Rf Circuit - Furuno FAR-2107 Series Service Manual

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.
Mag. Current
Heater
RF PWRp.c.b
Tx-HV
HV-9017p.c.b
MDp.c.b
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.
L3
Discharge
R1,2
Charge
K1
L1
Trigger
Q1-4
MOS FET
Switching
Heater
Pulase trans
Tx-HV
1 5
2 6
Short pulase
K1
7-11
7.2 Antenna Unit
Limitter
MIC
Pulase trans
Tx-HV
Magnetron
Trigger
3
Circulator
4 C801
CR802
Magnetron
CR803
R801
Antenna