Furuno FR-2155, Service Manual

Page: 51 / 152

2-37
The circulator HY801 is a passive directional coupler with three ports. It contains a permanent
and a core of ferrite material and bends the electromagnetic wave in a specific direction. In the
Figure 2.20, the microwave energy produced by the magnetron enters the circulator from port 2.
It is bent in the specific direction and emerges from port 3 with little loss, port 1 being isolated.
In the same manner, the received signal entering into port 3 is transferred to port 1, isolating port
2. This operation of the circulator protects the receiver during transmission and minimizes loss of
the received signal during reception.
The TR limiter CR810 is a self-activating switch made of two PIN diodes. Its function is to at-
tenuate the strong transmission signals from the magnetron and those received from other radars
through the antenna and to protect the MIC (Microwave IC). The PIN diode has certain charac-
teristics as shown in Figure 2.20 and conducts at a certain level of microwave power. When the
diode is in the cut-off state, the input impedance of the TR limiter matches the characteristic im-
pedance of the waveguide, and the microwave energy is delivered to the MIC. When the diode is
input into a conductive state, the waveguide is short-circuited and most of the input energy is
reflected back to the transmitter. A strong signal is thus weakened to about 50 mW by TR limiter.
The MIC incorporates a local oscillator and mixer diodes. The received microwave signal com-
ing from the diode limiter is mixed with the local oscillation signal in the mixer diodes and con-
verted to an IF signal of 60 MHz.
Attenuation
Output RX Time TX Time
50 mW
50mW
Input Voltage Input
Figure 2.21 Signal attenuation