Pag. 45
SIRIO 2000T
SIRIO 2000T
SIRIO 2000T
SIRIO 2000T
SIRIO 2000T
Electronic Broadcast Equipment
THE LF AND RF CONTROL AND PROCESS MAIN-BOARD
This is the most complex board in the transmitter and supports the LF input processing, with level
adjustment, audio-pass filtering and limiting. It also includes the RF control section and the I/O interfaces
and interconnecting the various transmitter modules with flat-cables.
Its electrical diagram is splitted in two sheets for clarity purpose and we shall examine in sequence.
In the first diagram are located the I/O interfaces, both the audio and the digital ports, and the analog
RF control. Let's start to briefly consider each block diagram.
In the upper left side of the diagram are located the audio channels amplifier/buffers made with 6 op-
amp sections of IC1 and IC2. Two impedance selector jumpers for the audio channels lead the pack
and a network made by resistors and diodes protects the inputs from occasional static discharges, as
required for
CE
compliance. Four unity-gain active buffers follow, and then two balanced to unbalanced
signal converters, which drive the electronic attenuator visible in the 2nd sheet.
The last op-amp in IC2 (d) amplifies the auxiliary channel input with a -0.1dB upper corner band >>200
kHz and drives the third channel of the electronic attenuator.
On the lower left of the diagram are the RS232 interface (IC5) and the parallel remote I/O active interface
with its protection network, built around TR1, TR2 and TR3. A wired or pull-up makes the logic levels <1V
as 0 and 10-12V for 1.
On the lower right section of the diagram it is located the RF power controller.
The RF direct and reflected power signals samples coming from the output directional coupler are
amplified by IC3 in two symmetrical circuits.
The direct-power control circuit, built around IC4a, continuously drives the RF output stage gain, varying
the supply voltage to the RF driver transistor and the regulated voltage of the output transistor. The reflected-
power limiting circuit IC4b only acts on the same loop when the IC3b output voltage is greater than the
threshold set by the voltage on the R49/R50 network. A third and fourth section of IC4 filters and buffers
the signal coming from the CPU and set the reference level for the output power loop. TR4 disables the RF
output when the synthesiser is not locked.
In the upper right section of the sheet is shown the control bus connector to the CPU, which carries the
digital control lines on the lower pins and the analog lines on the upper ones. From this connector comes the
power supply too. Only +12.5V and -12.5V are used in the board.
Let's now go to the second sheet of this diagram.
Beginning from the upper left side, we find IC8, which forms a 3-channel digitally controlled attenuator.
It separately manages left, right and auxiliary channels, while the external multiplex signal is processed in the
same channel as the right one. Three buffers/amplifier follow each channel: IC7a, IC8a and IC12a. The
output of the first two amplifiers drive the pre-emphasis stages, whose time constants can be digitally set at
0, 50 and 75µs, through the analog gates of IC9. A limiter stage follows, built around D8 and D9 diodes
acting as clippers. By varying the limiter’s reference voltage driven by the CPU though IC13a, the limiter
threshold level +Vl & -Vl can be adjusted. RT4, if present, imposes a ceiling to the limiter.
