Broadcast Warehouse TX 25/50, Техническое руководство

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TX25/50 FM Transmitter technical manual
page 29
Technical data
CON1 provides a 10 way interface to the power amplifier board and its sensors. Some of the connections of
CON1 are routed around to the power supply modules. CON2 provides a 16 way interface to the combo board
providing power to the combo board as well as routing signals to the back panel D-type socket for alarms/RS232
and returning modulation and limiter gain reduction signals to the main microcontroller.
The voltage regulator REG1 and decoupling capacitors C1 and C2 takes the 18 volt auxillary supply from CON1
and regulate it down to 5 volts for the microcontroller and LCD circuitry. The microcontroller is a 40 pin 8 bit type
running at 8 MHz. The microcontroller has several ports that have various functions and connect to external com-
ponents.
PORTA (6 bits) is primarily used for the analogue voltage inputs. Bit 0 is the limiter gain reduction, Bit 1 the modu-
lation, Bit 2 the AUX volts and Bit 3 the Fwd RF power. Bit 4 is not used and bit 5 of PORTA has a control con-
nection to the LCD display.
PORTB (8 bits) has a few different functions. Bits 7,6 and 3 of the port are the alarm signal outputs and are rout-
ed off via CON2 to the D-type on the back of the combo board. Bit's 5,4 and 2 are connected to the front panel
buttons to allow navigation of the LCD functions. Bits 1 and 0 provide a 2 wire interface to the PLL microcontroller
on the combo board.
PORTC (8 bits) bits 0 and 1 are used to write information to the LCD display. Bit 2 is the pulse width modulator
output (PWM) and connects into unity gain DC amplifier op-amp IC2A. The voltage generated by the PWM is set
by the software in the microcontroller. This PWM level is then fed through potentiometer VR2 and smoothed by
C13 before being buffered by the aforementioned op-amp. PORTC bits 3-5 are not used. Bit’s 6 and 7 of PORTC
are used by the UART inside the microcontroller. These pins are RXD and TXD for the RS232 interface. They are
routed via Con2 to the back panel D-type.
PORTD's 8 bit's are interfaced to the 8 bit data bus of the LCD display. The data byte on PORTD can be latched
into the LCD display by the LCD control bits on PORTC.
PORTE (3 bits) has the remaining A/D inputs. Bit 0 is the PA volts, Bit 1 is the Rev RF power and bit 2 is for the
PA temperature.
The Microcontrollers software reads all the analogue voltages, converts and displays them where neccessary and
outputs alarm signals in the event of a transmitter error.
There are various passive components associated with IC1. Each A/D port has input current limiting resistors R1-
11 and decoupling capacitors C5-11. X1, C1 and C2 provide the 8 MHz signal for the clock of the microcontroller.
R12 and R13 provide pull downs for correct operation of the inter processor communications channel. C12 pro-
vides supply decoupling for the microcontroller and VR1 sets the contrast of the LCD display.
IC2 is a dual op-amp whose purpose is to control the output power of the transmitter. Side a of the op-amp is con-
figured as a unity gain buffer for the PWM and was described previously. VR2 provides an adjustable DC level to
side B of the op-amp. Side B is configured as a DC amplifier with the gain set by R17, R18, R19, R20 and R25.
The larger the DC signal provided by side A and the larger the DC signal at the output of Side B. This DC signal
is used to turn down the output of the Power amplifiers power supply and with it the RF power output level.
LED3 and R20 provide a connection from the reverse rf power sensor into the input the non inverting input of side
B of the op-amp. LED3 only allows DC through above a certain level. Any DC above this level will increase the
output voltage from the op-amp and the RF power level will decrease. This forms the VSWR protection for the
transmitter. LED1 and LED2 also provide the same function but with the DC level this time being supplied by the
temperature sensor on the PA board. Too high a temperature will cause the DC level to exceed the turn on point
of the two diodes and voltage will once again turn up the op-amps voltage and turn down the RF power level
which should lower the temperature of the transmitter. The output from side B is fed to the switched mode power
supply of the power amplifier via R21 and CON1. R22 and C15 provide supply decoupling for the op-amp.
C14,15,16,17,18 provide further decoupling and feedback for the power control feedback loop formed around side
B of the op-amp.