Parts List

Audio Processor Board Instruction Manual 2-1
2 S I G NALS
The MT-3 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.
2.1 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
there can ever only be one source modulating the transmitter. In standard configuration, the
Modulation Output port is used so switch U5 is always on.
2.2 Modulation Output
The Modulation Output port is the output port used by all voice input signals. The voice inputs are
passed to U1A and U1B, a dual programmable compandor which is configured as an automatic
level control amplifier. Op-amp U3A provides the limiting action for the FM Audio Processor.
After the audio signals have been combined limited and buffered, they are filtered by 10
th
order
Linear Phase Lowpass Filter (U4). The output signal from the filter is then level adjusted by the
deviation control pot, R29, before buffer amplifier U3D.
In special applications, jumper JU6 can be disabled and JU7 enabled, this allows the transmitter to
be modulated directly from the auxiliary input. This should not be done without external filtering
since jumper JU7 bypasses the limiting and filter circuits.
2.3 L ow Frequency / Direct Mod. Output
The Low Frequency / Direct Modulation Output port has two functions depending on whether the
transmitter is synthesized or crystal controlled. In a synthesized transmitter, this port is used to
modulate the synthesizer reference frequency. The frequency response of this port is typically DC
to 300 Hz. In a crystal controlled transmitter, this port can be used by the Direct Modulation Input
to directly modulate the crystal control oscillator module. The frequency response of this port is
essentially flat from DC to 5 kHz with no limiting or filtering.
2.4 Microphone Input
The microphone input has an automatically level controlled (ALC) preamplifier U1 whose input
level is controlled by R2. The microphone input level control (R2) can accommodate a -25 dBm to
0 dBm input signal. The microphone input is limited and filtered and is output at the standard
modulation output port. The microphone input can have a standard 6dB/octave pre-emphasis
response or a flat-audio response, jumper JU1 at 'y' and 'x' position respectively.
2.5 Balanced Input
The 600 ohm balanced input uses the ALC preamplifier U1B, with input level control pot (R31).
The balanced input level control can accommodate a -25 dBm to 0 dBm input. (install JU17 when