Comb Board (05P0869): Fs-5075T - Furuno FS-1575 Service Manual

7.2.5 COMB Board (05P0869): FS-5075T

The COMB board combines TX output power of 250W from the PA-1 and PA-2 boards
to output power of 500W.
If either of the two PA boards fails to conduct normal operation, the COMB board will
get an imbalance of input impedance (of 50 ohms). UN_BAL voltage rises as the
impedance gets increasingly imbalanced.
Because of this characteristic, UN_BAL voltage is added to part of the control signal of
the transmission protection circuit.
For the transmission protection circuit diagram, refer to "4. TX output power control
circuit" on page 7-12.
When UN_BAL voltage exceeds the threshold, the DRV and PA boards are protected by
controlling the bias of 1st amplifier of the DRV board to reduce output power to the
threshold.
RF IN 1
RF IN 2
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.
CR1
T3
COMB
Fig. 7.2.13 COMB Board Block Diagram
7-28
7.2 Transmission Circuit
UN_BAL
T1, T2
R1
25 ohm
140W
RF OUT
T4