Transition time converter

Abstract: A transition time converter consists basically of a first reactance (23) in a horizontal path between an input terminal (21) and an output terminal (22), as well as a second reactance (24) opposite to the reactance of the first reactance element (23), and a dissipative resistance element (25) in series therewith, in a vertical path between input terminal (21) and a reference potential (26). This transition time converter provides considerably improved reflection characteristics, as compared to prior art transition time converters.

Description: FIELD OF THE INVENTION The present invention relates to a circuit for changing, in particular increasing, the transition time of an electric or electronic pulse. DESCRIPTION OF THE PRIOR ART So-called transition time converters are used to change, namely to increase, the transition time, i.e., the rise time and/or the fall time, of electric or electronic pulses. An important application of transition time converters of this type is in the field of pulse generators, or, in more general terms, logical signal sources. Most prior art signal sources of this kind provided adjustable rise or fall times. This is an important feature as the transition times of output pulses generated by the signal source have to be adapted to the operating speed of the circuitry which is supplied by the signal source (for example, pulses with unmatched transition time may overdrive the supplied circuitry). Adjustable rise or fall times impose no major implementation problems in case of signal sources with limited bandwidth, say 300 MHz. However, there is a recent demand for signal sources with extremely high bandwidth, or, what is effectively the same, for signal sources which are able to generate pulses with very fast transition times. Such signal sources operate at bandwidths of 1 GHz, or even 3 GHz and higher frequencies. It is impossible (or at least extremely costly) to provide adjustable transition times in such high-frequency signal sources. Therefore, logical signal sources of this kind usually do not provide transition time adjustability. Instead, they generate pulses with fixed (very fast) rise and fall times. Adjustment of the transition time is performed in that a multiplicity of external transition time converters are provided for connection to the output jack of the signal source. All of these transition time converters decelerate (increase) the transition times of pulses for a certain, but different, amount. In use, the transition time converter providing adequate transition time deceleration for a particular application is selected and connected to the output of the signal source. The output of the transition time converter then supplies pulses with adequate rise and fall times. One known possibility to increase the transition time of an output pulse is simply to provide a capacitor between the signal output and a reference output (usually ground) of the signal source. Its capacitance (together with the resistance of the circuit) determines the increase of the transition time. An equivalent solution is the provision of a coil in the signal output path (a "coil" as used herein may also be a simulated coil, e.g., using a gyrator or a circulator, or any other kind of inductance element, like a simple wire which acts as an inductance in high frequency applications). It will be appreciated that a single capacitor, or a single coil, does not provide the desired symmetric transitions, but rather an exponential output function. Therefore, most prior art transition time converters used a more sophisticated approach, namely an LC filter (for example, a T-type or π-type low pass filter), in order to increase the transition time. The LC filters were mounted in separate housings each, and the one suited for a particular application was connected with the output of the signal source. However, it turned out that even this solution - although suited to provide quite symmetric transitions - did not completely fulfil all needs. In particular, it turned out that the LC filter caused reflections in response to an incoming step wave. As a match of the LC filter's characteristic impedance to the output impedance of the signal source is only possible at a particular frequency, and as a step function, in turn, contains a multiplicity of frequencies, a match of the LC filter can generally not be performed. The reflections, with an amplitude usually in the amount of 70% of the incoming step wave amplitude, caused various signal integrity problems. For example, a reflected wave could interfere with the next pulse or transition and distort the same. On the other hand, as the output impedance of the signal source could not match with the LC filter's characteristic impedance (see discussion above), the reflections were reflected at the output of the signal source again, resulting in a wave travelling forth and back which even distorted the original pulse, e.g., its transition. Consequently, there is a need for an improved transition time converter which avoids the above drawbacks. SUMMARY OF THE INVENTION It is thus a major object of the present invention to provide a circuit for changing the transition time of an electric or electronic pulse of the kind described above which does not generate reflections, or at least reduce the reflections, in response to an incoming signal, but still provides accurate conversion of the transition time. According to the present invention, this object is solved, in a circuit as defined in claim 1. (A "reactance element" means essentially an impedance element with imaginary resistance. An "opposite reactance" in the terms used herein means that its imaginary resistance has the opposite sign, but the same absolute value, as the reactance to which it refers.) DE-A-37 30 578 already depicts a low pass filter with coils in the horizontal path and capacitances and resistors in the vertical path. However, this low pass filter is not designed to operate as a transition time converter, nor is its input matched. In a transition time converter destined to increase the transition time, the first reactance element (in the vertical path) will be a capacitor, and the second reactance element (in the horizontal path) will be a coil (or a simulated coil, see discussion above). The resistor is thus arranged in series with the capacitor in the vertical path. With respect to the high-frequency components of an incoming signal, the capacitor acts as a short circuit. Such high-frequency components pass therefore along the capacitor path, i.e., along the vertical path of the circuit. This path also contains the dissipative resistance element mentioned above; it is actually the only effective element in the vertical path. Therefore, incoming high-frequency components are fed via this dissipative resistance element. Detailed tests have shown that reflections are drastically reduced, or even completely avoided, by means of this measure. The dissipative resistance element can be selected to match at least approximately the output impedance of an electronic signal source, or the output impedance of the signal source can be selected to match the dissipative resistance element. This is a particularly advantageous embodiment in which no reflections occur at all because the input impedance of the transition time converter is matched to the output impedance of the signal source. The use of microstrip lines provides several related advantages such as compact design. This is an important point because the transition time converter will preferably be a separate component for connection to the output jack of a logical signal source such as a pulse or data generator. However, there is also another important point: It will be noted that, in the case of an intended increase of the transition time, the high-frequency path runs along the vertical paths, i.e., the capacitors. The above disclosed preferred design provides basically straight high-frequency paths, such that essentially no internal HF reflections in the transition time converter can occur. This increases the performance of the output pulse considerably. It will also be understood that this concept may be expanded to the case of a third reactance element, such as a capacitor. In this case, it is preferred to have two basically straight microstrip lines, one connecting an input terminal of the converter with the first reactance element (e.g., capacitor), and the other connecting the third reactance element (preferably also a capacitor) with an output terminal of the converter. In another advantageous embodiment of the present invention, the circuitry is mounted on a printed circuit board or a hybrid with metallized reference potential back plane, the dissipative resistance elements are arranged at or near the edge of the printed circuit board or the hybrid. This design provides an easy and space-saving connection of the dissipative resistance elements to reference potential (preferably ground), wherein possible internal reflections are avoided. The metallized back plane establishes a stable reference potential and thus increases the accuracy and performance of the circuit further. In order to increase handling efficiency and to shield the transition time converter against electromagnetic radiation, the circuit is, in another preferred embodiment of the invention, mounted in a separate package (preferably a metallized or metallic casing), for connection to the output of an electronic signal source. It is understood and expressly noted that the present invention relates to all useful and novel combinations of the above disclosed features, whether alone or in any other or arbitrary combination. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be explained with reference to the accompanying drawings, in which: Figs. 1a to 1d show various prior art transition time converters, Fig. 2 depicts the circuit of a first embodiment of the present invention, Fig. 3 is the circuit of a second embodiment, Fig. 4 illustrates the circuit of a third embodiment of the present invention, Fig. 5 is a timing diagram relating to a prior art transition time converter and illustrating the transmitted, as well as the reflected wave in case of an incoming pulse, Fig. 6 is a similar timing diagram illustrating the effects of the present invention, and Fig. 7 illustrates a preferred circuit layout for a transition time converter according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1a depicts an example of a very simple prior art transition time converter suited to increase the transition time. The transition time converter, generally outlined as 1, consists basically of a coil 2 which is incorporated in a housing (indicated by dotted line 3). An input terminal 4 is connected with the output of a signal source which generates fast, but not adjustable edges or transitions. An output terminal 5 of the transition time converter is connected with a load (indicated by dotted symbol 6). The second terminal of load 6 is connected with a reference potential, preferably ground (reference no. 7). Coil 2 establishes a resistance primarily effective with respect to high-frequency components, as is well known in the art. The coil does therefore increase the transition time of an incoming (positive or negative) edge of a pulse. A transition time converter of this type is particularly suited for adaptation of a logical signal source with fast, but unadjustable transition times to logic circuitry requiring slower edges. For this purpose, a multiplicity of transition time converters of the type shown in Fig. 1a is provided, each with a different inductance of the coil. The user may simply select the converter with the inductance providing adequate transition time conversion for his particular application circuit, and connect the corresponding transition time converter with the output jack of his signal source. It will be appreciated that the circuit shown in Fig. 1a does not provide very symmetric conversion of an edge, as the output voltage of coil 2 will be an exponentional function which can, for practical purposes, only be regarded as piecewise linear. The same applies to the second prior art solution shown in Fig. 1b which uses a capacitor 8, instead of coil 2 (the other reference numbers of Fig. 1a have been kept in Fig. 1b, but with an additional "a" after the corresponding reference numbers). Fig. 1c depicts a more sophisticated approach of a transition time converter according to the prior art. Basically, the transition time converter 1b consists of two inductances 9 and 10, as well as a capacitor 11. Those skilled in the art will note that the LC components of transition time converter 1b are essentially wired as a low pass in basic T-configuration. The characteristics of transition time converter 1c as an LC filter improves considerably the characteristics of the output pulse, in particular in terms of linearity. However, the LC filter does still cause considerable reflections at its input terminal. Reference is particularly made to Fig. 5 which depicts a timing diagram of a voltage 12 at the output of a voltage source feeding input terminal 4b, as well as a voltage 13 at output terminal 5b, of transition time converter 1b in Fig. 1c (the horizontal axis indicates time, and the vertical axis voltage). The diagram of Fig. 5 is based on an actual recording with an oscilloscope with built-in time domain reflectometer. Transition 14 is the voltage transition generated by the signal source. This steep edge is adequately transformed into a smoother transition 15 of output voltage 13. However, a reflection occurs at the output terminal of the signal source, see reference number 16 (it is delayed with respect to edge 14 because the diagram of Fig. 5 has been recorded directly at the output of the signal source). The reflection 16 disturbs the output voltage of the signal source and may superimpose with the next transition, thus disturbing the same. Even worse, the reflected wave may be reflected at the output terminal of the signal source again, resulting in a wave traveling forth and back which may, in turn, affect even the original transition. Another prior art approach is depicted in Fig. 1d. The transition time converter 1c shown in figure uses also a low-pass LC filter; however, the low-pass is wired in basic n-configuration and consists of inductance 17, as well as capacitors 18 and 19. The operational characteristics of transition time converter 1c corresponds to that of converter 1b in Fig. 1c. A first embodiment of a transition time converter according to the present invention is shown in Fig. 2. Reference number 20 relates to the converter as a whole. The respective input and output terminals are labeled as 21 and 22. The new transition time converter consists basically of an inductance 23, a capacitor 24 and an ohmic resistor 25. Capacitor 24 and resistor 25 are connected in series, and further with ground 26 (or another reference potential). The load is indicated in dotted lines and labeled as 27. All of the components in this circuit are impedance matched. In particular, inductance 23 and capacitor 24 are adapted to the output impedance of the signal source connected to input terminal 21; i.e., they are selected such that they fulfil the equation R Z = L C wherein R Z is the characteristic output impedance of the signal source, L is the inductance of coil 23 and C is the capacitance of capacitor 24. Likewise, the input impedance of load 27 is matched to the characteristic impedance of the transition time converter, as well as to the output impedance of the signal source. It is common in the art to choose all of these impedances as 50Ω. Further, resistor 25 is selected such that it fulfils the equation R = R z = L C . This selection is optimal with respect to reflections, i.e. selecting R (the resistance of resistor 25) according to the above equation avoids any reflections at input terminal 21. This can be easily seen from the following consideration: The input impedance of transition time convertor 20 is, in complex notation, R in = ( 1 jωC + R) (jωL + R z ) 1 jωC + R + jωL + R z wherein R in is the input impedance of the converter, R is the resistance of resistor 25, R Z is the input impedance of load 27 (which has been selected equal to the output impedance of the signal source), C is the capacitance of capacitor 24 and L is the inductance of coil 23. If we demand that the input impedance is equal to the characteristic impedance, i.e., R in = R Z , and if further the reactances are impedance matched such that jωL + 1 jωC = 0, then the original equation is simplified to R Z = L C + jωLR - jωLR Z + RR Z R + R Z . This leads to the equation R = R Z 2 jωL - L C jωL + R Z = R Z 2 jωL - R Z 2 jωL + R Z = R Z . That is, the resistance of resistor 25 has to be selected equal to the output impedance of the signal source, in order to obtain optimum reflection characteristics. If we regard the parallel paths constitued by capacitor 24 and resistor 25, and inductance 23 and input impedance 27, respectively, we note that the two impedances in the two paths are conjugate-complex to each other, i.e., the amounts of the (real) ohmic resistances are equal, and the imaginary parts have the same absolute value, but different signs. Another, more sophisticated embodiment of the transition time converter according to present invention is shown in Fig. 3. Components or items which are identical to the circuit in Fig. 2 have been assigned the same reference numbers, but with an additional "a". One will note that two additional components have been introduced in the embodiment of Fig. 3, namely capacitor 32 and resistor 33. Their values are identical to capacitor 24a and resistor 25a, respectively. The purpose of additional components 32 and 33 is to make the structure of the transition time converter symmetrical or reciprocal such that it does not matter which side of the transistion time converter is used as the input, or the output, respectively, and further to increase the linearity of a transition. Under certain circumstances, the performance of the transition time converter - in terms of its reflection characteristics - may be slightly decreased, as compared to the embodiment of Fig. 2; however, symmetry may sometimes be a more important consideration. A further embodiment of a transition time converter 20b, which provides even more linear transitions, is shown in Fig. 4. In this embodiment, an additional inductance 34, an additional capacitor 35, and an additional resistor 36 are provided. The inductance of coil 35 is equal to the inductance of coil 23b; similarly, the capacitance of capacitor 35 is equal to the capacitance of capacitors 24b and 32b, and the resistance of resistor 36 is equal to the resistance of resistors 25b and 33b. It will be appreciated that further inductances, capacitors and resistors may be added, in the same scheme as shown in Fig. 4, in order to obtain improved transfer characteristics of the transition time converter. The timing diagram of Fig. 6 depicts traces actually recorded with the new transition time converter (specifically, the circuit of Fig. 4) in a similar representation as Fig. 5. Reference number 28 relates to the pulse generated by a signal source, and reference number 29 designates the output pulse of the transition time converter. Edge 30 of the incoming pulse is a very fast (40 ps) positive transition. This transition is transformed into a slower transition 31 of the output pulse. It will particularly be noted that no or only minimal reflections (as compared to reflection 16 in Fig. 5) appear in trace 28. Only a small ripple can be observed. Further, even the output pulse 29 shows a smoother transition than in the prior art embodiment. Fig. 7 depicts one preferred example of the physical layout of a transition time converter as described hereinbefore. The circuit is mounted on a printed circuit board or hybrid 37. A signal source (not shown) is connected, via lead 38, with a microstrip line 39. Microstrip line 39 runs straight; in the shown example, it runs parallel to the edges of the PC board or hybrid 37. The inductance or coil in this case is a simple wire 40 connecting microstrip line 39 with a further microstrip line 41. The output signal of the transition time converter is fed, via lead 42, to a load (not shown). Both sides of inductance 40 are connected, via a respective miniaturized capacitors 43 and 44, with microstrip lines 45 and 46. A resistor 47 connects microstrip line 46 with the back plane of PC board or hybrid 37; this back plane is metallized and connected with a reference potential such as ground. Similarly, microstrip line 45 is connected with the back plane via a resistor (not shown) which is positioned at the sidewall of PC board or hybrid 37. Capacitors 43 and 44, as well as resistor 47, are directly soldered onto the microstrip lines. The metallized back plane provides an accurate reference potential and thus avoids jitter or ripple of the output pulse. The special position of the resistors helps to save space. Further, it will be noted that the paths for the high-frequency components, namely microstrip lines 39 and 41, run basically straight, such that no high-frequency reflections may occur. The whole circuit shown in Fig. 7 is preferably integrated in a metallized or metallic tube. A multiplicity of transition time converters of different type may be provided, in order to generate different transitions. The degree of increase of the transition time may be determined by appropriate selection of the reactive components, that is, the inductances and capacitors. In a preferred embodiment such as shown in Fig. 4, the values of the single components are as follows: L 23b = L 34 = 17.5 nH C 24b = C 35 = 4.7 pF C 32b = 9.4 pF R 25b = R 36 - 50 Ω R 33b = 25 Ω These values have been selected for a transition time converter which generates a 1 ns (1000 ps) transition.

Claims: A transition time converter circuit (20) for changing, in particular increasing, the transition time of an electric or electronic pulse, in particular the output pulse of an electronic signal source such as a pulse generator, wherein said electric or electronic pulse is the output pulse of an electronic signal source with a characteristic output impedance R Z , said circuit (20) comprising: (1.1) a first capacitor (24) and a first dissipative resistance element (25), preferably an ohmic resistor (25-25b), both arranged in series in a first vertical path between an input terminal (21-21b) of the circuit and a reference potential (26-26b), preferably ground potential, (1.2) a first coil (23) having an impedance conjugate-complex to the impedance of the first capacitor (24), the first coil (23) being arranged in a horizontal path with a first terminal of the first coil (23) connected to said input terminal (21-21b) of the circuit and a second terminal of the first coil (23) connected to an output terminal (22-22b) of the circuit, and (1.3) said first coil (23) and said first capacitor (24) are selected such that they fulfil the equation R Z = L C wherein L is the inductance of the first coil (23) and C is the capacitance of the first capacitor (24). The circuit (20) according to claim 1, wherein the output of said circuit is connected or connectable with a load (27-27b) which has a characteristic impedance basically equal to said characteristic output impedance R Z of said electronic signal source. The circuit (20) according to claim 1 or 2, wherein said first dissipative resistance element is selected such that it fulfils the equation R = R Z wherein R is the ohmic resistance of said first dissipative resistance element. The circuit (20) according to any one of the above claims, further comprising: a second capacitor (32) and a second dissipative resistance element (33), both arranged in series in a second vertical path between the second terminal of the first coil (23) and said reference potential (26a). The circuit (20) according to claim 4, further comprising: (5.1) a second coil (34) arranged in said horizontal path between the second terminal of the first coil (23) and said output terminal (22b) of said circuit, and (5.2) a third capacitor (32) and a third dissipative resistance element (33), both arranged in series in a third vertical path between said output terminal (22b) of said circuit and said reference potential (26a). The circuit (20) according to any one of the above claims, wherein: (6.1) said circuit is mounted on a printed circuit board or a hybrid (37), (6.2) the interconnections between said capacitor(s), said coil(s) and said dissipative resistance element(s) are made by means of micro strip lines (39,41,45,46) on the upper plane of said printed circuit board or hybrid (37), and (6.3) the micro strip line (39) between said input terminal and said coil(s) is essentially straight. The circuit (20) according to any one of the above claims 1 to 5, wherein (7.1) said circuit is mounted on a printed circuit board or a hybrid (37) with metallized reference potential back plane, (7.2) the interconnections between said capacitor(s), said coil(s) and said dissipative resistance element(s) are made by means of micro strip lines (39,41,45,46) on the upper plane of said printed circuit board or hybrid (37), and (7.3) said dissipative resistance element(s) are arranged at or near the edge of said printed circuit board or said hybrid (37). The circuit (20) according to any one of the above claims, wherein said circuit is mounted in a separate package, preferably a metallized or metallic tube, for connection to the output of an electronic signal source.

Inventors: SCHINZEL PETER, SCHINZEL, PETER
Applicants: HEWLETT PACKARD GMBH, HEWLETT-PACKARD GMBH
Application year: 1992
Main classification: H03K5
IPC codes: H03K5, H01P1, H03K3