©1994 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised: 1/31/00
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FUNCTIONAL DESCRIPTION.
The TD-3 is a CTCSS Decoder and
Encoder designed especially for use
with Hamtronics Receiver modules in
repeater use, but allowing also for
general use in other radios. It is tun-
able to cover the entire range of tone
frequencies from 63 to 250 Hz.
The unit normally is used to inhibit
a repeater transmitter from responding
to received signals which do not have
the appropriate subaudible tone to gain
access. This is done by inhibiting the
COS signal from the receiver squelch
circuit which is required to make the
transmitter respond. It may also be
used in non-repeater receiver systems
where muting of the receiver audio is
required so that only stations with the
proper tone are heard.
While the unit normally is used
only for receiver control by decoding
tones on the received signal, circuitry
is also included to generate that tone
to be applied to the Exciter module in
a repeater or transceiver so that the
subaudible tone is also transmitted.
THEORY OF OPERATION.
Refer to the schematic diagram.
The heart of the unit is the 567 phase-
lock loop decoder chip, U2. The fre-
quency of the loop detector is set by
R6/R7 and C9-C12. Audio input for
the chip is derived from the discrimi-
nator of the receiver and processed
through active low-pass filter Q1 to
reduce the level of audio in the normal
voice range and enhance the relatively
low level of the subaudible tone.
Since the response bandwidth of the
phase-lock loop in U2 is proportional
to the incoming audio level, R4 is ad-
justed to provide adequate sensitivity
without overdriving U2, thereby opti-
mizing the bandwidth of tones which
will activate the unit.
Bias for Q1 normally is provided
by the +4V riding on the discriminator
output of the detector ic in Hamtronics
receivers. If another type of receiver
is used, it is necessary to supply bias
for this stage as described under in-
stallation procedures.
U2 pin 8 normally rests at a logic
high with no tone signal present. The
bias current through R5 turns on Q2,
which provides a ground to the output
at E2. When the proper tone is de-
tected, pin 8 goes low, and Q2 is
turned off, removing the ground from
E2.
The ground at E2 normally is used
to shunt the COS signal from the re-
ceiver to ground, thereby preventing
the COS signal from activating the
COR board in the repeater. This dis-
ables the transmitter unless the proper
tone is received. Another way to use
the TD-3 in a transceiver is to mute
the audio from a receiver unless the
tone is present. This is done by using
the ground at E2 to short the audio
path before the speaker amplifier in
the receiver.
Q3 provides a tone signal for use
by a transmitter, when required. Some
of the square-wave audio tone signal
from the phase-lock oscillator in U2 is
low-pass filtered by R8/C13 and
R9/C14 to provide a quasi-sinewave
tone signal. This is amplified by Q3
and applied through level control R11
to the mic. input of the exciter. The
series resistance of R13 prevents the
amplifier from loading down the nor-
mal audio applied to the mic. input of
the exciter.
ASSEMBLY.
Construction is fairly simple. Re-
fer to the parts list and component lo-
cation diagram, and solder all parts in
place, using short leads. Following
are some special points to consider.
a. Install the ic socket with the
notch facing to the left as shown.
Then, plug in the ic, being careful not
to let pins bend over instead of going
in the socket.
b. Resistors mounted vertically
are shown with the body of the resistor
designated by a circle.
c. For the most common fre-
quency range, R7 is a combination of
a 10K resistor in series with a 2.2K re-
sistor. Install one in each hole, and
join them at the top. Twist the top
leads together, solder, and clip off the
excess. For the other frequency
ranges, a single resistor (not supplied)
can be substituted. See table 2 for
values.
d. Install R8 with a 1/4 inch high
loop formed in the right-hand lead to
act as a test point.
e. Observe polarity on elec-
trolytic capacitors, diodes, transistors,
and ic's.
f. Cut hookup wires to length,
strip each end 1/4 inch, and then sol-
der one end to terminal pads as fol-
lows. Wire lengths are as required to
conveniently mount the TD-3 behind a
Hamtronics receiver module. Note
that no wire is prepared for E5, the
transmit af output, since this function
normally is not used. If you do use it,
attach a hookup wire to it as required
for your installation.
TABLE 1. HOOKUP WIRES.
COLOR LENGTH PAD FUNCTION
ORN
9"
E1
DISC AUDIO
BLU
8½
"
E2
RCVR COS
BRN
6¾
"
E3
CTCSS DISABLE
RED
8¾
"
E4
B+ INPUT
g. Check over construction to be
sure all parts are installed in the proper
holes with the proper polarity. Check
all solder joints for bad joints, solder
splashes, etc.
SELECTING FREQUENCY
RANGE.
The value of R7 sets the tuning
range of TONE FREQ pot R6. The
value of the resistors used for R7 sup-
plied with the unit (10K in series with
2.2K) provides a tuning range which
covers the 100 Hz tone which is used
most often. If you are using another
tone frequency, consult the chart be-
low, and install a resistor (or two re-
sistors in series by placing one in each
hole and tack-soldering the two leads
at the top). The information is given
as a guide. If you don't have the ex-
act value(s) required, use a combina-
tion of resistors which will come close,
and tune the unit before mounting, so
the resistors can easily be changed.
TABLE 2. RESISTOR SELECTION.
TONE RANGE RES R7
63-72 Hz
22K
72-80 Hz
18K
80-95 Hz
15K
95-125 Hz
10K + 2.2K
125-150 Hz
10K
150-250 Hz
6.8K
MOUNTING.
The usual location for the TD-3
HAMTRONICS
HAMTRONICS
®
TD-3 SUBAUDIBLE TONE DECODER MODULE
TD-3 SUBAUDIBLE TONE DECODER MODULE
