Module and method for generating a pulsed microwave output signal

Abstract

A module (10, 10.1) for generating a pulsed microwave output signal and for controlling a microwave solid-state power amplifier module (110, 112, 114, 124, 126, 128) comprises a. An input member (S1), which has a microwave signal input (12) and at least a first and a second microwave signal output (14, 16), b. An output member (S2), which has at least a first and a second microwave signal input (18, 20) and a microwave signal output (22), c. A first signal path (24) between the first microwave signal output (14) of the input member (S1) and the first microwave signal input (18) of the output member (S2), d. A second signal path (26) between the second microwave signal output (16) of the input member (S1) and the second microwave signal input (20) of the output member (S2), e. A damping member (18) provided in the second signal path (26), wherein i. the input member (S1) has a control signal input (30) and is configured to connect its microwave signal input (12) to its first or second microwave signal output (14, 16) depending on a pulsed signal (40) arriving at the control signal input (30), and / or ii. the output member (S2) has a control signal input (32) and is configured to connect its first microwave signal input (18) or its second microwave signal input (20) to its microwave signal output (22) depending on a pulsed signal (40) arriving at the control signal input (32).

Description

[0001] Module and method for generating a pulsed microwave output signal

[0002] The invention relates to a module for generating a pulsed microwave output signal and for controlling a solid-state microwave power amplifier module, a solid-state microwave power amplifier, and a solid-state microwave power generator. The invention further comprises a method for generating pulsed microwave signals of varying amplitudes.

[0003] In the manufacture of semiconductor integrated circuits (ICs), components such as transistors are formed on a semiconductor substrate. During the manufacturing process, various materials are deposited or etched onto different layers to create the desired integrated circuit. In certain plasma-assisted processes, charges can accumulate on the surface of the substrate, which can negatively impact the plasma process. To reduce such charges, high-frequency power can be applied to the plasma process in pulsed form. This is just one example among many others of the need for a pulsed plasma.

[0004] However, in certain applications, not only single-stage pulsing but multi-stage pulsing in rapid alternation is desired.

[0005] Particularly short pulses are important for the generation of non-equilibrium plasma, non-thermal plasma, or cold plasma, and all processes where such plasma is used. These plasma forms are characterized by the fact that the electrons can follow the field, while the ionized atomic nuclei cannot follow the rapid field changes. The plasma remains "cold." One requirement is that the pulses are shorter in time than the time constant for the recombination of the ions with the electrons. This time constant depends on many influencing factors such as pressure, temperature, or the elemental nature of the ionized atoms themselves.

[0006] The object of the present invention is to provide a module that enables fast multi-stage pulsing with very short pulse durations.

[0007] This object is achieved by a module for generating a pulsed microwave output signal and for controlling a microwave solid-state power amplifier module with a. an input element which has a microwave signal input and at least a first and a second microwave signal output, b. an output element which has at least a first and a second microwave signal input and a microwave signal output, c. a first signal path between the first microwave signal output of the input element and the first microwave signal input of the output element, d. a second signal path between the second microwave signal output of the input element and the second microwave signal input of the output element, e. an attenuator provided in the second signal path, wherein i.the input element has a control signal input and is configured to connect its microwave signal input to its first or second microwave signal output depending on a pulse signal arriving at the control signal input and / or ii. the output element has a control signal input and is configured to connect its first microwave signal input or its second microwave signal input to its microwave signal output depending on a pulse signal arriving at the control signal input.

[0008] An attenuator is an electrical component capable of reducing an electrical signal by a specified value, e.g., reducing a voltage, current, and / or power by a certain factor. This can be a resistor, a voltage divider, an active circuit, or a single component. It can be a component that can be adjusted manually or via an electrical signal.

[0009] A microwave signal here refers to a signal with a frequency of 300 MHz to 20 GHz.

[0010] Microwave output signal here means a microwave signal at an output.

[0011] A pulsed microwave signal refers to a microwave signal that is pulsed. The pulses can be very short. "Very short" here means pulses in the range of 10 ns to 500 ns. The pulses can be in the range of 10 ns to 40 ns, preferably in the range of 15 ns to 30 ns. This means that the pulses are shorter in time than most time constants for the recombination of ions with electrons.

[0012] Furthermore, a pulse can have a rise time and / or fall time of < 10 ns. This ensures that the pulses have a sufficiently strong intensity, which is necessary to generate the plasma. The pulses can, in particular, be multi-stage, i.e., pulse between at least two power values, at least one of which is non-zero, and in particular, both are non-zero. This will be described further below.

[0013] Pulsed microwave output signal here means a pulsed microwave signal at an output.

[0014] A microwave solid-state power amplifier module is an electronic amplifier suitable for amplifying a low-power microwave signal into a higher-power microwave signal. The amplifying element used is a solid-state device, particularly a semiconductor solid-state device, such as a transistor. The transistor can be a HEMT, a MOSFET, an LDMOS transistor, or a transistor based on GaN or SiC. This terminology is used in microwave technology to differentiate it from conventional non-solid-state amplifier types, such as tube and / or magnetron amplifiers, which can be controlled much less precisely in terms of power and frequency and are subject to high levels of wear. Such microwave solid-state power amplifiers are described, for example, in US Pat. Nos. 9,595,930 B2 and 11,222,770 B2 as "solid-state microwave power amplifiers" or "solid-state microwave power generators."

[0015] With such a module for generating a pulsed microwave output signal, the microwave signal arriving at the input element can be pulsed depending on the pulse signal. The microwave signal can be transmitted to the output element via the first signal path with a first, e.g., undamped, amplitude. The microwave signal can be transmitted to the output element via the second signal path with a different / second amplitude.

[0016] Switching between the at least two paths can be performed by the input and output elements, controlled by the pulse signal. This allows a pulse function to be realized. The at least two different path attenuations can create at least two amplitude levels.

[0017] This allows for a dual-level pulse function. This function is particularly advantageous for plasma applications and other similar dynamic processes. This makes it possible for a single generator to deliver both low base power and short power peaks.

[0018] The pulse signal can be generated, for example, by a microcontroller with a high-resolution timer, thus enabling the generation of very short pulses. "Timer" here refers to a circuit component that outputs a timing signal, e.g., a timing pulse with a specified frequency. The timer can be configured as a PWM timer, so that no software intervention is necessary to generate regular and / or periodic pulses.

[0019] The signal lines from a pulse signal generator to the input element and to the output element can have the same length.

[0020] Both the input element and the output element can be constructed in many different ways. They are designed to switch the microwave signals from one signal path to the other at the pulse frequency speed. They preferably each have two transistors, in particular two MOSFETs. The control signal input of the input element and / or the output element is preferably connected to at least one of these transistors at its control input, e.g., at the gate or the base.

[0021] The input element and the output element may have the same design, in particular an identical one.

[0022] Both the input and output elements can be designed as so-called "single-pole, double-throw (SPDT) switches".

[0023] The input element and / or the output element can be designed as a particularly reflection-free high-frequency switching unit. In this case, it can be sufficient if the input element or the output element is designed as a high-frequency switching unit. The other element can, for example, be designed as a signal splitter. 'Signal splitter' here refers to a circuit component that is configured to connect a signal from an input terminal to either a first or a second output terminal or to establish a connection from an output terminal to either a first or a second input terminal. In addition to the signal inputs and signal outputs described above, the high-frequency switching unit can have further terminals, e.g.

[0024] Have supply voltage and ground connections.

[0025] As already mentioned above, the attenuator can be adjustable, e.g. designed as an adjustable resistor.

[0026] The attenuator can be designed as an analog attenuator. However, particular advantages arise when it is designed as a digital attenuator. An analog attenuator is understood to be an attenuator that can be continuously adjusted within a range, preferably via an analog control signal. A digital attenuator is understood to be an attenuator that can be adjusted between several discrete attenuation values, preferably via a digital control. The second signal path can have a second input element with a microwave signal input and a first and a second microwave signal output as well as a mode signal input and can be configured to connect its first microwave signal input to its first or second microwave signal output depending on a mode signal arriving at the mode signal input.Thus, based on the mode signal, it can be selected whether the pulsed microwave signal received by the first input element is passed to the first or second microwave signal output of the second input element.

[0027] The module can be configured so that the microwave signal is pulsed only at the output and not necessarily at the attenuator(s). This can be the case, for example, if only a divider is provided at the input.

[0028] The second signal path can have a second output element with two microwave signal inputs and one microwave signal output, as well as a mode signal input, and can be configured to connect its first microwave signal input or its second microwave signal input to its microwave signal output depending on a mode signal received at the mode signal input. Thus, the mode signal can be used to select which of the microwave signal inputs is connected to the microwave signal output. The attenuator can be arranged between one of the microwave signal outputs of the second input element and one of the microwave signal inputs of the second output element.

[0029] Alternatively, it can be provided that the second signal path has: a. A second input element which has a microwave signal input and at least a first and a second microwave signal output, b. A second output element which has at least a first and a second microwave signal input and a microwave signal output, c. A third signal path between the first microwave signal output of the second input element and the first microwave signal input of the second output element, d. A fourth signal path between the second microwave signal output of the second input element and the first microwave signal input of the second output element, e. Wherein the attenuator is provided in the third signal path, and where i.the second input element has a mode signal input and is configured to connect its microwave signal input to its first or second microwave signal output depending on a mode signal received at the mode signal input and / or ii. the second output element has a mode signal input and is configured to connect its first microwave signal input or its second microwave signal input to its microwave signal output depending on a mode signal received at the mode signal input.

[0030] One of the microwave signal outputs of the second input element and one of the microwave signal inputs of the second output element can be connected to a termination circuit. This can prevent reflections at the input and output elements. The termination circuit can include a resistor and, if necessary, a capacitor in series. The capacitor serves to block DC components.

[0031] The second input element and / or the second output element can be designed as a high-frequency switching unit. Thus, multi-stage pulsing of a microwave signal can be achieved.

[0032] The invention also includes a microwave solid-state power amplifier (MW-SSPA) with a module according to the invention, wherein a microwave solid-state power amplifier module is connected downstream of the module. A microwave solid-state power amplifier module is also referred to as a microwave solid-state power amplifier module and abbreviated to MW-SSPA module.

[0033] A splitter can be provided between the module and the microwave solid-state power amplifier module. Following the splitter, several amplifier paths, each with at least one microwave solid-state power amplifier module, can be provided, which are recombined by a combiner at the power output.

[0034] The invention further comprises a solid-state microwave power generator. In such a solid-state microwave power generator, a small-signal microwave generator, also called a synthesizer, can be connected upstream of a module according to the invention. A small-signal microwave generator here refers to a microwave signal generator capable of delivering an output power of less than or equal to 2 W.

[0035] A splitter can be provided between the small microwave signal generator and the module. Thus, the signal generated by the small microwave signal generator can be delivered to multiple modules. In one aspect, the invention relates to a method for generating pulsed microwave signals of different amplitudes, comprising the following method steps: a. generating a microwave signal, b. feeding the microwave signal to a module having a first and a second signal path with different attenuation, c. generating a pulse signal, d. transmitting the microwave signal via the first or second signal path to an output of the module depending on the pulse signal.

[0036] This makes it easy to generate high-frequency pulse signals with different amplitudes.

[0037] According to one method variant, a mode signal can be generated, and the microwave signal can be transmitted via the second signal path or routed to a termination circuit depending on the mode signal. The mode signal can thus be used to select whether a pulsed radio-frequency signal is transmitted and, if so, at which power level.

[0038] The pulse signal can be generated with a pulse length of the pulses of maximum 1 ps, in particular in the range of 10 ns to 100 ns, preferably in the range of 15 ns to 50 ns, and in particular in the range of 30 ns.

[0039] Furthermore, the pulse signal can be generated with a rise time and / or fall time of the pulses of < 10 ns.

[0040] The pulse length of the pulse signal can be adjusted in steps of < 1 ns. This allows for a wide range of fine-tuning options for pulse width modulation.

[0041] Further features and advantages of the invention will become apparent from the following detailed description of exemplary embodiments of the invention with reference to the figures of the drawing, which illustrate details essential to the invention, as well as from the claims. The features shown therein are not necessarily to scale and are presented in such a way that the special features of the invention can be clearly seen. The various features can be implemented individually or in combinations in variants of the invention.

[0042] The schematic drawing shows embodiments of the invention at various stages of use and explains them in more detail in the following description. They show:

[0043] Fig. 1 shows a first embodiment of a module;

[0044] Fig. 2 is a representation of a pulse signal and a two-stage pulsed microwave signal;

[0045] Fig. 3 shows a second embodiment of a module;

[0046] Fig. 4 shows a first embodiment of a microwave solid-state power amplifier;

[0047] Fig. 5 shows the use of a module for controlling an amplifier arrangement;

[0048] Fig. 6 shows a second embodiment of a microwave solid-state power amplifier;

[0049] Fig. 7 shows a first embodiment of a microwave solid-state power generator;

[0050] Fig. 8 shows a second embodiment of a microwave solid-state power generator;

[0051] Fig. 9 an amplifier arrangement.

[0052] Figure 1 shows a module 10 for generating a pulsed microwave output signal and for controlling a solid-state microwave power amplifier. The module 10 has an input element S1 with a microwave signal input 12 and a first and a second microwave signal output 14, 16. The module 10 also has an output element S2 with a first and a second microwave signal input 18, 20 and a microwave signal output 22. A first signal path 24 is provided between the first microwave signal output 14 of the input element S1 and the first microwave signal input 18 of the output element S2. A second signal path 26 is provided between the second microwave signal output 16 of the input element S1 and the second microwave signal output 18 of the output element S2. An adjustable attenuator 28 is provided in the second signal path 26.

[0053] The input element S1 and the output element S2 each have a control signal input 30, 32. The input element S1 is configured to connect its microwave signal input 12 to its first or second microwave signal output 14, 16 depending on a pulse signal received at the control signal input 30. The output element S2 is configured to connect its first microwave signal input 18 or its second microwave signal input 20 to its microwave signal output 22 depending on the pulse signal received at the control signal input 32. In the illustrated embodiment, the input element S1 and the output element S2 are each designed as a high-frequency switching unit. However, it is conceivable that either the input element S1 or the output element S2 is designed as a divider.

[0054] The same pulse signal is applied to the control signal inputs 30, 32. A microwave signal, which can originate from a small microwave signal generator, is applied to the first microwave signal input 12. A pulsed microwave signal is output at the microwave signal output 22, with the amplitude of the pulsed microwave signal depending on whether the microwave signal travels via the first signal path 24 or the second signal path 26.

[0055] Serial capacitors 33, 34, 35, 36, 37, 38 at the inputs and outputs of the input and / or output elements serve to separate DC components that can be coupled in, particularly by the controlled input and / or output elements. These capacitors 33, 34, 35, 36, 37, 38 are required or optional, depending on the switch used.

[0056] Figure 2 shows a pulse signal 40, which can be applied to the control signal inputs 30, 32. The pulse length of the pulses is typically 1 ps, in particular in the range of 10 ns to 100 ns, preferably in the range of 15 ns to 50 ns, and in particular in the range of 30 ns. The pulses have a rise time and / or fall time of <10 ns. The pulse length of the pulses of the pulse signal 40 can be adjusted in increments of <1 ns.

[0057] Furthermore, Figure 2 shows a two-stage pulsed microwave signal 42 which has a small amplitude in the region 44 corresponding to the pulse signal 40, i.e. has passed through the second signal path 26, and has a larger amplitude in the region 46, which corresponds to the higher level of the pulse signal 40.

[0058] The amplitude of microwave signal 46 in range 44 is adjustable by attenuator 28. It is conceivable to also provide an adjustable attenuator in signal path 24. Then, the amplitude of microwave signal 42 in range 46 is also adjustable.

[0059] If an attenuator is also provided in the first signal path 24, the attenuators of the first and second signal paths 24, 26 preferably have different values ​​or attenuation properties, so that a pulsed microwave signal can be generated which has different amplitudes.

[0060] Figure 3 shows an embodiment of a module 10.1, wherein elements corresponding to those in Figure 1 have the same reference numerals. In contrast to the embodiment according to Figure 1, the second signal path 26 has a second input element S3 and a second output element S4. The second input element S3 has a microwave signal input 50 and first and second microwave signal outputs 52, 54. Correspondingly, the second output element S4 has a first microwave signal input 56 and a second microwave signal input 58. Furthermore, the second output element S4 has a microwave signal output 60. The second input element S3 has a mode signal input 62 and the second output element S4 has a mode signal input 64.

[0061] The first microwave signal output 52 is connected to the first microwave signal input 56 of the second output element S4 via a third signal path 66, which is part of the second signal path 26 and contains the attenuator 28. The second microwave signal output 54 is connected to a termination circuit 68. The second microwave signal input 58 is connected to a termination circuit 70. The termination circuits 68, 70 each have a resistor 72, 74 and, in series therewith, an optional capacitor 76, 78.

[0062] The second input element S3 is configured to connect the first microwave signal input 50 to the first microwave signal output 52 or the second microwave signal output 54 depending on a mode signal received at the mode signal input 62. Accordingly, the second output element S4 is configured to connect the first microwave signal input 56 or the second microwave signal input 58 to the microwave signal output 60 depending on the mode signal received at the mode signal input 64. Depending on the mode signal, the signal received at the microwave signal input 50 is transferred via the third signal path 66 and the second output element 64 to the output element S2 or fed to the termination circuit 68. Thus, in addition to the option shown in Figure 2 of outputting a microwave signal with different amplitudes, there is the option of outputting no signal at the microwave signal output 22.

[0063] Figure 4 shows a microwave solid-state amplifier (MW-SSPA) 100 comprising a module 10, 10.1 according to Figures 1 or 3. A splitter 102 is connected downstream of the module 10, 10.1, which, in the illustrated embodiment, divides the pulsed microwave signal from the module 10, 10.1 into three amplifier paths 104, 106, 108. Each amplifier path 104, 106, 108 comprises a microwave solid-state power amplifier module 110, 112, 114. The signals amplified in the amplifier paths 104, 106, 108 are recombined in a combiner 116, so that an amplified pulsed microwave signal is output at output 118. Figure 5 shows the use of a module 10, 10.1 for controlling an amplifier arrangement 120. The amplifier arrangement 120 has a splitter 122 which divides the output signal of the module

[0064] 10. 10.1 in the illustrated embodiment is divided into three amplifier paths for 124, 126, 128, each of the amplifier paths 124, 126, 128 having a microwave solid-state power amplifier module 130, 132, 134. The signals from the amplifier paths 124, 126, 128 are combined in a combiner 136, so that an amplified pulsed microwave signal is output at output 138.

[0065] Figure 6 shows an alternative embodiment of a microwave solid-state power amplifier

[0066] 100.1. In contrast to the microwave solid-state power amplifier 100 of Figure 4, the microwave solid-state power amplifier 100.1 of Figure 6 does not have a module 10, 10.1 provided in front of the splitter 102, but rather a microwave signal is fed directly to the splitter 102. Modules 10,

[0067] 10.1 are arranged in the amplifier paths 104, 106, 108 and thus between the splitter 102 and the microwave solid-state power amplifier modules 110, 112, 114. The signals of the amplifier paths 104, 106, 108 are in turn fed to the combiner 116 so that an amplified pulsed microwave signal can be output at the output 118.

[0068] Figure 7 shows a microwave solid-state power generator (MW-SSPG) 200. This comprises the microwave solid-state power amplifier 100, which is preceded by a microwave small-signal generator 202.

[0069] Figure 8 shows an alternative embodiment of a microwave solid-state power generator

[0070] 200.1, which has a microwave solid-state power amplifier 100.1, preceded by a microwave small-signal generator 202. In the illustrated embodiment, the microwave small-signal generator 202 is arranged directly on the splitter 102 or integrated into it.

[0071] The arrangement shown in Figure 9 shows that a microwave small signal generator 202 including splitter 102 and modules 10, 10.1 connected thereto can be connected upstream of an amplifier arrangement 300 comprising microwave solid-state power amplifier modules 110, 112, 114 and a combiner 116.

[0072] A PWM module may be arranged and connected between the microwave small signal generator 202 and the microwave solid-state amplifier modules 110, 112, 114.

Claims

Claims Module (10, 10.1) for generating a pulsed microwave output signal and for controlling a microwave solid-state power amplifier module (110, 112, 114, 124, 126, 128) comprising: a. An input element (S1) comprising a microwave signal input (12) and at least one first and one second microwave signal output (14, 16); b. An output element (S2) comprising at least one first and one second microwave signal input (18, 20) and a microwave signal output (22); c. A first signal path (24) between the first microwave signal output (14) of the input element (S1) and the first microwave signal input (18) of the output element (S2); d. A second signal path (26) between the second microwave signal output (16) of the input element (S1) and the second microwave signal input (20) of the output element (S2), e. An attenuator (18) provided in the second signal path (26), wherein i.The input element (S1) has a control signal input (30) and is configured, depending on a pulse signal (40) received at the control signal input (30), to connect its microwave signal input (12) to its first or second microwave signal output (14, 16), and / or the output element (S2) has a control signal input (32) and is configured, depending on a pulse signal (40) received at the control signal input (32), to connect its first microwave signal input (18) or its second microwave signal input (20) to its microwave signal output (22). The module according to claim 1, characterized in that the input element (S1) and / or the output element (S2) is designed as a high-frequency switching unit. The module according to any of the preceding claims, characterized in that the attenuator (28) is adjustable. Module according to one of the preceding claims, characterized in that the attenuator (28) is designed as a digital attenuator. Module according to one of the preceding claims, characterized in that the second signal path (26) has a second input element (S3) with a microwave signal input (50) and a first and a second microwave signal output (52, 54) as well as a mode signal input (62) and is configured to connect its microwave signal input (50) to its first or second microwave signal output (52, 54) depending on a mode signal received at the mode signal input.Module according to one of the preceding claims, characterized in that the second signal path (26) has a second output element (S4) with two microwave signal inputs (56, 58) and a microwave signal output (60) as well as a mode signal input (64) and is configured to connect its first microwave signal input (56) or its second microwave signal input (58) to its microwave signal output (60) depending on a mode signal received at the mode signal input (64). Module according to one of the preceding claims 1 to 4, characterized in that the second signal path (26) has: a. A second input element (S3) having a microwave signal input (50) and at least one first and one second microwave signal output (52, 54), b. A second output element (S4) having at least one first and one second microwave signal input (56, 58) and one microwave signal output (60), c.A third signal path (66) between the first microwave signal output (52) of the second input element (S3) and the first microwave signal input (56) of the second output element (S4), d. A fourth signal path between the second microwave signal output (54) of the second input element (S3) and the second microwave signal input (56) of the second output element (S4), e. Where the attenuator (28) is provided in the third signal path (66), and wherein. i. the second input element (S3) has a mode signal input (62) and is configured to connect its microwave signal input (50) to its first or second microwave signal output (52, 54) depending on a mode signal received at the mode signal input (62), and / or ii. the second output element (S4) has a mode signal input (64) and is configured to connect its first microwave signal input (56) or its second microwave signal input (58) to its microwave signal output (60) depending on a mode signal received at the mode signal input (64). Module according to one of the preceding claims 5, 6, or 7, characterized in that one of the microwave signal outputs (52, 54) of the second input element (S3) and one of the microwave signal inputs (56, 58) of the second output element is connected to a termination circuit (68, 70).Module according to any one of the preceding claims 5 to 8, characterized in that the second input element (S3) and / or the second output element (S4) is configured as a high-frequency switching unit. Microwave solid-state power amplifier (100, 100.1) with a module (10, 10.1) according to any one of the preceding claims, wherein a microwave solid-state power amplifier module (110, 112, 114) is connected downstream of the module (10, 10.1). Microwave solid-state power amplifier according to claim 10, characterized in that a splitter (102) is provided between the module (10, 10.1) and the microwave solid-state power amplifier module (110, 112, 114). Microwave solid-state power generator (200, 200.1) with a module (10, 10.1) according to one of the preceding claims 1 to 9, characterized in that a microwave small-signal generator (202) is connected upstream of the module.Microwave solid-state power generator according to claim 12, characterized in that a splitter (102) is provided between the microwave small-signal generator and the module (10, 10.1). A method for generating pulsed microwave signals (42) of different amplitudes, comprising the following steps: a. generating a microwave signal, b. supplying the microwave signal to a module (10, 10.1) with a first and a second signal path (24, 26) of different attenuation, c. generating a pulse signal (40), d. transmitting the microwave signal via the first or second signal path (24, 26) depending on the pulse signal (40) to a microwave signal output (22) of the module (10, 10.1). The method according to claim 14, characterized in that a mode signal is generated and the microwave signal is transmitted via the second signal path (26) or routed to a termination circuit (68, 70) depending on the mode signal.A method according to any one of the preceding claims 14 or 15, characterized in that the pulse signal (40) is generated with a pulse length of at most 1 ps, in particular in the range of 10 ns to 40 ns, preferably in the range of 15 ns to 30 ns. A method according to any one of the preceding claims 14 to 16, characterized in that the pulse signal (40) is generated with a rise time and / or fall time of < 10 ns. A method according to any one of the preceding claims 14 to 17, characterized in that the pulse length of the pulse signal (40) is set in increments of < 1 ns.

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