Sailor Inmarsat B, Руководство по ремонту

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3 MODULE DESCRIPTION Inmarsat B
PAGE 3-12 9901
LOOP 1
PD :32 VCO
:N :64/65
:A
PD :63 VCO
:N
:A
:20/21
:A
:N :20/21
:64 PD VCO
VHF
L Band
Lock
det.
LOOP 2
10.08 MHz
from Triplexer
To TSP
from PLL
Lock Signals
35534
Fig. 3.11.
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.