Protection Circuit; Imd Improvement; Bias Adjustment - Furuno FS-1575 Service Manual

1. Protection circuit

1) The transmission protection circuit reduces TX output power by detecting the
V-SWR VR/VF signals outputted from the Q4 and Q5 and the DIV UNBAL signal
(05P0866A: FS-5075) outputted from the DRV board and controlling the bias of
the 1st amplifier "Q1." The PROTECT signal is added to this control line.
2) The DIV UNBAL signal (05P0866A: FS-5075) detects the occurrence of a failure
in the PA1 and PA2 or a problem with the connector of RF OUT 1 and 2. In other
words, this signal detects an error in load of the DRV board.
3) The maximum power of the RF signal from the TX board is +12dBm. To prevent
the excessive power input in the RF signal, limiters "CR2" and "CR3" are mounted
to the input circuit of the DRV board. The limiters will activated when the DRV
board input power reaches +8 to +10dBm.
4) When PA board temperature remains at the threshold for a period of one minute,
output power will automatically reduce and output an error message saying
"ERROR: Tx power reduced. Main AMP heated."
Lower than 77°C
77°C to 84°C
84°C or above
Note) Output power will reduce when any of temperatures detected by the PA-1, PA-2,
and DRV boards remains at the threshold for a period of one minute or more.

2. IMD improvement

As DRV board output power increases, IMD (intermodulation distortion) gets worse. To
avoid this problem, the CR15 limiter detects output power from the Q4 and Q5 to
reduce the bias of these amplifiers.

3. Bias adjustment

For the adjustment procedure, refer to information on page 5-41. To make check or
adjustment of bias current, be sure to disconnect the [TX OUT] connector from the TX
board.
ADJ.
TR/FET
Point
Q4 and Q5 R79
Q3 and Q4 R78
Q1 R77
CONFIDENTIAL (internal use only)
This confidential document is used only by FURUNO authorized persons.
It is strictly prohibited to reproduce the document in whole or in part without prior written permission of FURUNO.
FS-5075
Hi MID LOW1
MID MID LOW1
LOW1 LOW1 LOW1
LOW2 LOW2 LOW2
Table 7.2.5 Bias Currents of DRV Board
Bias
Measurement Point
Current
TP4 - TP5: 4 mV
400 mA Note)
500 mA for current measurement of the 24V line
450 mA TP2 - TP3: 100 mV
150 mA TP1 - TP2: 75 mV
7-21
7.2 Transmission Circuit
FS-2575
LOW2 Hi MID
LOW2 MID MID
LOW2 LOW LOW
LOW2 LOW LOW
LOW
LOW
LOW
LOW