WO1986005032A1

WO1986005032A1 - Device for shaping and generating electric pulses and use of said device as a protective circuit for electronic components

Info

Publication number: WO1986005032A1
Application number: PCT/SE1986/000069
Authority: WO
Inventors: Rune Persson; Walter Margulis
Original assignee: Rune Persson; Walter Margulis
Priority date: 1985-02-19
Filing date: 1986-02-18
Publication date: 1986-08-28
Also published as: EP0211932A1; AU5544186A; SE8500790D0; SE8500790L

Abstract

A device for passive shaping and generation of electric pulses from an electric signal incoming to the device, comprising a first coaxial conductor, which is connected along its extension to one or several pairs of coaxial conductors, of which one or several at their end located remotest from the first coaxial conductor are either short-circuited or open. According to the invention, said pair of coaxial conductors consist of only one coaxial conductor, viz. a second coaxial conductor (5). Said first coaxial conductor (2) is provided with a recess (6) extending from its shell surface (7) to its central conductor (8) and along an axial distance substantially corresponding to the outer diameter of said second coaxial conductor (5). Each of said second coaxial conductors (5) is provided with a recess (9) corresponding to said recess (6). The recesses (6, 9) of the two coaxial conductors (2, 5) so co-operate, that the central conductors (8) of the two coaxial conductors (2, 5) abut each other, and so that the shells (7, 10) of the two coaxial conductors (2, 5) are in electric connection with each other, where a connecting member (17, 18) is provided for retaining the two coaxial conductors (2, 5) in said electric connection with each other.

Description

"Device for shaping and generating electric pulses and use of said device as a pro tect i v e circuit for electronic components".

This invention relates to a device for shaping and generating electric pulses of short duration and to a use of said device.

The development of high-speed electronics and electro-optical components has rendered it possible to generate electric transients of the magnitude picosec onds (ps) and even fractions of picoseconds. It is, however, often desired to shape a generated signal by means of an independent device in order to produce a certain necessary shape of a pulse.

It has previously been proposed for the shaping and generation of pulses to utilize a coaxial conductor, which was connected along its extension to two coaxial conductors extending from the firstmentioned coaxial conductor, which two connected coaxial conductors either were short-circuited at the ends or had so-called open ends, i.e. were not connected to any further element. As example thereof can be mentioned devices consisting of four coaxial conductors, which were so coupled together that two constitute a transient conductor and two constitute reflecting elements.

Such devices, however, could not be used for the generation and shaping of pulses with a length of the magnitude picoseconds.

Such known devices, furthermore, were complicated to change in respect of, for example, the length of the reflecting elements, because the components comprised are coupled by soldering. At many applications of a pulse shaper of the kind here referred to it is necessary to be able to change the length of the reflecting elements. This requirement is especially essential when the pulse shaper is used for the generation of pulses, which are utilized for studying other electronic components, such as different types of semiconductor elements. The present invention provides, a device for shaping and generating pulses which is a considerable improvement over known similar devices. The foremost advantages of the present device are that it is extremely simple in its structure and that it is capable to operate with pulses having a rise time and/or duration in the range of picoseconds.

The present Invention, thus, refers to a device forppassive shaping and generation of electric pulses from an electric signal entering the device, comprising a first coaxial conductor, which along its extension is connected to one or several pairs of coaxial conductors, of which one or several at their ends located remotest from the first coaxial conductor either are short-circuited or open. The device is characterized in that said pair of coaxial conductors consist of only one coaxial conductor, viz. a second coaxial conductor, that said first coaxial conductor is provided with a recess extending from the outer surface of the conductor to its central conductor and along an axial distance corresponding substantially to the outer diameter of said second coaxial conductor, that each of said second coaxial conductors are provided with a recess corresponding to said recess, and that the recesses of the two coaxial conductors co-operate so that the central conductors of the two coaxial conductors abut each other , and so that the outer surfaces of the two coaxial conductors are electrically connected to each other, and that connecting means are provided for retaining the two coaxial conductors in said electric connection with each other.

The invention further refers to a use of a device comprising a first coaxial conductor, which along its extension is connected to one or several pairs of coaxial conductors, of which one or several at their ends located remotest from the first coaxial conductor are short- -circuited, where said pair of coaxial conductor con sist of only one coaxial conductor, viz. a second coaxial conductor, and a recess is provided in the first coaxial conductor and, respectively, the second coaxial conductor or conductors, so that the recesses of the two coaxial conductors so co-operate, that the central conductors of the two coaxial conductors abut each other, and so that the outer surfaces of the two coaxial conductors are in electric contact with each other, which said useis characterized in that the device is coupled-in as a protective circuit for electronic components, which protective circuit filters out electric signals, which vary slowly in relation to the time required for transporting electric signals in the two branches of said second coaxial conductor . The invention is described in greater detail in the following, with reference to the accompanying drawings showing and illustrating exemplified embodiments of the invention. In said drawings Fig. 1 is an exploded view of the device, Fig. 2 is a view of a detail of the lower coaxial cable in Fig. 1, Fig. 3 is a view from above of the coaxial cable in Fig. 2, Fig. 4 is a lateral view of the coaxial cable in Fig. 2, Fig. 5 shows an input signal to the present device. Figs. 6-11 show an output signal from different variants of the present device. In Fig. 1 a device 1 for passive shaping and generation of electric pulses is shown when an electric signal is supplied to it.

The device 1 comprises a first coaxial conductor 2, which is provided with high-speed connections 3,4, such as so-called SMA-connections. The coaxial conductor 2 can, for example, be a so-called 50 Ω -coaxial conductor with a length of 5 cm and an outer diameter of 3,6 mm. This coaxial conductor 2 is connected along its extension to one or several pairs of coaxial conductors. According to the invention, each such pair of coaxial conductors consist of only one coaxial conductor 5, which also is a so-called 50 Ω-coaxial conductor. The coaxial conductors consist of a metallic central conductor and a metallic shelly between which an insulation of a plastic material is located. The firstmentioned coaxial conductor 2 is provided with a recess 6, see Fig. 2, extending from its shell surface 7 to its central conductor 8. As appears from Fig. 2, the recess has such a depth that the central conductor 8 has been ground off partially, or preferably to about its half. The recess is carried out along an axial distance corresponding substantially to the outer diameter of the second coaxial conductor 5. Each of said second coaxial conductors 5 also is provided with a recess 9 corresponding to the recess in the first coaxial conductor. The recesses 6,9 of the two coaxial conductors 2,5 are arranged to co-operate so that the central conductors of the two coaxial conductors abut each other and thereby are connected to each other, and so that the respective shell surfaces 7,10 of the two coaxial conductors are connected to each other.

In Figs. 3 and 4 said recesses are shown in greater detail. Preferred measures concerning coaxial cables with an outer diameter of 3,6 mm are also shown. In order to prevent the central conductor 11 of the second coaxial conductor 5 to contact the shell 7 of the first coaxial conductor 2, the shell 7 is ground off along the straight edges of the recess 6 , so that the distance between the straight edges 12 and the upper surface 14 of the insulation 13 is about 0,2 mm, as best shown in Figs. 2 and 4. The second coaxial conductor 5 is provided with an identical recess as the first coaxial conductor 2.

The coaxial conductors preferably have a recess, the depth of which is slightly smaller than half its diameter, for example 1,6-1,7 mm.The shell surface of the first coaxial conductor 2 as well as that of the second coaxial conductor 5 preferably are ground off on both sides of the recess, so that plane surfaces 15, 16 are formed, as shown in Fig. 2 in order to facilitate joining of the two coaxial conductors as indicated in Fig. 1. The coaxial conductors 2,5 are combined to a cross by means of a connecting member, which retains the coaxial conductors in the aforesaid electric connection with each other. The connecting member comprises two portions 17, 18 matching each other, which in assembled state together form two intersecting channels 19,20, which respective channels have a diameter corresponding to the outer diameter of the coaxial conductors 2,5, and intohwhich channels the respective coaxial conductor is intended to be placed. The portions are made of an electrically conductive material, for example brass. The portions 17, 18 preferably are held together by means of a screw joint 21,22; 23,24; 25, 26. After the formation of the channel parts in the portions 17, 18, the grooved surface of one portion is ground off slightly, for example 0,1 mm, so that the channels in the assembly direction of the portions have a diameter slightly smaller than the outer diameter of the coaxial conductors. The channel conductors hereby can be pressed against each other by tightening the screws 21,22. According to one embodiment, the recess in said second coaxial conductor 5 is located centrally along the length of the coaxial conductor 5, whereby two branches of exactly the same length L are formed, where the entire length of the coaxial conductor 5 is 2L. Both free ends 27,28 of said second coaxial conductor 5 according to one embodiment are short-circuited or open. In Fig. 1 the right- hand end 27 is shown open. Short-circuiting of an end preferably is carried out, for example, by coating the end with an electrically conductive layer containing silver, whereby the central conductor 11 is electrically connected to the shell 10. According to another embodiment, one end 27 is open while the other end 28 is short-circuited.

The invention has above been exemplified by the first coaxial conductor 2 having only one recess, with which a second coaxial conductor co-operates. According to a further embodiment, however, several recesses are provided in the first coaxial conductor 2 in spaced relationship. Each of said recesses is intended to co-operate with a second coaxial conductor 5. The second coaxial conductors can have different length and/or be provided with open and/or short-circuited ends. Hereby an infinite number of modifications of an incoming electric signal can be carried out by the pulse shaper according to the present invention. The second coaxial conductor 5 candiave an angle to the first coaxial conductor 2 other than a right angle. It is, however, preferable to place the coaxial conductors at a rignt angle, because then the remaining shell has .exactly the same length from the recess to the respective free end.

By the extremely simple way of coupling two coaxial conductors as described above a.o. soldered joints are avoid- ed, but nevertheless a close and very good electric contact between the coaxial conductors is obtained. It was found that by the arrangement described above the stray inductance is reduced substantially, which has a highly important effect, viz. that the device becomes very rapid and thereby can operate in the time range faster than 100 picoseconds, which heretofore was not possible with devices of the type concerned, and especially in the time range of single picoseconds.

The present device, thus, implies that very short and rapid pulses can be processed by the present device. The present device is intended to receive an electric signal at its input 29 and thereby emit a signal at its output 30, which signal is changed compared to the input signal due to the number of second coaxial conductors 5, the branch length L thereof and whether their free ends are open or short-circuited. The present device, thus, is a passive element. For determining the output signal from the pulse shaper 1 for a given input signal in the form of a step pulse with the amplitude V_o, it is necessary to study expressions concerning the amplitude of the pulse V_t, transmitted through the first coaxial conductor and the pulse V_r reflected by the second coaxial conductor in response to an incoming pulse V_i. The following expressions concerning a transmission line including a discontinuity can be written: V_t/V_i = 2 Z_t/(Z_t + Z_i ) and V_r/V_i = (Z_t - Z_i)/(Z_t + Z_i) where Z_i and Z_t are the impedance for an incoming and, respectively, a transmitted pulse. For a short-circuited 50 Ω -coaxial conductor Z_i = 50 Ω and Z_t = 0Ω . This yields V_r = -V_i.

For a coaxial conductor with open end Z_i = 50 Ω and Z_t =∞Ω. This yields V_r = +V_i.

For an arrangement according to above where an incoming pulse can choose one of three identical ways Z_i = 50 Ω and Z_t = 16.67Ω . Thus, the pulse propagating in any one of them is V_t = V_i/2 .

An incoming step pulse, thus, with the amplitude V_o gives rise to a positive step with the amplitude V_o /2 at the output. It is assumed that both ends of the second coaxial conductor 5 are short-circuited to earth. It follows from the aforesaid, that each branch of the second coaxial conductor 5 gives rise to a signal with the amplitude -V_o/4 at the output. As a consequence thereof, thus, a positive step is followed by an identical step with inverse sign, where the later step follows after a time delay corresponding to the propagation time along a branch with the length L. At an experiment each branch at the second coaxial conductor 5 had a length in the interval 2 to 150 mm, which corresponds to a delay of about 20 ps to 1,5 ns.

Provided that the source of the incoming pulse and the load at the output both are adjusted to 50 Ω , the present device thereby did not give rise to any undesirable reflected additional pulses. With the present device, for example, a square pulse with a duration in response to the length of the second coaxial conductor can be created from an incoming step pulse. This is a very essential application of the present invention, especially as regards the aforesaid time range. In the attached Figures 5-10 examples of pulses are shown. These Figures are signal progresses described by means of oscilloscope. Fig. 5 shows the signal incoming to the input 29 of the device, and Figs. 6-10 show signals appearing at the output 30 of the device. In the Figures the following scales apply:

Fig.No. Vertical scale Horizontal scale (voltage,mV) (time, ps)

5 50 50

6 20 20 7 20 50

8 < 50 50

9 <50 50

10 20 100

The Figures 6-9 are taken from a device with only one second coaxial conductor 5, both ends of which are short- circuited. The Figures 6-9 show the output signal from the first coaxial conductor 2 for different branch lengths of the second coaxial conductor 5, which correspond to time delays according to below: Fig.No. Branch length (L) of Corresponding second coaxial conductor time delay (ps) (mm). . . .

6 3 28 7 5 47

8 10 93

9 15 140

As appears from the Figures 6 -9, an output signal is obtained which in respect of pulse duration and amplitude (V_o/2) corresponds to what is said above. Furthermore, the fact that even the fastest signals have symmetric shape, proves that the short-circuiting at the ends of the second coaxial cable reflects the electromagnetic waves into the coaxial cable without appreciable losses. This applies at least up to a frequency of about 15 Gc/s. A comparison between the Figures 6,8 and 9 shows clearly how the branch length (L) of the second coaxial cable influences the appearance of the output signal, where Fig. 9 almost can be regarded as a square wave. A great number of different pulses, thus, can be shaped by the present device in a simple way and during very short times.

Another essential application field for the present device is as derivator. Provided that the entire length of the second coaxial conductor 5 is sufficiently short, the output signal namely is proportional to the derivata of the input signal. This is due to the fact that for an input signal f(t) an output signal is obtained which is equal to

where δt is small.

It was found extremely advantageous to use the present device as derivator at the measuring of laser pulses in the time range picoseconds. As an example can be mention- ed that, when the input signal f(t) is a gauss-pulse with a width of 100 ps at half the top value (PWHM = Pull width half maximum), a differentiation of the pulse with an accuracy better than 5% takes place when the branch length of the second coaxial conductor corresponds to a time delay corresponding to 50 ps.

Provided that the ends of the second coaxial conductor 5 are not short-circuited to earth but open, the delayed reflected pulse will be added to the input signal and that continues to have the same sign and amplitude as the input signal. For an input signal in the shape of a step pulse the output signal is in staircase form where the length of the horizontal sections between the vertical steps is determined by the length of the second coaxial conductor 5. When, however, the input signal is gauss- -shaped or consists of a square wave, the output signal consists of two pulses of the same shape as the input signal separated at a distance which easily can be chosen by means of the branch length of the second coaxial conductor in the range of about 20 ps to 1,5 ns.

As mentioned above, the present device can comprise more than one second coaxial conductor. As an example an output signal is shown in Fig. 10 which is obtained from the input signal shown in Pig. 5. At this experiment a first coaxial conductor was used, in which two recesses were formed. Each one of the recesses co-operated with a second coaxial conductor. Each of the second coaxial conductors had a branch length of 7,5 mm, and their both ends were short-circuited to earth. The present device can also be used for generating a pair or more of separated pulses with very short duration. The device in this case is provided with two or more of said second coaxial conductors spaced from each other along the first coaxial conductor. With three of said second coaxial conductors four pulses are generated, with four of said second coaxial conductors eight pulses are generated, a.s.o. Seen in the wave propagation direction for the first coaxial conductor,the first one of the second coaxial conductors has short-circuited ends, while the second one of the second coaxial conductors has open ends. The conductor with short-circuited ends determines the duration of the pulses while the conductor with open ends determines the distance, i.e. the separation between the pulses. By this embodiment of the invention both the duration of the pulses and the separation between the pulses can be changed easily and rapidly.

In Fig. 11 a series of output signals taken up by an oscilloscope from such a device are shown, where between each take-up the zero-level was lowered. The input signal was a single step pulse of the type shown in Fig. 5 . The duration of the pulses was determined to be 50 ps while the separation between the pulses was varied from about 100 ps, which is illustrated by the uppermost curve, to about 1,4 ns, which is illustrated by the lowermost curve. The utilization of the device for the generation of two separated pulses with very short duration is advantageous at measurements on electronic components, such as transistors, integrated circuits, semiconductors, lasers, optical wave-guides etc., in respect of the capacity of the components to react on short pulses. It is, thus, apparent from the aforesaid that the present invention provides a pulse shaper, which owing to its structure can be utilized for the shaping and generation of pulses with very short duration and very short rise time. Its structural design renders it possible that the appearance and number of pulses easily can be varied by combining different coaxial cables of said first and, respectively, second type.

As mentioned above, the present invention also comprises a special use of the present device. According to this use, the present device is utilized as a protective circuit for electronic components, which are sensitive to transients of a voltage high for the components. The use is based in the fact that the device acts as a superfast filter for direct voltage components, because the output signal always is electrically earthed for signals, which vary slowly in relation to the time required for transporting the signal in the second coaxial conductor. At this use the ends of the second coaxial conductor are short-circuited. According to a preferred embodiment, at the use of the devvice as a protective circuit the said time delay is lower than 1 ns, preferably 50 ps, in each of the two branches of a second coaxial conductor. When, for example, the said time delay, due to the distance covered by the signal in the second wave-guide, is about 50 ps, voltage transients with a rise time of about 1 ns or longer are filtered out substantially entirely. More precisely, the output signal is only 5% of the input signal. Second coaxial conductors longer than corresponding to 50 ps can be used. The attenuation, however, depends on the ratio between said time delay and the rise time of the input signal.

As an example can be mentioned that at experiments anvoltage transient with the maximum voltage 50V and with a duration of several nanoseconds was applied on the input of the present device, while its output was connected to an electronic component adapted for a maximum input voltage of 5V. The transient was filtered out, and the electronic component was not affected. The present device, thus, can be used with great advantage as protective circuit for electronic components, where it has the special advantage of filtering out very fast transients, which are detrimental for the electric component or components in question. The present invention can be modified in many ways obvious to the expert. The detail design, for example, of said recess can be varied, a.o. in response to the utilization of coaxial conductors of a different type.

The present invention, thus, must not be regarded restricted to the embodiments described above, but can be varied within the scope of the attached claims.

In the above, coaxial conductors with a diameter of 3,6 mm were chosen as example. Other coaxial conductors, however, can be used. In certain cases the use of coaxial conductors with a diameter of 1,8 mm is preferred, because such conductors imply that shortare branch lengths can be obtained.

Claims

1. A device for passive shaping and generation of electric pulses from an electric signal incoming to the device, comprising a first coaxial conductor, which is connected along its extension to one or several pairs of coaxial conductors, of which one or several at the end located remotest from the first coaxial conductor are either short-circuited or open, c h a r a c t e r i z e d i n that said pair of coaxial conductors consists of only one coaxial conductor, viz. a second coaxial conductor (5), that said first coaxial conductor (2) is provided with a recess (6) extending from its shell surface (7) to its central conductor (8) and along an axial distance corresponding substantially to the outer diameter of said second coaxial conductor 5, that each of said second coaxial conductors (5) is provided with a recess (9) corresponding to said first recess (6), that the recesses (6,9) of the two coaxial conductors (2,5) so co-operate that the central conductors (8) of the two coaxial conductors (2,5) abut each other and so that the shells (7,10) of the two coaxial conductors (2,5) are in electric connection with each other, and that a connecting member (17,18) is provided for retaining the two coaxial conductors (2,5) in said electric connection with each other.

2. A device as defined in claim 1, c h a r a c t e r i z e d i n that the recess (9) in said second coaxial conductor (5) is located centrally on the coaxial conductor (5) along its length.

3. A device as defined in claim 1 or 2, c h a r a c t ¬e r i z e d i n that both free ends of the second coaxial conductor (5) are short-circuited or open.

4. A device as defined in claim 1 or 2, c h a r a c t e r i z e d i n that one free end of the second coaxial conductor (5) is short-circuited and the other end is open.

5. A device as defined in claim 1,2, 3 or 4, c h a r a c t e r i z e d i n that several recesses (6) are located along the length of the first coaxial conductor (2), each of which recesses is arranged to co-operate with a second coaxial conductor (5).

6. A device as defined in claim 1,2,3,4 or 5, c h a r a c t e r i z e d i n that the second coaxial conductor (5) forms a right angle with the first coaxial conductor (2).

7. A device as defined in claims 1,2,3,4,5 or 6, c h a r a c t e r i z e d i n that said connecting member comprises two portions (17, 18), which in assembled state form two intersecting channels (19,20), which respective channels have a diameter substantially corresponding to the respective outer diameter of the coaxial conductors (2,5), into which channels (19,20) the respective coaxial conductor are intended to be placed.

8. A device as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the length of each of said coaxial conductors is shorter than 150 mm.

9. A use of a device according to claim 1 comprising a first coaxial conductor (2), which is connected along its extension to one or several pairs of coaxial conductors, one or several of which at their ends located remotest from the first coaxial conductor are short-circuited, where said pair of coaxial conductors consist of only one coaxial conductor (5), viz. a second coaxial conductor, and where a recess (6;9) is located in the first (2) and, respectively, the second coaxial conductors (5), so that the recesses of the two coaxial conductors so co-operate, that the central conductors (8) of the two coaxial conductors (2,5) abut each other, and so that the shells (7,10) of the two coaxial conductors (2,5) are in electric contact with each other, c h a r a c t e r i z e d i n that said device is coupled in as a protective circuit for electronic components, which protective circuit filters out electric signals, which vary slowly in relation to the time required for transporting electric signals in the two branches of said second coaxial conductor (5).

10, A use as defined in claim 9, c h a r a c t e r i z e d i n that the device is designed so that said time is shorter than 1 ns, preferably 50 ps.