Rohde & Schwarz 1138.4000, Service Manual

Page: 86 / 213

Instrument Design and Function Description Radio Unit R&S CRTU-RU
1140.0050.82 3.12 E-4
DIGITAL BOARD
The DIGITAL BOARD constitutes the central control and measurement board which contains all circuit
parts for conversion and further processing of the analog IF (receiver side) into digital I/Q values.
On the transmitter side, analog as well as digital I/Q values are modulated upon the carrier and provided
as analog IF.
Design
The DIGITAL BOARD is designed as HVC2000 plug-in module. The
HVC2000 cabinet is equipped with a screwed-on cover both at the front
and rear to ensure optimum electrical shielding of the module.
Cooling system The casing is provided with ventilation holes on the right and left side.
The instrument fan can generate a strong cooling flow through these
holes and the module.
This efficient cooling is necessary, since a great number of highly
integrated fast digital devices is used on the module and eight
sandwich modules are fitted so that the board is densely packed.
Control and supply The control, bus lines and voltage supply of the DIGITAL BOARD are
effected via four 96-pin and one 48-pin FUTURE-bus connectors from
MOTHERBOARD1.
Function
For each RF channel a set of sandwich boards ADC MODULE1/2,
DDC MODULE1/2, TXDSP MODULE1/2 and AUC MODULE1/2 is
fitted.
On the DIGITAL BOARD, all necessary clocks are generated and
provided from the netclocks 1 and 2 in a clock conditioning and
distribution circuit. These clocks are required for sampling, filtering and
down converting the digital IF internally and for the LINKHANDLER1/2.
Besides, two programmable logic devices (FPGA) are used on the
DIGITAL BOARD, which provide address decoding, interrupt control
and host interfaces between ISA bus and internal circuit parts
(DDC/TXDSP) as well as the control of the RF FRONTEND (via FE
I
2
C-Bus), the REFERENCE BOARD and AUDIO BOARD (via buffered
ISA bus) and the RXTX BOARD1/2 (via serial bus). Further serial
interfaces lead from the programmable logic devices to both
linkhandlers LINKHANDLER1/2 and vice versa.
An AD converter on the DIGITAL BOARD is used for polling the
diagnostic voltages of the CRTU Radio Unit modules, which can
indicate module states and errors.
ADC MODULE1/2 The sandwich modules ADC MODULE1/2 are connected to the
DIGITAL BOARD via a multipoint connector. In addition, they are
screwed to a solid ground block to obtain a good ground connection.
The modules ADC MODULE1/2 receive the analog IF from the RXTX
BOARD1/2 directly via a MMCX connector (IF3RX1 and IF3RX2
respectively) and convert it into a 12-bit data stream by means of a fast
AD converter.
DDC MODULE1/2 The sandwich modules DDC MODULE1/2 are directly plugged onto the
DIGITAL BOARD via three multipoint connectors and process the
digital data stream of the receiver. In a special ASIC chip, the I/Q
shaping, the matching of the data rate and the respective filtering
(bandwidth shaping) of the digital data stream are produced. Then
follows a DSP (RX DSP) with further evaluation of the digital I/Q data
for measurement purposes. Besides, the digital I/Q data are passed on
via the MOTHERBOARD1 to both linkhandlers LINKHANDLER1/2 for
evaluation.