GB2188786A - Coaxial line pulse-shapers - Google Patents

Abstract

A pulse-shaper for generating very short pulses, eg down to 100 ps, from a fast input step-voltage, comprises first and second coaxial transmission lines 1, 2 having the same characteristic impedance z, and an open-ended stub coaxial transmission line 5 of one-way propagation-time tau connected between one or each pair of similar conductors (ie the inner and outer conductors respectively) of the first and second lines at one end of each line. As shown one such stub line is connected between the pair of inner conductors and the characteristic impedance is 2z; alternatively such a stub line, of characteristic impedance z, can be connected between each pair of similar conductors. When fed from a step-waveform generator G of output impedance z and into a load L of impedance z, a voltage pulse of half the step height and of duration 2 tau is produced across the load. In practice, the several lines are arranged coaxially with one another to alleviate the effect of stray capacitance. A unit for producing pulses of selectable duration is described. <IMAGE>

Description

SPECIFICATION Improvements in or relating to pulse-shapers This invention relates to pulse-shapers and has one application in generating very short pulses, eg down to 100 ps duration, from a fast input step-voltage.

According to the present invention a pulseshaper comprises: first and second coaxial transmission lines having the same characteristic impedance z; an open-ended stub coaxial transmission line of one-way propagation-time T connected between one or both pairs of similar conductors (inner and outer) of said first and second lines at one end of each said line, said stub line or lines being thereby effectively in series with said first and second lines, and presenting, in total, a series impedance equal to 2z; whereby when the other end of the first said line is connected to a step waveform generator of internal output impedance z and the other end of the second said line is connected to a load of impedance z, a voltage pulse of half the step amplitude and of duration 2r is produced across the load.

Where a said stub line is connected between only one pair of similar conductors, its characteristic impedance is 2z; where a said stub line is connected between both pairs of similar conductors, the characteristic impedance of each is z.

Preferably, in order to alleviate the effect of stray capacitance, said first and second lines, and said stub line or lines, are all arranged coaxially with one another to overlap over the electrical length of said stub line or lines, with the outer conductor of said first or second line outermost and the inner conductor of said first or second line innermost.

The present invention also provides a stubline pulse-shaper unit consisting essentially of only the overlapping portions of said lines and having a coaxial cable coupling at each end of said unit.

The present pulse-shaper may provide pulses of variable length by arranging, where only a single stub line is included, that the innermost conductor is a removably insert of selectable length. Alternatively, such variablelength pulses may be obtained by arranging that the appropriate conductors are tubes having telescopic portions; the latter construction can also provide a pulse-shaper including two stub lines, in which the first said transmission line has total DC isolation from the second said transmission line and a floating output is produced.

The present invention will now be described, by way of example, with reference to the accompanying drawings wherein: Fig 1 is a circuit diagram of an unbalancedoutput embodiment of the invention.

Fig 2 shows reflected waveforms in the circuit of Fig 1.

Figs 3 and 4 are equivalent forms of the circuit of Fig 1 using lumped components.

Fig 5 shows diagrammatically a coaxial form of the circuit of Fig 1.

Fig 6 is a simplified sectional elevation of a practical coaxial pulse-shaper unit corresponding to the circuit of Fig 5.

Fig 7 is a circuit diagram of a floating-output embodiment of the invention.

Fig 8 is a semi-diagrammatic sectional elevation of a coaxial pulse-shaper unit corresponding to the circuit of Fig 7.

Fig 9 shows the unit of Fig 8 adjusted to give a different pulse-length.

Fig 1 shows two coaxial transmission lines 1 and 2 having a characteristic impedance z and having input and output ends 3 and 4 respectively. Their outer conductors are earthed and between the other ends of their input conductors is connected an open-ended stub transmission line 5 of characteristic impedance 2z and one-way propagation-time T 50 that line 5 is effectively in series with lines 1 and 2. A resistive load L of impedance z is connected to end 4 and a step-waveform generator G having a resistive internal output impedance z is connected to end 3 as shown.

Suitably, for the invention to produce output pulses of 100 ps duration or less (full width at half maximum height), the generator G is a Tektronix TDR/Sampler 7S12 using an S-6 Sampling head and an S-52 Pulse Generator Eead. The S-52 has a rise-time of 25ps and an output of 250mV and the S-6 has a risetime of 30ps, giving a system rise-time of 45ps.

The operation of the circuit is as follows.

Assuming an output pulse of total height +V is required, a step (A in Fig 1) of height +2V is applied by generator G at end 3. When this step reaches stub 5 the equivalent circuit of Fig 3. applies, and +3V appears between points a and c. (Note that, applying Thevenin's Theorem, in the equivalent circuit of a line delivering a step output 2V, the equivalent internal generator has an output step voltage, 4V, which is twice the line input step, 2V.) +V (step B in Fig 2) is reflected back to G and there absorbed. +2V appears across stub 5 and +V is transmitted along line 2 to load L (step C in Fig 1) where it is absorbed.

The 2V step propagates to the open end of stub 5, is reflected and reappears after a time 2T between points a and b, and the equivalent circuit of Fig 4 now applies. The stub internal generator voltage 4V is balanced about earth and drives a split balanced internal impedance z+z=2z, that the 2V transient between points a and b is balanced about earth. +V (step D in Fig 2) returns to generator G and -V (step E in Fig 1) is transmitted to the load L and absorbed. Thus the load L sees a pulse of amplitude V and duration 2T and the generator G sees two reflected steps.

Where it is important to reproduce as far as possible the fast edge of the input step in the front and back edges of the output pulse, as is usually the case, the arrangement as shown in Fig 1 is not satisfactory. This is because both the inner and outer conductors of stub 5 are "live", and the stray capacitance of the outer conductor to its surroundings will slow these edges. This difficulty can be overcome by effectively locating stub 5 coaxially within either line 1 or line 2, and such an arrangement is shown in Fig 5, where the outer conductors of lines 1 and 2 form a continuous outer conductor and the stub 5 is located within line 2. The rods or tubes forming the inner conductors of lines 1 and 2 overlap to form the stub 5, with a reduced-diameter portion of the one extending within the other but insulated from it.

A practical embodiment of Fig 5 is shown in Fig 6. The unit shown is provided at each end with couplings 6, 7 for connection to standard coaxial cables (not shown) of characteristic impedance 50Q. It comprises an outer metal tube 8 joining the couplings 6 and 7, and insulated from an inner metal tube 9 connected to the central socket 10 of coupling 7 by dielectric material 11. Tube 9 terminates short of pin 12 of coupling 6. Pin 12 is connected to a central rod 13 which is a loose fit within a tube of dielectric material 14 which insulates pin 12 from tube 9.Rod 13 terminates short of socket 10 so that the electrical length of stub 5 extends between the free ends of tube 9 and rod 13 with characteristic impedance 100Q. Pin 12, carrying rod 13, is a screw fit in a fixed metal insert 15, so that rods 13 of different lengths can be inserted to vary the value of r.

In Figs 1, 5 and 6 the generator and load connections to the pulse shaper are reversible, ie the generator can be connected to line 2 (socket 7 in Fig 6) and the load to line 1 (socket 6 in Fig 6), or vice versa.

The arrangements of Figs 1, 5 and 6 provide an unbalanced output, ie one side of the output is at a fixed neutral potential such as earth. For some purposes an output which can be balanced (positive and negative) about earth is required, eg for operating a four-diode sampling bridge. The embodiments shown in Figs 7 and 8 provide such an output. Fig 7 is similar to Fig 1 except that, instead of their being joined together and earthed, a second open-ended stub line 16 is connected between the adjacent ends of the outer conductors of lines 1 and 2 as shown. Because stubs 5 and 16 are now effectively connected in series, each has characteristic impedance z so that in series they total 2z as in Fig 1.

For fast edges, Fig 7 has even greater disadvantages than Fig 1 as regards stray capacitance, since the adjacent ends of all four outer conductors are "live". This difficulty can be overcome by the construction shown seni-diagrammatically in Fig 8 from which the dielec tric insulation between the radially separated tubes is largely omitted for clarity.

In Fig 7 the outer conductor of stub 5 is connected to the inner conductor of line 2 and can therefore be located within line 2. Similarly, the outer conductor of line 1 can be located within stub 16. The structure can thus be arranged as in Fig 8, comprising three concentric tubes 17, 18 and 19. Line 1 is formed by tubes 18 and 19. Tube 18 is divided by a short annular insulator 21 into a LH portion 18c, a central portion 18b and a RH portion 18a. Tube 19 is divided into a LH portion 19b which forms withportion 1 8c the line 1, and a RH open-ended portion 1 9a which forms with portion 18b the stub 5. Tube 17 is divided into a LH open-ended portion 1 7a which forms with section 18c the stub 16, and a fixed RH portion 1 7b which forms with section 18a the line 2.The RH end of section 19a is tied by annular insulator 20 to the LH end of fixed section 18a, over which the RH end of section 1 8b can telescope; the RH end of portion 19 can telescope over the LH end of portion 1 9a and the RH end of portion 1 7a can telescope over the LH end of portion 17b.

Portion 1 8c is shown notionally tied to portion 1 9b by an annular insulator at 22 so that they move together, but in practice they are tied elsewhere in order to avoid reflective etc effects.

Fig 8 shows the arrangement set to form stubs 5 and i6 of length 2x and propagationtime T. If only a non-variable stublength and propagation-time are required, the telescoping facilities shown can be omitted, ie all the tube portions can be axially fixed, instead of only portions 17b, 18a aid 19a.

Fig 9 shows the arrangement of Fig 8 adjusted, by means of the telescopic portions, to stubs of length x and propagationtimes of l/2. The axial positions of insulator 20, with attached portions 18a and 19a, remain fixed, as does portion 17b. Portions 18c and 19b are moved rightwards by a distance x so that stub 5 is reduced to length x, portion 18b telescoping over portion 1 8a and portion 1 9a telescoping within portion 19b. Stub 16 is likewise reduced to length x by moving portion 1 7a rightwards by a distance 2x, portion 1 7a telescoping over portion 17b.

It will be seen that for a given change in stub length, portion 1 7a must move rightwards through twice the distance of portions 18c and 19b. These two movements can be effected simultaneously by means of a suitable mechanical linkage (not shown), but this is not essential. Using an oscilloscope, either stub can be adjusted to give the desired pulse length, which will have a back edge which decreases multi-stepwise until the other stub is also adjusted to the same length, as indicated by the conversion of the back edge to a single step.

It is important that the axial gap between portions 18b and 18c at insulator 21 should be as short as possible, in order to avoid a discontinuity in line 1 which would degrade the risetime of the front edge of the pulse.

With the arrangement of Fig 8, the RH ends of tubes 17 and 18 (line 2) have total DC isolation from the LH ends of tubes 18 and 19 (line 1), allowing either conductor of line 2 to be held at any desired potential.

In Figs 7 and 8, the generator and load connections are again reversible.

Instead of using thin-walled tube portions which telescope one within another in sliding electrical contact, as shown in Figs 8 and 9, in order to avoid impedance discontinuities (tending to unwanted reflections) it is preferred that the tube portions should be of equal diameter and be divided circumferentially into arcuate sectors which interdigitate with their longitudinal edges in contact and can slide axially relative to one another.

Claims (8)

1. A pulse-shaper comprising: first and second coaxial transmission lines having the same characteristic impedance z; an open-ended stub coaxial transmission line of one-way propagation-time T connected between one or both pairs of similar conductors (ie the inner and outer conductors respectively) of said first and second lines at one end of each said line, said stub line or lines being thereby effectively in series with said first and second lines, and presenting, in total, a series impedance equal to 2z; whereby, when the other end of said first line is connected to a step waveform generator of internal output impedance z and the other end of said second line is connected to a load of impedance z, a voltage pulse of the half the step amplitude and of duration 2T is produced across the load.

2. A pulse-shaper as claimed in claim 1 wherein a said stub line is connected between only one pair of similar conductors and its characteristic impedance is 2z.

3. A pulse-shaper as claimed in claim 1 wherein a said stub line is connected between both pairs of similar conductors and the characteristic impedance of each stub line is z.

4. A pulse-shaper as claimed in any of claims 1 to 3 wherein said first and second lines, and said stub line or lines, are all arranged coaxially with one another to overlap over the electrical length of said stub line or lines, with the outer conductor of said first or second line outermost and the inner conductor of said first or second line innermost.

5. A stub-line pulse-shaper unit consisting essentially of only the overlapping portions of the lines of a pulse-shaper as claimed in claim 4 and having a coaxial cable coupling at each end of said unit.

6. A pulse-shaper or pulse-shaper unit as claimed in claim 4 or claim 5, for providing pulses of variable length, wherein only a single stub line is included and wherein the innermost conductor is a removable insert of selectable length.

7. A pulse-shaper or pulse-shaper unit as claimed in claim 4 or claim 5, for providing pulses of variable length, wherein the appropriate conductors are tubes having telescopic portions.

8. A pulse-shaper or pulse-shaper unit substantially as hereinbefore described with reference to the accompanying drawings.