Furuno FS-1570, Service Manual

Page: 165 / 322

5.8 Antenna coupler, AT-1560
5-48
Self-test
See Chapter 5.
Matching circuit
The following signals control the antenna matching circuit. Either gamma or pi
matching circuit is selected.
·
FREQ signal (U4, Counter)
U4 measures the frequency of the input signal to the matching circuit which tunes
1.6 MHz to 30 MHz signal. The frequency data is also used to store the matching
data.
· Vr and Vf (T1, VSWR detector)
Forward power (Vf) and return power (Vr) are used to calculate VSWR. The
matching circuit of the minimum VSWR is selected.
· V+ and V- (T2, Phase detector)
The phase sensor detects the phase of antenna impedance, capacitive or inductive
reactance of an impedance matching network.
· Ia (T3, Ia detector)
Antenna current, Ia is used for Ia indication and monitoring the matching condition.
Table 5.8.2 Matching Condition
Frequency Tune OK Tune NG
3.36 MHz or less VSWR<3.0 VSWR>3.0
3.36 MHz or more VSWR<2.0
Matching data
is memorized. VSWR>2.0 Matching circuit
short circuits.
The maximum inductance of the matching circuit is about 50 uH. To tune 2 MHz band
signal to about 7m antenna, most coils in the matching circuit are used. C171 and C172
are used when the antenna capacitance is 40 pF or less and when the supplied voltage to
the antenna is 6 kV or more.