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16
Theory of Operation
fm audio Processor modulation outputs
The Modulation Output port is used by all
voice input signals. The voice inputs are
passed to U1A and U1B, a dual programmable
compressor-expander that is configured as
an automatic level control amplifier. Op-Amp
U3A provides the limiting action for the audio
processor.
After the audio signals have been combined,
limited and buffered, they are filtered by a 10th
order Linear Phase Low Pass Filter (U4). The
output signal from the filter is then level adjusted
by the deviation control pot, R29, at the input
of buffer amplifier U3D. In special applications,
jumper JU6 can be disabled and JU7 enabled
allowing the transmitter to be modulated directly
from the auxiliary input. External filtering may be
required since jumper JU7 bypasses the limiting
and filter circuits.
The Low Frequency / Direct Modulation Output
port has two functions depending on whether
the transmitter is synthesized. 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.
fm audio Processor microphone Input
The microphone input has an automatically level
controlled (ALC) pre-amplifier 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 6 dB/octave pre-emphasis
response or a flat-audio response, jumper JU1
at Y and X position respectively.
MT-3 FM Transmitter Mainboard Instruction Manual
IM20-MT3TXMN
fm audio Processor Balanced Input
The 600 Ω balanced input uses the ALC
pre-amplifier U1B with input level control pot
(R31). The balanced input level control can
accommodate a -25 dBm to 0 dBm input. (Install
JU17 when using the lower input levels). Like
the microphone input, the balanced input is
limited and filtered and is output at the standard
modulation output port.
If no compression is required (i.e., customer
is providing their own), JU11 can be enabled
providing a path through R48 (Auxiliary Input
Level Control) to amplifier U2C where pre-
emphasis or flat audio can then be selected
from its output.
fm audio Processor auxiliary Input
The auxiliary input is a special input and does
not have an ALC. This input can be configured
for a pre-emphasis response (enable JU9 Y) or
a flat-audio response (enable JU9 X). The level
for this input is set by R48. The auxiliary output
is normally summed with the voice signals by
Op-Amp U3A, limited, then filtered and output
at the standard modulation output port. The
value of R57 (Select) can be tailored for specific
applications.
When jumper JU6 is disabled and jumper JU7
is enabled, the auxiliary input can be used to
directly modulate the transmitter. Care should be
taken when directly modulating the transmitter
with the auxiliary input because the MT-3
transmitters use direct FM modulation and there
is no filtering or limiting action provided by the
auxiliary input. The input level to the auxiliary
input should be -18 dBm and can be driven by
one of three inputs:
•
balanced input – JU11
•
tone / digital input – JU12 X or Y enabled
•
direct modulation input – through JU28
When the 600 Ω balanced input is connected
to the auxiliary input, the balanced input level
control can be used to adjust the level for the
auxiliary input.
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