©2002 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Printed: 12/14/01
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FUNCTIONAL DESCRIPTION.
The DE-202 is designed to provide
interface between computer equi p-
ment and a vhf or uhf fm receiver to
allow reception of “202” modem type
digital fsk transmissions at rates up
to 1200 baud. The ‘202” type modem
scheme uses audio tones of 2200 Hz
(mark) and 1200 Hz (space) to trans-
mit digital information on simplex and
half-duplex radio links. This is the
type of transmission commonly used
for packet radio and other computer
based communications on the vhf and
uhf bands.
The DE-202 is ideal for use with
packet radio TNC’s which have no in-
ternal modem, and it is also suitable
for many other amateur and commer-
cial applications, such as telemetry,
wireless alarm systems, and teletype.
In addition to receiving and demodu-
lating the mark and space tones, the
DE-202 also provides frequency com-
pensation to counteract the de -
emphasis circuits in the receiver, so
that the two tones are applied to the
detector at equal levels. This provides
maximum sensitivity for clean rece p-
tion of weak signals.
In addition to the DATA output
from the DE-202, it also gives a re-
ceive carrier detect (ROD) handshake
to the computer equi pment when a
valid data signal is being received.
Outputs are available for use with ei-
ther RS-232 or TTL computer inte r-
face. LED indicators on the board
provide visual check of DATA and
RCD status.
CONSTRUCTION.
Assembly
Is straight forward using
the di agrams and parts list.
Following are some Items which
should receive special attention.
a. Observe polarity of electrolytic
capacitors, LED’s, IC, and socket.
b. The LED’s have a longer lead
for the anode (positive side). Align
them for polarity as shown in the di a-
gram.
c. Resistor R16 has been added
to the de sign after the pc board was
made. it is installed in series with the
base lead on Q1 as shown in the de-
tail drawing. Install Q1 on the board
with emitter and collector leads in the
board and the base lead out of the
board. After installing R16 in the hole
previously provided for the base lead,
splice and tack solder the free end to
the Q1 base lead as shown.
d. Resistors mounted standing up
are shown on the diagram with a ci r-
cle indicating the position of the body
and a line indicating the top lead go-
ing over to the other hole.
e. Terminal pins are supplied on
a strip. Cut each one off the flashing
metal, and install it in the board by
pushing the pointed end through the
board until the first set of dimples
pops through the board to lock it in
place. Be care ful not to crush the ba r-
rel. The open end of the terminal will
be up when properly installed. You
can grip one wall of the open end with
needle-nose pliers to press the termi-
nal into the hole in the board.
f. In the original design of the ic,
pin 9 was not used; so the pcb layout
has a line running through that pad.
We later found some ic’s in which pin
9 is connected internally. Therefore,
it
is necessary to remove the metal
insert from pin 9 of the ic socket
before installing it.
THEORY OF OPERATION.
The heart of the unit is UI, which
is a phase-locked loop detector. The
center frequency is set by C2 and
R1/R3. The bandwidth is set by the
resistors and capacitors between pins
7, 8, 11, and 12. The dc output at pin
7 is low for mark (2200 Hz) and hi for
space (1200 Hz). Inverter Q1 provides
the correct output sense for TTL
loads, and inverter/switch Q2 pr o-
vides output for RS-232 loads.
Pin 5 provides a lo when a valid
carrier is received, either mark or
space. Q3 inverts this RCD signal for
RS-232 loads. Note that R12 and R15
provide -12V signal paths for RS-232
outputs when Q2 or Q3 is turned off,
so that the RS-232 output signals are
+12V instead of just +12V and
ground. Such a symmetrical voltage
swing above and below ground is be t-
ter than using ground as one of the
logic states to avoid problems with
noise and hum on the ground leads
between equipment.
The audio input for U1 is taken
usually from the detector (discrimina-
tor) of the fm receiver. An fm receiver
designed for voice operation has de -
emphasis, which provides roll-off of
high freque ncies to be the reciprocal
of a pre-emphasis curve in the trans-
mitter. This is done to enhance voice
communications with weak signals. It
turns out not to be advantageous to
do this for data signals transmitted in
fsk modulation; so C1 is deliberately a
small value in the DE202 in order to
counteract the high frequency drop-off
in the receiver. This avoids having to
modify the receiver in order to opti-
mize it for data reception.
TABLE 1. DC SIGNALING LEVELS.
COMMAND DATA/TONE
TTL RS-232
OFF
MARK 2200Hz
HI
-12V
ON
SPACE 1200Hz
LO
+
12V
TABLE 2. RS-232 25-PIN CONNECTOR
REFERENCE.
CONN TO/FROM
MODULE*/
FUNCTION PIN
COMPUTER TERMINAL
Protective gnd
1
<
DE-202/E6
Transmit data
2
>
MO-202/E1
Receive data
3
<
DE-202/E1
Req to send
4
>
MO-202/E5
Clear to send
5
<
MO-202/E8
Signal grounds
7
< >
DE-202/E6,
MO-202/E4
Rcv carrier detect 8
<
DE-202/E3
*MO-202 is the companion Data Modulator.
MOUNTING THE MODULE.
Once construction is completed
and the board has been checked for
wiring errors and bad solder joints,
the module may be mounted to the
chassis. The preferrable location is
close to the receiver to provide short
hook-up wire connection the the de-
tector in the receiver. The unit was
designed for easy mounting at the
right-hand side of Hamtronlcs receiver
modules, adjacent to the detector ci r-
cuitry. The module may be mounted
with 4-40 screws in the four 1/8-inch
mounting holes in the corners of the
board. Although the length of leads to
the computer equipment and power
supply are not critical, it is good prac-
tice to keep the audio lead from the
detector circuits as short as possible
to avoid getting noise and hum into
the receiver audio output stage; so it
would be good to keep the DE-202
next to the receiver. In se lecting the
location, be sure to allow for access to
the trim pot used to adjust the fre-
quency of the phase -locked loop, R3.
When the unit is to be mounted in an
rf tight box, the DE-202 is designed
for mounting at the right-hand side of
the receiver with room behind the DE-
202 for potentiometer adjustment.
POWER CONNECTIONS.
Power connections depend on
whether the unit will be used with TTL
or RS-232 computer inte rface. For
TTL interface, only a positive supply
HAMTRONICS® DE-202 DATA DEMODULATOR:
ASSEMBLY, INSTALLATION, & MAINTENANCE
