Theory of Operation
15
MT-3 FM Transmitter Mainboard Instruction Manual
IM20-MT3TXMN
turn-on time
Turn-on Time is the time it takes the FM Audio
Processor to output a stable audio signal to
P3-1 once the +8.0 VDC power is enabled.
The turn-on time can be virtually eliminated by
configuring the audio processor for continuous
audio standby; however, this results in
increased current consumption. Powering of the
compression amplifier contributes to most of the
time delay, so transmitters configured with the
compression amplifier disabled (for data or non-
compressed audio) will exhibit the fast
turn-on time. The response measurement is
made with the standard factory settings with a
1 kHz tone applied to the balanced input.
low frequency modulation
The transmitter has an additional option
to address low frequency user modulation
requirements. A phase modulated bandwidth
from 0 (DC) to 100 Hz (PLL loop filter
bandwidth) allows specialized applications such
as paging or trunking where a separate low
frequency digital / analog modulation channel
is required. Low Frequency Modulation allows
external access to the low frequency modulation
capabilities of the synthesizer module. The
DIRECT MODULATION inputs on the J1 control
connector of the MT-3 Motherboard will be used
(B20 for TX A, and A20 for TX B).
turn on time
mode
turn-on time
audio Processor Standby Current
fast turn on – higher current
approx 1 mS
15 ma
Current save – slower response
approx 150 mS
450 µa
fm audio Processor Signals
The FM Audio Processor has six audio inputs,
two audio outputs and one audio control input.
Five of the audio inputs are used primarily for
voice and tone signals. The sixth, the Direct
Modulation Input, is used primarily for data
signals. The audio control input is used to switch
audio outputs so the transmitter can transmit
voice or data.
The audio inputs on the audio processor are:
• Dynamic microphone input
• 600 Ω balanced input
• Subtone inputs
• Auxiliary input
• Direct modulation input for data signals
fm audio Processor outputs
Both the audio outputs, Modulation Output
(P3-1) and Low Frequency / Direct Modulation
Output (P4-2), are gated by audio switches U5
and U6 respectively which are controlled by the
Transmit Audio Control Input (P4-4). The audio
switches can be operated complimentary to
each other so that only one source modulates
the transmitter. In standard configuration, the
Modulation Output port is used so switch U5 is
always on.
