If Receiving Circuit - Furuno Felcom 250 Service Manual

11.1 Communication unit: FB-2000
The maximum transmitting output value for the IF board is +12 ± 3 dBm.
The Antenna unit input level (when EIRP is set to the maximum level) is –5 dBm. The
input levels for FB-1500 and FB-1250 are the same, too.
FL3 (Duplexer) has the transmitting / receiving signals for L-Band separated. The isolation
ratio for FL3 transmitting / receiving signals have the level of 40 dB or more ensured. The
IF Module for the Antenna unit has a Duplexer installed, too.

2. IF Receiving Circuit

The receiving output level for FB-1500 Antenna unit is -79.4 to -49.4 dBm. If you consider
the Antenna coaxial cable loss (-9 to 0 dB), the receiving level that will be input into the IF
board will be -88.4 to -49.4 dBm. The value for FB-1250 will be -6.2 dB lower than these
values.
U18 is a receiving ATT (RX ATT). If the loss for the Antenna coaxial cable is small, the
receiving circuit may get saturated.
To prevent from having such status occur, the receiving signal level is adjusted in U18
based on the TX ATT value captured during cable calibration.
The RX ATT value can be confirmed on the "BDE" screen in Service menu of the Web
MMI. The RX ATT value will not change even during communication.
U20 has the 1.5 GHz receiving signal and 1785.95 ± 17 MHz from RX 1
st
Lo mixed to
st
convert it into the frequency of 1 IF: 243.95 MHz.
st
U25 is an IC where frequency conversion and AGC circuit are built in. 1 IF: 243.95 MHz
nd
and 491.68 MHz(1/2: 245.84MHz) from Rx 2 Lo are mixed to have the frequency
nd
converted into 2 IF: 1.89 MHz, and is then output to the MODEM board.
The "L-Band Out" output level is 50 Ω/-60.1 to -94.3 dBm.
3. 42 V
42 V supplied from the POWER board is the Antenna unit power supply.
The power consumption for the Antenna units: FB-1500 and 1250 are both approx. 40 W.
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