MT-MR PLATINUM. POINT TO POINT AURAL RADIO RELAY SYSTEM
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stage, followed by another transistorized amplifier stage. Both stages uses BFR93L transistors. An RF sample is
drawn out from the buffer stage’s output to feed the frequency pre-scalers (dividers) of PLL circuit.
RF sample drawn out of VCO’s output is fed to pre-scaler or frequency divider circuits at U3 (MB1501), who divides
incoming signal’s frequency, feeding in turn the resultant low frequency signal to the phase-comparator circuit of the
same IC. This IC is fed, by the other hand, with the 12.8 MHz signal incoming from the external TCXO Reference
Oscillator, this signal being also divided in frequency (at internal programmable dividers into the same IC) by the
rate ordered by Microcontroller unit, this rate chosen by the Microcontroller in order to reduce the reference
frequency to a value that can be compared with the value of the signal obtained from the RF sample to reduce and
keep the phase difference between both signals to zero when the VCO output falls in the desired frequency value.
Phase comparator circuit ( also within the same IC) produces a DC error voltage proportional to the phase
difference between both signals; the reference signal and the RF sample drawn out from VCO’s output. This DC
error voltage is amplified in a differential amplifier and fed to varicaps D1-D5 at VCO in order to keep its bias
voltage, and consequently its capacitance in a such value that VCO’s output signal frequency will be the previously
ordered by the Microcontroller unit. To adjust the operating frequency to any value within the prescribed frequency
sub-band, only it’s required to follow the instructions given in this Manual, watching the Microcontroller’s Display
screen, and operating the two access push-buttons of the Microcontroller unit in order to set the desired frequency.
3.5.- Microcontroller or Control Unit & Attached Display Unit:
This unit is directly attached to the back side of Equipment’s Front Panel, containing the main supervision & control
elements that checks and controls all transmitter's operational parameters. This unit works closely together with
PLL section, sending directly to this unit the frequency-change orders concerning frequency programming of PLL
internal pre-scaler circuits located at PLL Integrated Circuit, in accordance with frequency that is chosen and
entered to Microcontroller. This unit also receives information from PLL concerning its phase-lock condition, and
allows to release the RF power at equipment’s output only with phase-locked condition is reached
.
Control Unit contains the Microcontroller device, an specialized Microprocessor type T89C51RD2 having a clock
frequency of 11.0592 MHz.This device controls, through ADC0834 Integrated circuit, the possible selection points
of the PLL’s internal programmable divider, that determines the binary frequency-division rate, that is, the
operating frequencies of the VCO. It also has an series interface MAX232 coupling to the RS232 output interface
to remotely-control equipment. Microcontroller’s display device is the Liquid Crystal Display CMC216E05, who is
coupled to Microcontroller’s output ports through its “D” outputs (D0, D1, D2, .....D7).
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