several other  important  beneficial 
characteristics for the DCD system.

First,   since   the assumptions used when the TNC  software 
was  written  depend  on DCD representing  the  presence  or 
absence  of a data carrier on the channel,  it is  important 
that  the DCD circuit be able to distinguish a data  carrier 
from  noise  or  other non packet signals  to  a  reasonable 
degree.   The  DCD circuits which simply detect the presence 
of  ANY  type of signal or noise on the channel  are  simply 
inadequate  to this task.  Since the DCD  circuit  presented 
here  is  based  on the update signals in  a  Digital  Phase 
Locked  Loop (DPLL) which recovers both baud clock and  data 
from an NRZI packet data stream,  its output represents true 
detection of the data carrier.

Second,  once  a  data carrier decision has  been  correctly 
made,  it  is important that the DCD indication remain valid 
through  short fades,  collisions,  and while a  signal  too 
marginal to decode is on the channel.   This is accomplished 
by  providing  a  DCD "hang time" of approximately  5  to  8 
character  periods  (this can be optimized) to hold the  DCD 
output  true through short dropouts from the  above  causes.  
This  prevents  a queued up TNC from piling  on  collisions, 
transmitting over a station which has a marginal signal, and 
beginning   to  transmit  over  a  station  which  is  still 
transmitting but whose signal received a short multipath hit 
during the packet.

Third,  it is important that the DCD system NOT be sensitive 
to audio amplitude variations.  It should respond in exactly 
the  same  way for any signal that the modem is  capable  of 
decoding regardless of absolute input amplitude.  Since this 
DCD  circuit operates from the data recovered by the  modem, 
all  amplitude  information  is suppressed  before  the  DCD 
circuit even sees the signal.

                           NOTE!
    If  your TNC uses the EXAR 2211  demodulator,  this 
    new circuit is unnecessary for you.   Your existing 
    DCD circuit can be more easily modified for correct 
    operation  without this circuit.   The modification 
    procedure  for  the 2211 demodulator  is  presented 
    elsewhere and not repeated here.

(continued in file #2)

EOF