MT-MR PLATINUM. POINT TO POINT AURAL RADIO RELAY SYSTEM
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3.3.- Audio Processor Unit.
This unit receives the main program audio signal, together with the three possible upper-baseband SCA channels.
Program audio can be introduced into this unit following two different ways:
a) Monaural mode.
b) Stereo Multiplex mode.
Normally, Stereo MPX mode is employed by almost all FM Broadcast stations, but MT Platinum transmitter offers
the option of link broadcast station with monaural audio (p.e., for AM stations links) using the standard pre-
emphasis characteristics. This monaural signal enters Audio Processor Unit via a balanced XLR female connector,
with two live inputs incoming to the two inputs (inverter & non-inverter) of an Opamp to get a common, unbalanced
output. This first amplifier stage also works as an active pre-emphasis circuit using its own feedback loop. Pre-
emphasis time constant can be changed by a jumper from 50 to 75 usec., or vice versa, to comply with U.S. or
Europe transmission standards. Pre-emphasis curve can be exactly adjusted by a potentiometer. Once pre-
emphasized, monaural audio signal passes through a 19 KHz Low-Pass Filter, and is furtherly amplified by the
other section of the same OpAmp.
At this stage’s output, Monaural & Stereo audio signals are joined together. Obviously the presence of one of the
two audio signals excludes the other. That is, transmitter uses Stereo, or Monaural audio, once at a time. At this
circuit point are also added the three sub-carrier signals from the three SCA optional channels, being these SCA
signals previously filtered by series capacitors, that is by simple RC high-pass filter elements. Now, composite
baseband is amplified again by the two sections of an OpAmp U5. Between both amplifier sections there is a
variable capacitor to optimize high-frequency end of baseband response when stereo multiplexed signal is used.
Finally, this unit includes an optional clipper circuit who can be internally connnected or disconnected by a jumper.
This clipper circuit avoids any carrier overdeviation produced by high audio peaks incoming from this unit. Action of
this circuit considerably distorts audio signal peaks, so audio level must be always kept under the clipping
thresholds. This circuit is used as a security element only.
At clipper’s output, composite baseband signal has increased its level to exceed the required to frequency-modulate
Equipment’s VCO between the allowable limits of ± 75 KHz. Overall level of composite baseband can be adjusted
by a potentiometer, whereas each baseband component has its own level adjustment potentiometer (monaural or
stereo program audio, and the three SCA channels.
3.4.- VCO (Voltage-Controlled Oscillator) & PLL (Phase-Locked Loop) Units.
These units generates the RF carrier signal, directly at the assigned transmission frequency. It comprises a
Voltage-Controlled Oscillator who is frequency-modulated by the audio baseband signal incoming from the Audio
Processor Unit. This unit is located within RF Amplifier's box. Oscillator’s frequency is deviated up and downwards
from its center value, according to the alternating baseband signal level. Center frequency value is kept constant by
the AFC (Automatic Frequency Control) circuit, this comprising a PLL (Phase-Locked Loop), this located at Mother
Board unit. Allowable deviation limits of carrier frequency are ± 75 KHz from its center value. Once carrier is
generated by an NFET, and frequency-modulated by a group of varicap diodes located at FET’s gate circuit in
order to modify oscillator’s working frequency. Some varicaps are used in parallel in order to linearize the circuit’s
modulation characteristic curve. Output of oscillator is properly isolated to avoid amplitude changes with any
eventual variation of the load conditions at circuit’s output. This isolation is reached using a first transistor buffer
