Furuno FAR-2107 Series - RF Circuit

7.2 Antenna Unit
7-11
4. RF circuit
Magnetron
While the Magnetron is not operating, TX high voltage (TX-HV) runs through R1/2,
and is changed on capacitor: C1-4. When trigger pulse is applied to MOS-FETs Q1 to
Q4, the FET is turned on and discharges C1 to C4. Voltage is increased on the primary
winding of the pulse transformer and high voltage of 6 kV to 8 kV is generated on the
secondary winding to make the Magnetron oscillate.
Fig. 7.2.7 Block diagram of high frequency circuit
Major parts of TX circuit
- CR1, 2 (MD board)
CR1 and 2 serve to protect MOS-FET. When the TX trigger is switched from ON
to OFF, counter-electromotive force (overshoot) generated on the primary winding
of the pulse transformer is released to the TX-HV power supply.
- R41, 42 (MD board)
R41 and 42 serve to decrease the main bang. When the TX trigger is switched from
ON to OFF, there remains residual voltage on the primary winding of the pulse
transformer. R41 and 42 serve to consume this residual voltage.
- L1 (MD board)
When the pulse length is S1 (short pulse), the relay is turned on to connect L1 to
the TX-HV line. The coil impedance becomes low in a certain time after the pulse
trailing, shorting the primary winding of the pulse transformer, forcibly dissipates
residual voltage of the Magnetron to make the pulse trailing edge steep.
MIC
Magnetron
Magnetron
Circulator
Antenna
Pulase trans
Pulase trans
Trigger
Trigger
Charge
Discharge
Tx-HV
Tx-HV
Tx-HV
CR802
CR803
R801
R41,42
CR1,2
C801
Heater
Heater
C1-4
L1
K1
Q1-4
MOS FET
Switching
HV-9017p.c.b
RF PWRp.c.b
K1
Short pulase
MDp.c.b
1
2
3
4
5
6
R34,35
Limitter
R1,2
R3,4,40
L3
Mag. Current