Theory of Operation
6
MT-3 FM Transmitter Mainboard Instruction Manual
IM20-MT3TXMN
transmitter Push-to-talk
All three of the Push-To-Talk (PTT) inputs that
key the transmitter are active low (< +2 VDC).
One PTT input is on the front panel microphone
connector. The other two PTT inputs: PTT
WTO (PTT with Time-Out-Timer) and PTT
NTO (PTT No Time-Out-Timer) are on the
backplane connector of the transmitter board.
If required, the microphone’s PTT input can be
configured to activate the transmitter’s Time-
Out-Timer (TOT). An isolated PTT input can be
made available by installing an optional relay
(RELAY1) and configuring jumpers J1 to J4 so
that the relay controls the PTT circuitry.
microphone Ptt
Jumper J1 on the Front Panel Board configures
the microphone’s PTT input (MIC PTT) to
either bypass or activate the transmitter’s TOT.
Installing surface mount jumper J1 in the ‘X’
position (default) selects the MIC PTT NTO
line, which bypasses the TOT. Installing surface
mount jumper J1 to the ‘Y’ position selects
the MIC PTT WTO line, which activates the
transmitter’s TOT. When SW1 is in the KEYED
position, the MIC PTT NTO line is automatically
grounded.
Ptt with time-out-timer
Pins B10 and Z10 of the backplane connector
are the PTT WTO input. When the PTT WTO
signal, which is normally high, falls below
+2.0 VDC, the transmitter is keyed. The
transmitter is disabled when the PTT WTO input
rises above +2.3 VDC or if the TOT’s time-out
period is exceeded. If the time-out period is
exceeded, the PTT WTO input must go high
(> +2.3 VDC) and then low again in order to
re-key the transmitter.
The PTT WTO threshold of approximately
+2 VDC (0.3 VDC hysteresis) is set by U1a, R1,
R2, R3, R4, R9 and R10 while diodes D1 and
D2 provide over-voltage protection for U1a. The
PTT WTO signal output from U1a is ‘AND’ed
with the MIC PTT WTO by U2a. When either the
PTT WTO or the MIC PTT WTO is activated,
the output of U2a goes low, which triggers the
transmitter’s TOT located on the FM Audio
Processor. The TOT’s output is ‘AND’ed with the
MIC PTT NTO signal (U2c) and the PTT NTO
signal (U2d).
When any one of the preceding three signals
(TOT’s output, MIC PTT NTO, PTT NTO) go low,
the transmitter is activated by transistors Q1 to
Q7 which switch power to the various modules.
Ptt no time-out-timer
Pins B14 and Z14 of the backplane connector
are the PTT NTO input. When the PTT NTO
signal, which is normally high, falls below
+2.0 VDC, the transmitter is keyed. As
long as the PTT NTO signal remains below
+2.0 VDC, the transmitter will remain keyed.
The transmitter is disabled when the PTT NTO
signal rises above +2.3 VDC.
The PTT NTO threshold of approximately
+2 VDC (0.3 VDC hysteresis) is set by U1b, R5,
R6, R7, R8, R9 and R10 while diodes D3 and
D4 provide over-voltage protection for U1b. The
PTT NTO signal is ‘AND’ed with the output of
U2c (MIC PTT NTO signal ‘AND’ed with the TOT
output) by U2d. When the output of U2d goes
low, transistors Q1 to Q7 activate the transmitter,
which switch power to the various modules.
Ptt relay
The transmitter’s PTT circuitry can be
completely isolated by installing RELAY1.
Jumpers J1 to J5 configure the relay to provide
an isolated PTT input for either the PTT WTO
line or PTT NTO line. Energizing the relay
enables the isolated PTT input. The transmitter
board will accept any of the Aromat TF2E line
relays. These relays are DPDT, single side
stable, and have coil voltages ranging from
+3 VDC to +48 VDC. Only one set of relay
contacts is used to activate the PTT circuitry.
To configure the isolated input for PTT WTO
operation, jumpers J2, J3 and J4 must be in the
‘Y’ position. In this mode, pins B10 and Z10 no
longer function as the PTT WTO input; however,
pins B14 and Z14 continue to function as the
normal PTT NTO input.
To configure the isolated input for PTT NTO
operation, jumpers J2, J3 and J4 must be in the
‘X’ position. In this mode, pins B14 and Z14 no
longer function as the PTT NTO input; however,
pins B10 and Z10 continue to function as the
normal PTT WTO input.
