Furuno NX-700B Service Manual page 78

Signals received at 490 kHz
Signals received at 490 kHz are switched at U1, inputted into RX2 circuit and
converted into FSK signals of 1700 Hz + 85 Hz at U1 of this circuit. The local
oscillator frequency (488.3 kHz) at U1 is generated by dividing the frequency Y1
(3906.5 kHz) by 8 at U2.
(518 kHz + 85 Hz) - 488.3 kHz = 1700 Hz + 85 Hz
The FSK signals are converted into 0 and 1 signals at U5 (PLL detection). R75 at
U5 adjusts the signals by setting "Adjust Rx Unit" to "On" in "[MENU] ->
Initialize1" so that the duty of U5 output signal (BY signal) is 50 %. BY signals are
converted into RS-485 signals at U3 and sent to the Display unit.
Signals received at 4209.5 kHz
Signals received at 4209.5 kHz are converted into 490 kHz at CR3 and 4. The local
oscillator frequency of 3719.5 kHz inputted to CR3 and 4 is generated by dividing
the frequency X'tal of 29.865 MHz by 8 at U10.
4209.5 kHz - 3719.5 kHz = 490 kHz
Signals converted into 490 kHz are switched at U1 and inputted to RX2 circuit.
The signal flow after that is the same as described above for signals received at 490
kHz.
Receiving test
In the receiving test, test signals (1700 Hz + 85 Hz, Test start, Test off) sent from the
Display unit via RS-485 are received at U2.
To start the receiving test, turn on the LO SW DC signal by the Test start signal, turn
off the power (+9 VDC) to the antenna, and shut off signals from the antenna and
noises. Then 1700 Hz + 85 Hz signal is modulated to AM at CR5.
The carrier frequency is a local oscillator signal from RX-1, RX-2 and 4209.5 kHz
Local oscillator circuit. NAVTEX received signals are obtained from the component
of AM- modulated signals. The figure below shows the spectrum of the test signal at
518 kHz.
To switch to FREQ
Switching to FREQ is performed by the FREQ signal (490 kHz, 4209.5 kHz) sent
via RS-485 from the Display unit. This signal performs the control of switching
circuit (U1) of receiving circuit at 490 kHz and 4209.5 kHz, and the control of local
oscillator frequency 4209.5 kHz (U10).
1700+85Hz
516.3kHz
Fig. 5.3.2 Spectrum of Test signal at 518 kHz
518kHz+85Hz
1700+85Hz
5-7
5.3 Block Description