Functions Of Each Circuit Board - Furuno 1622 Service Manual

2.4!Functions of Each Circuit board

2.4 Functions of Each Circuit board
2.4.1 Display Unit
The display unit consists of DU board (03P9268), PANEL board (PNL9194), and LCD indicator
(03S9461).
PANEL Board
Mounted on this board are the operating keys, panel-illuminating LEDs, and electrical buzzer,
constituting the man-machine interface.
DU Board (03P9268)
The reference oscillator U125 is a quartz oscillator which generates an accurate and stable
reference frequency signal. This reference frequency signal is the source of the transmission
trigger, the clock for video signal processing in the display unit, the time-base signal, the
reference clock for LCD display, the reference clock for CPU, and so on.
According to the distance range selected in the panel operation, CPU computes the ratio of
dividing the reference frequency and sends the data to the gate array U111 (radar signal
processing circuit). Based on the data, U111 creates the transmission trigger from the reference
frequency signal.
This radar intentionally varies the pulse repetition frequency of the transmission trigger to
prevent the interference signals from other radars.
The IF video signal from scanner unit inputs to the video circuit on the DU board.
The video circuit includes FTC circuit that rejects the reflecting signals from the rain and snow.
CPU switches on or off the FTC function according to the panel operation.
The IF video signal is converted by a A/D converter to the three-level digital signal, which inputs
to the gate array U111 (radar signal processing circuit). In U111, the input digital IF video signal
is sampled with the sampling clock corresponding to the range setting. Further in U111, when
the interference rejector is turned on in the menu, each receiving video signal is correlated with
the corresponding transmission to reject the interference signals from other radars. The video
signal output from U111 is written in the VRAM U116 for the display.
The antenna bearing signal and heading signal are input to CPU from scanner unit. The heading
correction data, obtained at heading alignment (instllation menu) is stored in EEPROM U105,
and CPU computes the correct direction of the antenna, using the correction data. The data to
correct the difference between the transmitting and receiving timing is obtained at TX time delay
adjustment (installation menu) and is also stored in EEPROM U105. The correction is carried
out in U118.
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