3
MODULE DESCRIPTION
Inmarsat B
The integrated PLL circuits used in loop 1 and 2 do not include the prescaler. Therefore, external
prescalers with a division ratio of 21/22 are used. The loop filters are of same type as the one used in the
L-band synthesizer.
The VCO’s are a Colpitts-Clapp type, built up with BJT’s . Before the amplified VCO signals enter the loop
mixer, they are low-pass filtered to attenuate the harmonic contents. Loop mixing takes place in a passive
double balanced diode mixer, and at the output the mixer, a low-pass filter removes the sum frequency.
For initialisation and selection of frequencies, the PLL circuits need three signals each: clock, data and
strobe. The strobe signal is used for chip selection and therefore three separate wires are used. The clock
and data signals are the same for the three PLL’s, the strobe signals determining which one is loaded with
data. The strobe, data and clock signals are generated from the TSP board.
Each of the three PLL’s has a built-in lock detector indicating when a VCO is unlocked. These signals are
combined in a lock indicator circuit indicating if one or more VCO’s are unlocked. The lock condition is
signalled to the TSP board, which sends this information to the transceiver unit. The lock indicator is also
connected to a led, placed on the synthesizer board, which is lit when a VCO is unlocked.
The supply voltages ( +18V, +8V, -15.5V) from the SMPS are converted to +15V, +5V and -1.25V are
generated by standard voltage regulators, but the more critical supply voltages to VCO’s and phase
detectors are made by discrete components to lower the noise level.
3.1.11 SWITCH MODE POWER SUPPLY
The 40V DC supplied from the transceiver unit is converted to different voltage levels needed to supply
the modules and motors. This is done in the switch mode power supply. Actually the power supply consists
of two power units based on switch mode technology. The two switch circuits in power units 1 and 2 are
synchronised to minimise spurious and avoid beat tones, which can be a problem when a module is
supplied from both power units. A block diagram is shown in fig. 3.12.
Power Unit 1
Circuit
Synch.
Power Unit 2
Converter
D/A
Timer
Clock
Data
Strobe
DMB from HPA
DMB from TSP
+28VR
+18V
+24V
+28V
-16V
+8V
+12V
Connection Board
40V DC from
Register
Shift
35525
Fig. 3.12.
The first power unit generates all fixed voltages and the second power unit generates a variable voltage
between 4 and 28V DC to control the transmitter power of the HPA.
The level of the variable voltage is controlled from a microcontroller placed in the HPA. The microcontroller
is connected to a digital to analogue converter through a serial connection. The serial data stream is
organised in 8 bit groups clocked into a shift register and thus converted to 8 bits in parallel. The seven
most significant bits are used as input for the D/A converter and select voltage level. The least significant
bit is used to switch off the power of the HPA. This feature is used when the HPA is transmitting in burst
mode.
PAGE 3-13
9901
Summary of Contents for Inmarsat B
Page 1: ...SAILOR Inmarsat B Workshop Manual W4400GB0 ...
Page 2: ...Inmarsat B Workshop Manual 9905 ...
Page 7: ...CONTENTS 1 INTRODUCTION 1 1 1 1 SYSTEM COMPONENTS 1 2 1 2 TECHNICAL DATA 1 2 9849 Inmarsat B ...
Page 46: ...4 ACCESSORIES Inmarsat B PAGE 4 3 Inmarsat B 9849 Diagram Veritas connection box ...
Page 124: ...CONTENTS 8 PERFORMANCE CHECK AFTER REPAIR 8 1 8 1 START UP SEQUENCE 8 1 9936 Inmarsat B ...
Page 127: ...CONTENTS 9 SERVICE 9 1 9 1 CHECK OF OCXO 9 1 9936 Inmarsat B ...
Page 129: ...CONTENTS 10 PARTS LISTS 10 1 9901 Inmarsat B ...
Page 133: ...CONTENTS 11 ABBREVIATIONS 11 1 Inmarsat B 9849 ...