Sailor Inmarsat B, Руководство по ремонту

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3 MODULE DESCRIPTION Inmarsat B
PAGE 3-11 9901
L-band synthesizer
The L-band oscillator is based on a voltage controlled oscillator (VCO) with good close in phase noise
properties. Frequency stability is obtained using a PLL circuit, the VCO locked to the highly stable
reference frequency supplied from the triplexer board.
The integrated PLL circuit contains phase detector, divider for reference frequency, divider with modulus
two architecture and built in L-band prescaler with a division ratio of 64/65.
The 10.08 MHz reference frequency is divided by 32 in the reference divider to obtain a frequency
resolution of 315 kHz.
The loop filter is an active low-pass filter built up around a low noise operational amplifier. The output
signal from the VCO is amplified in two integrated L-band amplifiers (MMIC). The purpose of the amplifiers
is not only to obtain the required signal level but also to isolate the frequency determining components
in the VCO from the rest of the circuits.
VHF synthesizer
The VHF synthesizer is also based on PLL technology but because of the high frequency resolution of
2.5 kHz and demands for low phase noise, a dual loop synthesizer is used. The two loops are built up as
conventional synthesizers with the exception that loop 1 contains a mixer.
The reference divider in loop 1 is 63 giving a frequency resolution of 160 kHz and for loop 2 it is 64, giving
a frequency resolution of 157.5 kHz.
When the divider in the feed back loop of loop 1 is incremented by 1, the output frequency of the VHF
synthesizer is increased by 160 kHz. If at the same time, the divider in the feed back loop of loop 2 is
reduced by one, the output frequency of the VHF synthesizer is decreased by 157.5 kHz. The resulting
change of frequency at the output of the VHF synthesizer is therefore only 2.5 kHz.
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 mixer output, 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 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.
DOWN converter synthesizer
The two local oscillator signals used in the DOWN converter are generated on this synthesizer module.
The first local oscillator which is used to mix the receiver frequency to first IF is called L-band synthesizer
and the other is called VHF synthesizer.
The frequency coverage of the two synthesizers is listed below:
L-band synthesizer: 1345.680 - 1379.385 MHz in steps of 315 kHz
VHF synthesizer: 200.7200 - 201.0325 MHz in steps of 2.5 kHz
A block diagram of the synthesizer module is shown in fig. 3.11.