Furuno FS-1570 Service Manual page 145

Filter circuit
The received signal is input to PRESEL board (05P0746) to prevent signals at unwanted
frequencies from getting through and optimize the slectivity of a receiver. PRESEL
board includes four bandpass filters and a low-pass filter, insertion loss of 2 to 3 dB. 0.1
to 1.45 MHz signal passes through the low-pass filter. BPF2 allows signals at frequency
range from 1.45 to 4.99 MHz to pass. BPF2 is made up of LC tuned circuits. Capacitors
in BPF2 are selected by relays, receiving PR1 to PR6 data.
Fig.5.4.5 Block diagram of PRESEL Board (05P0746)
BPF2 tunes the signal at frequency bands tabulated in Table 5.4.4. CR7 and CR8 is a
receiver protector which protects receiver front-end circuits from damage when an input
signal of 15 Vp-p or more is present. C23 and L7 is a 72.455 kHz trap circuit which
allows IF signal not to leak from the antenna.
1.45 to 1.49999 1.8 to 1.84999
1.5 to 1.54999 1.85 to 1.89999
1.55 to 1.59999 1.9 to 1.94999
1.6 to 1.64999 1.95 to 1.99999
1.65 to 1.69999 2.0 to 2.04999
1.7 to 1.74999 2.05 to 2.09999
1.75 to 1.79999 2.1 to 2.14999
Table 5.4.4 Tune circuits in BPF2
Tuning frequencies(MHz)
2.15 to 2.19999 2.5 to 2.54999 3.1 to 3.19999
2.2 to 2.24999 2.55 to 2.5999 3.2 to 3.29999
2.25 to 2.29999 2.6 to 2.69999 3.3 to 3.39999
2.3 to 2.34999 2.7 to 2.79999 3.4 to 3.59999
2.35 to 2.39999 2.8 to 2.89999 3.6 to 3.79999
2.4 to 2.44999 2.9 to 2.99999 3.8 to 3.99999
2.45 to 2.49999 3.0 to 3.09999 4.0 to 4.29999
5-28
5.4 TX/RX Board (05P0733)
4.3 to 4.59999
4.6 to 4.99999