Communications: directive radio wave systems and devices (e.g. – With particular circuit
Reexamination Certificate
2002-09-10
2004-03-02
Sotomayor, John B. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
With particular circuit
C342S202000, C342S204000, C307S106000, C331S087000
Reexamination Certificate
active
06700532
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a drive circuit for a magnetron used in a pulse-modulated radar, for example.
2. Description of the Prior Art
A pulse-modulated radar (hereinafter referred to simply as the pulse radar) is a radar which determines the distance to a target from the time that elapses after transmitting pulsed waves toward the target before a portion of the pulsed waves reflected by the target is received. Given the velocity of light, c, and the elapsed time &Dgr;t between transmission and reception, the distance D to the target is calculated as follows:
D=c&Dgr;t/
2
Important factors for judging the performance of the pulse radar are such characteristics as bearing discrimination, maximum and minimum detecting ranges, as well as range discrimination. The range discrimination is defined as a minimum distance R at which two targets placed on the same bearing with respect to a radar antenna can be displayed separately. Since radio waves propagate with a speed of about 300 m/&mgr;s, they make a round trip of about 150 m within a period of 1 &mgr;s. Thus, there is a relationship expressed by the following equation between pulselength &tgr; (&mgr;s) and the range discrimination R:
R=
300&tgr;/2=150&tgr;(m)
Basically, the range discrimination is determined by pulselength, that is, the smaller the pulselength, the better the range discrimination and short-range detection.
The construction of a pulse radar is now generally described. The pulse radar is constructed of a magnetron for generating microwaves, a magnetron drive circuit for driving the magnetron, an antenna, a receiver circuit and other electronic components.
FIG. 3
is a circuit diagram generally showing the configuration of a conventional magnetron drive circuit
101
. As depicted in
FIG. 3
, a pulse transformer
11
is used in the conventional magnetron drive circuit
101
. One end of a primary winding
12
of the pulse transformer
11
is connected directly to a power source V and grounded through a capacitor
20
while the other end of the primary winding
12
is connected to a drain of a switching n-channel metaloxide-semiconductor field effect transistor (NMOSFET)
21
(hereinafter referred to as the switching FET
21
). A source of the switching FET
21
is directly grounded and its gate is grounded through a resistor
22
. An absorption resistor
23
is normally connected between both ends of the primary winding
12
. On the other hand, one end of a secondary winding
13
of the pulse transformer
11
is connected to a cathode of the magnetron (not shown) while the other end of the secondary winding
13
is connected to an anode of the magnetron.
In the magnetron drive circuit
101
constructed as described above, the switching FET
21
turns on when a transmission trigger having a specific pulselength is fed into the gate of the switching FET
21
. As a result, a high-voltage pulse having the same pulselength is generated. When the high-voltage pulse is applied to the magnetron, it oscillates and produces an extremely high-power microwave output (transmission pulse), which is radiated from the radar antenna (not shown).
It has been recognized that the combination of the aforementioned magnetron drive circuit
101
and the magnetron of the prior art has a problem known as a transmission missing phenomenon which occurs as follows. If the rising edge of a pulse for driving the magnetron is made too sharp to obtain a narrow pulselength, the magnetron will fail to oscillate, resulting in an inability to generate the transmission pulse.
Also, it is generally needed to sharpen the falling edge of the transmission pulse to obtain a narrow pulselength. In the conventional magnetron drive circuit
101
, the absorption resistor
23
is added as stated above to decrease residual energy left in the pulse transformer
11
to zero level in a short time. The absorption resistor
23
, however, acts as an extra load in a rising period of the transmission pulse. Therefore, the resistance value of the absorption resistor
23
can not be made so small that it is impossible to obtain so sharp a falling edge of the transmission pulse.
FIG. 4
is a diagram showing the waveform of an input pulse fed into the conventional magnetron drive circuit
101
(bottom) and the waveform of a transmission pulse applied to the magnetron (top). Although the transmission pulse ideally should have a rectangular shape, the actual transmission pulse has a sawtooth shape as depicted in FIG.
4
.
SUMMARY OF THE INVENTION
The invention is intended to provide a solution to the aforementioned problems of the prior art. Accordingly, it is an object of the present invention to provide a radar signal generator for producing a magnetron driving signal so that the aforementioned problems will be solved.
Accordingly, it is another object of the invention to provide a magnetron drive circuit which drives a magnetron to produce a generally rectangular-shaped narrow transmission pulse having sharply shaped rising and falling edges.
According to the invention, a magnetron drive circuit comprises a nonlinear load circuit, which becomes ON at a voltage approximately equal to a voltage at which a magnetron begins to oscillate, is connected to a secondary winding of a pulse transformer for generating a pulse for driving the magnetron in parallel with the magnetron.
This construction ensures that the flow of electrons from a cathode to an anode of the magnetron properly oscillates in an initial stage of oscillation. In other words, the construction of the invention helps prevent the so-called transmission missing phenomenon which could occur if the flow of electrons reaches the anode of the magnetron resulting in a failure of oscillation. Since the magnetron used in a pulse-modulated radar begins to oscillate typically at around 80% of a peak point of applied voltage, it is preferred to reduce the rate of increase of the magnetron input voltage within a time period during which the voltage applied to the magnetron rises from 80% to 100% of the peak voltage. On the other hand, because the transmission missing phenomenon does not occur even if the applied voltage is rapidly increased until the magnetron begins to oscillate, the rising edge of the magnetron input voltage is sharpened during a pre-oscillation period of the magnetron to obtain a narrow transmission pulse width.
In one aspect of the invention, the nonlinear load circuit includes a diode which breaks down at a voltage approximately equal to the voltage at which the magnetron begins to oscillate. The nonlinear load circuit of this feature can be produced by using an element easily available on the market.
In another aspect of the invention, the nonlinear load circuit is configured by connecting a parallel circuit, which is formed of a series circuit including a first resistor having a resistance approximately equal to the rated internal impedance of the magnetron and a capacitor having a specific capacitance and a second resistor having a resistance corresponding to two to three times the rated internal impedance of the magnetron, to the aforementioned diode in series.
In this construction, the first resistor, the capacitor and the second resistor together act as a temporary load, the capacitor serving as a particularly large load, when the diode has broken down. This helps decrease the rate of voltage rise close to the voltage at which the magnetron begins to oscillate, making it possible to prevent the transmission missing phenomenon in a more reliable manner.
In another aspect of the invention, the magnetron drive circuit further comprises a residual energy absorption circuit for absorbing residual energy left in the pulse transformer by short-circuiting one of its windings approximately at the same time as the voltage level of a transmission trigger fed into a primary winding of the pulse transformer begins to fall.
The residual energy absorption circuit absorbs the residual energy left in the pulse transformer without producing a
Funae Akio
Kitabatake Asao
Birch & Stewart Kolasch & Birch, LLP
Furuno Electric Company Limited
Sotomayor John B.
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