Furuno FR-2155, Service Manual

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2-35
2.32 Block Diagram of PF Board (03P6668/03P6669)
The PFN (1)/(2) Boards produce a narrow high tension pulse that drives the magnetron. It is
composed of a line-type-pulser.
Choke coil L801 forms a series resonant circuit with the pulse forming network (PFN) consisting
of C814 thru C824 and L811. The TX high voltage in the input is doubled by the electromotive
force of this coil and the PFN is charged up to twice the input voltage. The time taken for full
charge is roughly given by the equation T = 2
π√
L
×
C , pwhere L represents L801 and C, capaci-
tors of the PFN.
The PFN is a concentrated constant L-C circuit, which is an application of parallel two-line cir-
cuits with an open end. The modulation pulse that drives the magnetron is produced when the
energy in the PFN discharges through CR813 to CR815. The duration of the pulse is equal to the
time required for the voltage wave to go and return in the L-C network, and it is given by the
duration T = 2N
√
L
×
C, where N is the number of sections of the L-C network.
CR801 prevents the energy stored in the PFN from discharging to the input line. The advantage
of employing CR801 is that it allows a wide choice of CR813 to CR 815 firing timings and effi-
cient utilization of stored energy; CR813 to CR 815 can be fired at any time after the PFN has
been charged to the peak and fluctuation of trigger timing does not affect the amplitude of the
pulse produced.
The pulse transformer T801 boosts the pulse produced by the PFN. Since the characteristic im-
pedance of the PFN and the input impedance of T801 matches (about three ohms), a pulse the
half of the network voltage is developed across T801 primary winding. T801 boosts this pulse up
by 18 times and applies the resultant output to the magnetron.
L801 L802 CR801
C814~C824
L811
CR815, CR814, CR813
T801
Figure 2.17 PFN (1)/(2) Board