Field of the Invention

The present invention relates to a radar system comprising at least one magnetron, which is connected to at least one magnetron drive circuit, which magnetron drive circuit comprises at least one transformer, which transformer comprises at least a first secondary side, which is connected to the magnetron, and which transformer comprises at least one primary side, which primary side is connected to at least one switching element, which switching element is serially coupled to the primary side of the transformer, and at least one second switching element for generating a parallel circuit to the primary side of the transformer.

Background of the Invention

Pulsed radar systems have existed for a long time and are important tools for many types of applications such as target detection, navigation, range determination etc. A pulsed radar system is based on the transmitting of a sequence of short RP pulses and subsequently the receiving of the echoes reflected from the targets. The time difference between a transmitted pulse and a received echo is used to determine a number of parameters such as range, speed etc.

The waveform of the transmitted pulses is a very important parameter for the radar system, and steep pulses are necessary in order to obtain a good range resolution etc.

The waveform of the transmitted pulses is of great importance as it has great impact on the time accuracy of the radar. An ideal waveform of the transmitted pulses is a square pulse with zero rise and decay times. In order to generate a pulse with an ideal rectangular shape, an infinite bandwidth is required. However, in practice it is not possible to generate a pulse with an ideal square waveform due to the finite bandwidth of the electronic circuits in the radar system.

Despite the fact that it is impossible to achieve a radar system with an infinite band- width, arrangements can be made and an approximation to an ideal square pulse can be achieved.

In order to generate an RF pulse in a radar system, a magnetron is often used. The magnetron acts as an oscillator and is coupled to a drive circuit comprising switching means. The drive circuit is often referred to as a magnetron drive circuit or a modula- tor. The magnetron drive circuit comprises electronic circuits with one or more time constants and the leading edge and the falling edge of the generated pulses are therefore more or less distorted. The magnetron drive circuit also comprises one or more transformers, which are periodically charged and discharged for generating a train of pulses. When the transformer is discharged, the residual energy stored in the trans- former slowly decreases and the falling edge of the pulse is hereby seriously distorted. However, arrangements can be made so the waveform of the transmitted pulses is less distorted.

According to US2003/0058160 such arrangements have been made in order to achieve a pulse with a relatively sharp transmitted pulse falling edge. Incorporating an absorption circuit between a pulse transformer and the magnetron does this. This way the absorption circuit discharges the residual energy stored in the pulse transformer relatively fast, and the transmission pulse falling edge is hereby sharpened. The invention described according to US2003/0058160 has two separate primary sides; one for the trigger circuit and one for the tail damper circuit. However, the extra coil in the transformer reduces the yield of the transformer.

JP 2000353938 concerns a pulse generating circuit for generating a pulse in which the waveform of the falling part can be prevented from being disturbed by quickly dis- charging electromagnetic energy stored in a parasitic reactance. This can be solved by a pulse generating circuit which is provided with a main switching element Ql serially connected between a DC powor source and a transformator terminal, a further switching element Q2 for short-circuiting connected to the transformator terminal in parallel, and an on/off control circuit for turning on/off the main switching element Ql and the switching element Q2 for short-circuiting into the mutually inverse states. In this case, electromagnetic energy stored in a parasitic reactance can be discharged so as to be quickly attenuated.

This circuit is using a first direction for magnetizing the transformer and a second opposite current direction for demagnetizing the transformer. Different embodiments are described and shown in the figures.

The embodiment shown in fig 1 shows a short-circuit for the transformer, where the transformer generates a current flowing in the opposite direction through a semiconductor switch, which current might destroy the semiconductor switch.

In fig. 2 is the current direction through the semiconductor switch correct for the semi- conductor switch. No current limitation is indicated as well in the description and on the drawing. Two identical electronic driving circuits are used for generating control signals for the semiconductor switches.

The fig. 3 describes nearly the same circuit where only the driving circuit is different. Here is only one electronic circuit in use and a transformer inverts the output signals. The transformer can generate a short time-delay for the driving pulse. Herby can the Ql be closed before Q2 is open. The path for current flowing in the transformer coil is interrupted, and a pulse of high voltage is generated.

In fig. 4 is indicated a short-circuit in the form of a resister coupled parallel to the transformer.

Object of the Invention

An object of the invention is to provide a radar system, which generates a pulse in such a way, that the falling edge of the generated pulse approximates a falling edge of an ideal rectangularly shaped pulse.

Another object of the invention is to provide a highly efficient transformer for driving ^■ a magnetron for transmitting sharp pulses towards a magnetron and maintaining con- stant current in pulses.

Description of the Invention

The first object of the invention is achieved by a radar system as described in the introduction, wherein a coil is coupled serial between the first switching element and the primary side of the transformer, and a resistor is serial coupled between the primary side of the transformer and the second switching element.

The current that flows through the transformer and the first switching element also flows through the coil, in which a magnetic field is generated and the current flowing through the transformer is limited. When the second switch is opened the coil is dis- charged through a diode.

The second switch is used for generating a parallel circuit, which is parallel coupled to the primary side of the transformer and provides a very efficient absorption circuit for the residual energy stored in the transformer. This way, the falling edge of the trans- mitted pulse is sharpened considerably and approximates the falling edge of an ideal rectangular pulse.

Furthermore, the parallel circuit is located on the primary side of the transformer, and the parallel circuit is only activated at the end of each transmitted pulse. The parallel circuit is switched on by the second switching means, and thus the parallel circuit is not distorting the leading edge or the steady state of the transmitted pulse, which is a great advantage, compared to prior art. By using the present invention it is unnecessary to have two primary sides.

The parallel circuit comprises at least one resistor. The resistor acts as a temporary load of the transformer. This way the resistor absorbs the residual energy, which is stored in the transformer, and the transformer is hereby quickly and efficiently demagnetized. The falling edge of the transmitted pulse is hereby approximates the falling edge of an ideal rectangular pulse, and the accuracy of the radar system is improved considerably compared to traditional radar systems.

The present invention hereby provides a drive circuit with a tail suppression circuit, which does not affect the waveform of the pulse during transmission.

According to an embodiment of the invention the first switching means has a first and a second state for generating a pulse to the magnetron. When the first switching means is switched to the first state, a leading pulse edge is generated. When the second switching means is switched to the second state, a falling edge is created. The state of the first switching means is controlled by a first input signal of rectangular pulses, and the input signal is often referred to as a pulse-signal, as it is used to periodically pulse the radar.

According to an advantageous embodiment of the invention, the second switching means has a first and a second state, where the parallel circuit is switched off, when the second switching means is in the first state, and where the second switching means is generating a parallel circuit to the primary side in the second state. This way, the second switching means is used to switch the parallel circuit on and off. The activation of the parallel circuit can be synchronized with the transmission of a pulse for efficient suppression of the pulse tail.

The second switching means is controlled by a second input signal of rectangular pulses, and the second input signal is often referred to as a tail-signal, as it is used for suppressing the tails of the transmitting pulses.

According to a preferred embodiment of the invention the first and the second switching means are arranged to be in complementary states and the switching of the first ^■ switching means and the second switching means are synchronized. This way a con- fiicting state between the first switching means and the second switching means is avoided. When a pulse is transmitted, the second switching means must be in the second state to avoid that the parallel circuit is switched on and connected to the primary side of the transformer.

When the first switching means is switched to a second state, the voltage over the transformer is cut off and the pulse falling edge is generated. At the same time the first switching means is arranged in a first state, this means that the parallel circuit is switched on and the transformer is demagnetized in a fast and efficient manner. The

decay time of the pulse falling edge is hereby reduced considerably. The second switching means is subsequently switched off after a fixed time period, which typically equals the pulse length of the transmitted pulse. However, the time period may vary to achieve a specific pulse repetition time or to achieve a more or less demagnet- ized transformer.

According to a preferred embodiment of the invention the trigger signal and the tail signal are generated and controlled by digital logic.

It is preferred that a common transformer is generating driving pulses for the first switching element and second switching element. By using separated transformer coils signals having opposite shape can be generated for driving the switching elements. Herby can be achieved that the switches are operating opposite.

According to an advantageous embodiment of the invention transistor circuits implement the switching means. FET transistors are the preferred transistor technology, but others can be used as well.

According to a preferred embodiment of the invention the radar system comprises means for measuring the current flowing through the magnetron. Controlling means are added for adjusting the supply voltage to the magnetron drive circuit, where the adjustment is based on the measured current flowing through the magnetron. This way the current flowing through the magnetron is controlled. The supply current is a preferred embodiment of the invention adjusted periodically.

This way, the current that flows through the magnetron is held constant according to the specifications of the nominal current in the magnetron used. The power used by the magnetron and the magnetron drive circuit is hereby also held constant, which assures that overload of the magnetron drive circuit and the magnetron is avoided. Regulating of the current allows for tolerances in the magnetron current specifications, which tolerances result from the production of the magnetron. Small current variations

can result in large current flows in the magnetron drive circuit, which could lead to an overload of the electronic components. However, by using the present invention, safe operation of the magnetron drive circuit and the magnetron is achieved, which is a great advantage compared to prior art.

The magnetron is not an ideal component, and the current that flows through the magnetron changes in time. However, this undesired current drift can be allowed for by regulating the magnetron current and thus obtaining the desired current flow through the magnetron. According to a preferred embodiment of the invention the voltage for the primary side of the transformer varies between 300V and 380V.

The invention further comprises a transformer for a radar system, which transformer comprises of at least one primary side, which primary side comprises of a number of windings, and which primary side is coupled to switching means, and where the transformer has a secondary side comprising a number of windings, and which secondary side is at least coupled to one magnetron, where the primary side of the transformer is divided into at least two layers, which are separated by the windings of the secondary side.

Hereby is achieved low leakage and capacitance between primary and secondary coils.

According to a preferred embodiment of the invention the windings of the secondary side of the transformer are wires. Furthermore, the windings of the primary side of the transformer are made of foil. A reduced leakage is hereby achieved, and the transformer is easy to mass-produce.

According to an advantageous embodiment of the invention the secondary side of the transformer is coupled to an anode, cathode and a heater of the magnetron.

Description of the Drawing

The invention is explained in more detail with reference to the drawings, where

fig. 1. shows a schematic diagram of a radar system, fig. 2. shows an electronic diagram of the magnetron drive circuit , fig. 3. shows a pulse timing diagram of the radar system, and fig. 4. shows a cross-section of a transformer according to the present invention.

Detailed Description of the Invention

Figure 1 shows a schematic diagram of a radar system 2, the radar system 2 comprises a CPU 4, which is connected to a control circuit 6, where the control circuit is connected to a magnetron drive circuit 8 and a magnetron current measuring circuit 10. The magnetron drive circuit 8 comprises the functionality of a modulator. The magnetron drive circuit 8 is connected to a power supply 12, which means that a magnetron 14 can be pulsed with a high voltage signal.

The magnetron 14 is connected with a RF unit 16 comprising of a waveguide for proper connection to the radar antenna 18.

According to a preferred embodiment of the invention the control circuit 6 is implanted by a CPLD (Complex Programmable Logic Device).

The CPU 4 has there main purposes. The CPU 4 and the CPLD control the magnetron drive circuit 8 to assure that the transmitted pulses are correct. The CPU 4 and the CPLD start and stop a trigger pulse and a tail suppression pulse and repeats itself with a given frequency for generating a pulse train. The CPU 4 also controls the antenna motor (not shown) to obtain correct rotation speed of the antenna 18. Furthermore, the CPU 4 tunes the receiver (not shown), which provides the best possible video signal to a display unit.

The CPU 4 controls a flyback converter in the power supply 12 to regulate the voltage to the switching circuit. According to a preferred embodiment of the invention the voltage can be regulated between 300 and 380 volts.

Figure 2 shows an electronic diagram 102 of a preferred embodiment of the present invention comprising input connection means for trigger pulse 104 and a tail pulse

106. The trigger pulse input 104 and the tail pulse input 106 are connected to a transformer 108, which adapts the input signals to the switching circuitry located on the secondary side of the transformer 108. Furthermore, the transformer 108 serves to generate a galvanic insulation between the input connection means and the switching circuit. The transformer 108 is coupled to the first switching means 110 and the second switching means 112 in such a way that the first 110 and the second 112 switching means are not switched at the same time. A coil (111) is coupled serial between the switching element (110) and the primary side (318, 320) of the transformer (116, 316). The coil (111) has a diode (115) coupled in parallel so that the diode is conduct- ing when the first switching element (110) is closed. In this way the coil (111) is discharged.

A resistor (113) is serial coupled between the primary side (318, 320) of the transformer (116, 316) and the second switching means (112).

According to a preferred embodiment of the invention FET transistors implement the switching means 110 and 112, but the invention is not limited to the use of a specific transistor type.

The switching means 110 and 112 are coupled to the magnetron 114 via a transformer 116 to generate a high voltage over the magnetron 114 and thereby sending radar pulses through the radar antenna (not shown). The current through the magnetron 114 is found by measuring a voltage drop above a resistor 118. The current signal is then used to regulate the voltage over the magnetron 114 to offset the unwanted drift in the frequency contents of the pulse generated and transmitted by the magnetron 114.

The first switching means 110 are used for generating a pulse by switching the supply voltage to the magnetron 114 on and off. The second switching 112 means are used to generate a circuit 120, which is parallel coupled to the magnetron. The parallel circuit 120 is used to absorb the residual energy stored in the transformer 116. The parallel circuit 120 is connected and activated each time the trigger pulse ends.

The secondary side of the transformer 116 is coupled to a heater inside the magnetron 114 via a third transformer 124.

Figure 3 shows a pulse-timing diagram 202 of the pulses in the magnetron drive circuit, which magnetron drive circuit is illustrated in figure 2. Each pulse is measured and captured by an oscilloscope. The pulses are aligned so the timing of the different pulses is easily compared.

• The trigger pulse 204 is measured at the trigger input 104 shown in figure 2,

• the tail pulse 206 is measured at the tail input 106 shown in figure 2,

• the trigger pulse 208 is measured on the secondary side of the transformer 108 shown in figure 2

• the tail pulse 210 is measured at the secondary side of the transformer 108 shown in figure 2 β the trigger pulse 212 is measured at the gate of the transistor 110 shown in figure 2, » the tail pulse 214 is measured at the gate of the transistor 112 with reference to the source of the transistor 112 shown in figure 2, β the trigger pulse 216 is measured at the drain of the transistor 110 shown in figure 2, β the pulse 218 is measured at the primary side of the transformer 116 shown in figure 2 and

• the pulse 220 is measured at the secondary side of the transformer 116 shown in figure 2.

The trigger pulse 208 and the tail pulse 210 is 180 degrees out of phase assuring that the first 110 and second switching means 112 are not activated at the same time. Thus the parallel circuit 108 is not functioning before each trigger pulse ends. The length of the tail pulse is specified to obtain good tail suppression.

Figure 4 shows a cross-section of the transformer 316 used in the magnetron drive circuit 116 shown in figure 2. The transformer 316 is a sandwich type construction, where the secondary side 322 divides the primary sides 318 and 320 of the transformer. The two primary sides 318 and 320 and the secondary side 322 of the transformer are galvanic insulated by means of insulating tape. An insulation tape separates the windings of the primary side 318, 320 and the secondary side 322.

The windings of the primary side 318 and 320 are made of copper foil, and the windings of the secondary side 322 are made of copper wire.

On the basis of the measured current flowing through the magnetron (14,114), the supply current is periodically adjusted.