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STATE00.TXT
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1988-09-03
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93 lines
******************* FILE 1 of 6 FILES **********************
(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