A07A2200S
On the board it’s possible to activate an alarm signal which takes place when the modulation is
lower than 10KHz for a period of time longer than two minutes (the level 10KHz has been selected
because is a little higher than the value due to the stereo subcarrier). When the modulation absence
alarm is on, the output power is removed and the transmitter remains in stand-by until the
modulation will be restored.
5.5 DLCD board
All the input and output data concerning the transmitter are controlled by the DLCD board, to
which also the keyboard and the LCD display control and visualisation board are connected.
A Motorola microprocessor 68HC11 controls the whole transmitter through the J1 and J2
connectors: the keyboard is multiplexed by U6 and U9.
All the values to be measured are fed to the E port with the proper protections against overvoltages
or polarity inversions (DZ1 - … - DZ8 diodes).
A self supplied clock (U21) is connected to the D port through three lines.
The microcontroller serial port is switched by U2 and U25 on the RS232 connector placed on the
front panel, on the rear one and on the RS485 port on the rear as well; the driver for RS232 is made
of U18 (MAX232), while the one for RS485 is U19 (SN75176).
The ports B, C and F of the µP are connected to an external 128KB flash memory, where the DT
management program is present, which at any time can be loaded through the front COM1 by any
PC.
The G port is for all the outputs (pre-emphasis enabling, mono-stereo, alarms, measurement
selection, etc.). The two external outputs for the alarms are uncoupled by two reed relays with
closed or opened contacts selected by two jumpers placed on the MBA board (Z1 and Z2).
A part of the H port is used, as output in PWM, to control the forward and reflected power and the
fine correction of the transmission frequency. The PWM mean value is detected with accuracy, to
avoid errors due to supply voltage variations or saturation and interdiction of the H port outputs.
The operational amplifiers U11, U12, U13 and U14 are part of the powers control circuit The
control loop has a cut frequency of several hundreds Hertz, so in few milliseconds the power can be
controlled and eventually reduced or eliminated in extreme events. The microprocessor therefore
provides to the loop the forward and reflected power reference values, the quantities to be checked
are PWR and PWD, coming from the directional coupler, while the over stated operational
amplifiers represent the error amplifier.
The LCD display, driven by the A port, is a 40x4 alphanumerical characters type and allows an
useful displaying of data and transmitter settings.
380 221 R02 Pag. 31
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