Broadcast Electronics Marti Electronics ME-40, Manual

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©2011 Broadcast Electronics
4.3 METERING CIRCUIT.
Metering of important exciter operating parameters is provided by a TEST METER. Six steady-state
parameters are selected by a rotary switch and displayed on an analog meter.
A Peak-Hold Bar Graph LED display constantly monitors the AC composite signal applied to the modulated
oscillator. Indication of short transient peaks exceeding 100% are held for one second for viewing.
4.4 AFC/PLL CIRCUIT.
The AFC/PLL circuit synthesizes the exciter carrier frequency and maintains the phase and frequency of
the carrier. The ME-40 frequency synthesizer and comparator circuit provides
2000 synthesized frequencies within the commercial FM broadcast band in 10kHz increments.
Carrier sampled at the output of the modulated oscillator is returned to the AFC/PLL circuit as feedback. This
feedback is divided and compared to a scaled-down reference frequency within a programmable frequency
synthesizer and comparator logic circuit to develop a correction signal.
During normal operation, the AFC/PLL circuit constantly modifies the correction signal applied to the
modulated oscillator to maintain the stability of the carrier. If the carrier is off frequency, the AFC/PLL circuit
will mute the RF output and de-energize the AFC relay until the carrier is locked in phase and frequency to
the reference oscillator. A dual-speed loop filter provides rapid stabilization of the carrier and allows
modulation from 1 Hz to 100 kHz. When frequency stabilization is attained, a front-panel status indicator
will illuminate.
As a secondary function, the assembly accepts all audio inputs, corrects the audio, and sums the corrected
audio with AFC tuning bias which linearizes the modulation and adjusts the carrier frequency of the
modulated oscillator.
4.5 MODULATED OSCILLATOR CIRCUIT.
The modulated oscillator circuit generates the final carrier frequency, frequency modulates the carrier, and
amplifies the modulated RF carrier to a level sufficient to drive the RF amplifier. Additional circuitry
interfaced with the AFC/PLL circuit maintains the RF carrier center frequency as part of a phase-locked-loop.
4.6 RF AMPLIFIER ASSEMBLY.
The RF amplifier assembly consists of three stages of amplification designed to increase the 2 milliwatt RF
input signal from the modulated oscillator to an adjustable RF power level of 3 to
40 watts as required to drive an associated transmitter.
The first stage employs a broadband thick-film hybrid amplifier which provides a saturated output of
approximately one watt to the input of the driver stage. The driver provides 8 watts of RF to the power
amplifier which outputs an adjustable RF level of 3 to 40 watts.
A microstrip directional coupler on the RF amplifier printed circuit board supplies information to the exciter
control circuitry to automatically maintain RF power output and provide protection during high VSWR
operating conditions.
The RF amplifier transistors are mounted on a large heat sink positioned in the direct air flow from a
cooling fan. Heat sink temperature is monitored by the control circuitry. If an over-temperature condition
exists, the control circuit will automatically reduce RF power to maintain safe operation of the RF devices.
The broadband characteristics of the amplifier eliminates the necessity for adjustments for any frequency
within the FM band, assures that the exciter output is transparent to the signal generated by the
modulated oscillator, and enhances amplifier stability under varying load conditions.