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3.4 Power
3.4 Power
SIM914 350 MHz Preamplifier
comparators in U102 will be driven low when the voltage on C106 or
C107 exceeds
1.7 V. The driven comparator discharges C108 from
+5 V to
5 V. C108 will be slowly recharged to +5 V by R128, a
1 MΩ resistor, thereby stretching the overload signal to about 10 ms.
One of the comparators in U300 drives the front-panel overload LED
until the voltage on C108 recharges above ground. This overload de-
tection will detect overloads as short as 3 ns.
The overload detectors are wire-ord by D303 which will pull the sta-
tus pin (pin 2 on the rear-panel connector to the SIM mainframe) to
0 V via the 3.9 V Zener diode when an overload occurs. The status
pin may be polled via the SIM900 Mainframe to detect overloads in
the unit.
The 5 VDC power supplies are filtered at the rear panel (by L1, L3,
C1 & C2), again on the main PCB (by L2, L4, C300 & C301), and fi-
nally at each channel of the amplifier (L100, L101, C110 & C111 for
the top channel and L200, L201, C210 & C211 for the bottom chan-
nel.) Careful power supply filtering is important to reduce channel
crosstalk. The crosstalk from the output of Channel 1 to the input
of Channel 2 is less than 60dB (1:1000 of the amplitude) and peaks
around 300 MHz. The crosstalk from the output of Channel 2 to the
input of Channel 1 is less than 80dB (1:10,000 of the amplitude) and
occurs in a broad band between 180 MHz and 360 MHz.
3 – 3
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