Parts List

2-10 Transmitter Main Board Instruction Manual
2.3.4.2 Audio Processor Modulation Output
The Modulation Output port is the output port used by all voice input signals. The voice inputs are
combined by summing amplifier U4a which has an associated symmetry control (R14). Op-amp
U4a together with U5a provide the limiting action for the audio processor. After the audio signals
have been combined and limited, they are filtered by a 6 pole lowpass Butterworth filter (U5b, U5c,
and U5d). Jumpers JU24, JU25 and JU26 configure the lowpass filter for narrowband or
wideband operation: jumpers JU24(x and y) to JU26(x and y) are installed for wideband (25 kHz)
channels and are not installed for narrowband (12.5 kHz) channels. The output signal from the
filter is then level adjusted by the deviation control pot, R29, before modulating the transmitter.
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.
2.3.4.3 Audio Processor Low 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 60 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. Amplification, if desired, is provided
by Op-amp U8a which is powered independent of the audio card voice circuits by jumper JU1.
2.3.4.4 Audio Processor Inputs
As previously mentioned, the MT-3 Audio Processor has six audio inputs, five used primarily for
voice signals and one for data signals.
The 6 audio inputs on the audio processor are:
• dynamic microphone input
• 600 ohm balanced input
• 2 subtone inputs
• auxiliary input.
• direct modulation input.