Protection Circuit - Furuno FS-1575 Service Manual

Signal
Name
Power supply voltage for Q4 and Q5 push-pull amplifiers of the PA board.
24V The 24V power supply (0V) line and the housing are connected by a
capacitor to make RF path.
15V Power supply voltage for the Q1, Q2, and Q3 of the PA board.
5V Power supply voltage for the PA control circuit.
RF signal from the TX board. The maximum input power is
RF IN
+12dBm/50 ohms. Normally, power of approximately +5 to 8dBm is inputted.
PA OUT Outputs power of 150W.
Temperature detection signal of the PA board. When a specified temperature
TEMP PA
is detected, output power will reduce.
V-SWR Vr detection signal outputted from the PA board, which controls the
bias of the 1st amplifier of the PA board, when this signal exceeds the
VR PA
threshold, therefore, educes TX output power.
Detected value is displayed on the SELF CHECK window.
V-SWR Vf detection signal outputted from the PA board, which controls the
bias of the 1st amplifier of the PA board, when this signal exceeds the
CHK PA
threshold, therefore, reduces TX output power.
Detected value is displayed on the SELF CHECK window.
PA IC detection signal. Detected value is displayed on the Check meter or
the SELF CHECK window.
IC
This signal controls the bias current of the 1st amplifier, when PA IC exceeds
the threshold (19A or more), therefore, reduces TX output power.
PA Tr collector voltage detection signal. Detected value is displayed on the
VC
Check meter and the SELF CHECK window.
PA board hardware information. This information is displayed by selecting
PA HW VER
[MENU] -> 9.SERVICE -> 5.TEST -> 1.VERSION.
PA ON Activates the bias circuit in the PA circuit.

1. Protection circuit

1) The transmission protection circuit reduces TX output power by detecting the SWR
Vr/Vf signals outputted from the Q4 and Q5 and controlling the bias of the 1st
amplifier "Q1."
2) To prevent the excessive power input in the RF signal from the TX board, limiters
"CR2" and "CR3" are mounted to the input circuit of the PA board. The limiters
will activated when the PA board input power reaches +8 to +10dBm.
3) When PA board temperature exceeds the threshold for a period of one minute or
more, output power will automatically reduce and output an error message saying
"ERROR: Tx power reduced. Main AMP heated."
70°C to 77°C
77°C or above
84°C or above
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.
Table 7.2.7 Major Signals of PA Board
Function
Hi MID
MID MID
LOW LOW
LOW LOW
7-26
7.2 Transmission Circuit
LOW
LOW
LOW
LOW