Dual channel time domain reflectometer

Abstract: A dual channel time domain reflectometer includes a pair of input lines connected at respective nodes to a refer­ ence flat current pulse generator and a traveling wave sampling gate, respectively. The sampling gates are actuated by a balanced strobe generator which includes a wavelength coupler for coupling a high amplitude fast rise time pulse to each of the gates simultaneously. Pulses of requisite ampli­ tude and shape are generated by a circuit responsive to a strobe trigger input which drives a step recovery diode.

Description: BACKGROUND OF THE INVENTION The following invention relates to a dual channel time domain reflectometer in which two data channels may be synchronously sampled with an extremely high degree of data resolution and accuracy. Time domain reflectometers (TDR's) are used to measure impedance anomalies in cables and other devices under test. Typically, a time domain reflectometer comprises a current pulse source which is connected to a cable under test or a cable leading to a device under test. Another point on the cable is connected to an oscilloscope. The current source provides a pulse to the cable/device under test and any impedance discontinuities or mismatches may be viewed as reflections of the current pulse on an oscilloscope. An example of such a system is shown in Frye, U.S. patent No. 3,434,049. Advances in semiconductor chip technology have created a need for measuring and analyzing integrated circuit devices which respond very quickly and with minimal distortion to input waveforms. Such devices are difficult to test, however, because of the inherent data resolution limitations in the current generation of test equipment. For example, it would frequently be advantageous to know the propogation time of a pulse through an integrated circuit, and also the shape of the output waveform as a function of a test input. These measurements are not possible in a single channel time domain reflectometer, which can only measure a reflection of an input pulse at either the input or the output of a device. It is possible to tie together two identical time domain reflectometers in order to make differential measurements, or to measure both the reflection of a test pulse at the input of a device under test and the resulting output pulse. But, while in theory such an arrangement can be constructed, as a practical matter it would have little utility due to timing errors which would make it impossible to sample both the input and output of a device under test at the same time. In order to sample and analyze differential inputs or both a reflected input pulse and the resulting output pulse from a device under test, it is necessary to have extremely accurate timing resolution. Moreover, it is necessary to ensure that the test pulse is isolated from the sampling circuit so that internal reflections of the test pulse do not degrade the input waveforms. In prior art devices such as that shown in FIG. 1, a current pulse generator 1 is connected to a sampling head 2 by means of a transmission line 3. The sampling head 2 is then connected to a device under test (DUT) 4. At the interface between the transmission line 3 leading from the current pulse generator 1 to the sampling head 2, however, multiple reflections of a test pulse could occur which effectively degrade the wave shape of any input pulse to be sampled by the sampling head 2. Moreover, with this type of arrangement, a dual channel TDR having the necessary timing resolution would be difficult to construct because of ambiguities in the timing of pulses transmitted over the internal transmission line. SUMMARY OF THE INVENTION The dual channel time domain reflectometer of the present invention comprises pair of input channels in which each of the input channels includes an internal input line. A reference flat pulse current generator for generating test pulses is coupled to each channel along with a travelling wave sampling gate for sampling the reflections of the test pulses in each of the channels. Each input line, together with its respective pulse generator and sampling gate, is connected at a node so that the test pulses are generated at the same internal location at which the reflections of the test pulses are sampled. This avoids multiple internal reflections such as those caused by transmission lines, which, in the prior art, had been used to link the sampling head with the current test pulse generator. In order to precisely control the sampling of the reflected test pulse (or of a test pulse propagated through a device under test) on both channels, a balanced strobe generator is connected to each sampling gate so as to simultaneously actuate both sampling gates. Simultaneous actuation is provided by a waveguide coupler which provides actuating pulses in response to a pulse from a driver circuit. The waveguide coupler includes a ground plane conductor having two slots disposed at right angles to one another. A signal-carrying conductor is disposed within one of the slots to form a coplanar waveguide. The signal-carrying conductor is connected to the driver circuit and each of the other slots are connected to the sampling gates in each respective channel. The slots function as slot lines which are inductively coupled to the coplanar waveguide. When a pulse is generated by the driver circuit, it travels down the coplanar waveguide to a terminating resistance. This pulse induces a pair of sampling control pulses which are opposite in phase but equal in amplitude in each of the slot lines, which are disposed orthogonally with respect to the coplanar waveguide. These balanced pairs of pulses for each channel arrive at both sampling gates at the same time so that data on both channels is sampled simultaneously. In order to use the waveguide coupler referred to above, it is necessary to generate a high amplitude pulse having a very fast rise time. This is accomplished through the use of a step recovery diode driver circuit. A step recovery diode is connected between the ground plane and the signal line of the coplanar waveguide. Connected by an inductive bond wire connection to the cathode of the step recovery diode is a driver circuit including an electronic switch pair in which one of the electronic switches is pre-biased to provide extremely fast switching for the other electronic switch. This circuit is responsive to short, low amplitude trigger pulses provided by a strobe pulse generator, and provides pulses to the signal-carrying line of the coplanar waveguide having extremely fast rise times and high amplitude sufficient to generate the matched pair of sampling control pulses. Due to the presence of the waveguide coupler, the two channels are located adjacent one another at the front panel of the dual channel time domain reflectometer. Each input channel includes a signal line which is surrounded by a ground conductor and separated from the signal line by air. A connection node exists at opposide ends of the slot lines in the waveguide coupler where the sampling gates for each channel are located. At these node locations the coaxial ground is cut away from the signal line and a bond wire connection is made from the sampling head circuitry to the signal line. On the opposite side of the input line in each channel, a flat pulse current generator circuit is located which is also connected to each respective node. Thus the test pulses are generated in each respective channel and sampled at the same electrical and physical location. This insures that sampling occurs in both channels simultaneously, thereby providing a high degree of timing resolution with no internal degradation of the signal due to multiple internal reflections along connecting cables. It is a principal object of this invention to provide a dual channel time domain reflectometer having a very high degree of timing resolution and measurement accuracy. Yet a further object of this invention is to reduce distortion of both test and data signals in a dual channel time domain reflectometer by providing an input connection configuration in which each channel includes a common connection node to an input line for a traveling wave sampling gate and a reference flat pulse current generator. A still further object of this invention is to provide a waveguide coupler for providing actuating pulses to both channels of a dual channel TDR so as to sample data on each channel simultaneously. Yet a further object of this invention is to provide a driver circuit responsive to a strobe pulse generator for producing a fast rise time, high amplitude pulse for generating sampling control pulses in a waveguide coupler so that both channels may be sampled simultaneously. The foregoing and other objectives, features and advantages of the present invention will be more readily understood upon consideration of the following detailed description of the invention taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block schematic diagram of a prior art single-channel time domain reflectometer. FIG. 2 is a block schematic diagram of a dual channel time domain reflectometer of the present invention. FIG. 3 is a schematic diagram including details of the mechanical layout of the electrical connections in the diagram of FIG. 2 including a waveguide coupler for coupling the output of a driver circuit to each of the input channels of the dual channel time domain reflectometer of FIG. 2. FIG. 4 is a schematic circuit diagram of a step recovery diode driver circuit which is part of the balanced strobe generator of FIG. 2. DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 2, a dual channel time domain reflectometer (TDR) 10 includes a first input channel 12 and a second input channel 14. Internal input lines 16 and 18 are connected to nodes 20 and 22, respectively. A terminating impedance Z₀ is connected to each of the nodes 20 and 22 by way of a transmission line (not shown in FIG. 2). Connected to nodes 20 and 22 are a pair of traveling wave sampling gates 24 and 26. The sampling gates 24 and 26 are driven by a balanced strobe generator 28. Also connected to nodes 20 and 22 are a pair of reference flat current pulse generators 30 and 32, respectively. The physical layout of the block diagram of FIG. 2 within the chassis of the dual channel TDR 10 is shown in FIG. 3. Channel inputs 12 and 14 include a pair of input jacks 34 and 36, respectively, located on the outside of a chassis panel 38. Behind the chassis panel 38 and connected to jacks 34 and 36 are internal input lines 16 and 18, respectively. The internal input lines 16 and 18 are transmission lines which include an outer coaxial sheath 40 surrounding a signal-carrying line 42. Both internal input lines 16 and 18 include cutaway portions 44 and 46, respectively, where the coaxial shielding 40 is stripped away from the signal-carrying conductor 42. This is to expose a portion of each signal-carrying line 42 so that it can be connected at nodes 20 and 22, respectively (indicated by the dotted outlines), to the respective traveling wave sampling gate and reference flat current pulse generator for each channel. On internal input line 16 reference flat pulse generator 30 is connected to node 20 as is traveling wave sampling gate 24. At node 22, the internal input line 18 is connected to traveling wave sampling gate 26 and reference flat pulse generator 32. This arrangement places both nodes 20 and 22 just behind front panel jacks 34 and 36 adjacent one another. This physical layout helps to provide precise synchronization in the sampling of signals on internal input lines 16 and 18, and minimizes signal degradation internally of chassis panel 38 due to the effects of stray capacitance. Two connections at both nodes 20 and 22 are shown for connecting traveling wave sampling gates 24 and 26 and reference flat current pulse generators 30 and 32 to their respective nodes. The reason for this type of connection is due to the use of a diode bridge at the point of connection, an example of which may be found in U.S. Patent No. 4647795 issue 3 March 1987, entitled "Travelling Wave Sampler" and assigned to the same assignee. Similarly, a suitable reference flat current pulse generator of a type suitable for use in the present invention may be found in U.S. Patent No. 4758736 issue 19 July 1988 entitled "Fast Transition Flat Pulse Generator" and assigned to the same assignee. In order to assure that signals on input lines 16 and 18 are sampled synchronously, a waveguide coupler indicated generally at 48 is provided to couple a trigger pulse generated by a strobe pulse generator (not shown) to traveling wave sampling gates 24 and 26, respectively. The waveguide coupler 48 includes a diamond-shaped ground plane 52 including orthogonally disposed slots 54 and 56. (The shape of the ground plane 52 is not material to the purposes of the invention, and other considerations may dictate the most convenient shape.) Slot 56 includes a signal carrying line 58 which begins at step recovery diode 60 and terminates at a resistor R₀. The cathode of the step recovery diode 60 is connected to signal-carrying line 58 while its anode is connected to the ground plane 52. A driver circuit 62 is also connected to the cathode of step recovery diode 60 through inductor L1. The timing of a strobe trigger pulse applied to traveling wave sampling gates 24 and 26 is controlled by a strobe pulse generator (not shown) which may, in turn, be under the control of a computer or other microprocessor-based system (not shown). Each traveling wave sampling gate 24, 26 is actuated only upon receipt of a pair of relatively high amplitude, fast rise time, exponential pulses. It is important for the proper functioning of the dual channel TDR 10 that samples of signals on internal input lines 16 and 18 are taken at precisely the same time. It is the purpose of the driver circuit 62, its associated inductor L1, and step recovery diode 60 to provide a single-ended high amplitude pulse having a fast rise time, that is, one between 70-100 picoseconds. The waveguide coupler 48 transforms the single-ended pulse into a pair of sampling control pulses which are applied to each of the traveling wave sampling gates 24 and 26. Since the geometry of the wave guide coupler 48 is symmetrical, these pulses will arrive at precisely the same time at each gate 24, 26. The signal-carrying conductor 58 disposed within slot 56 and surrounded by ground plane 52 functions as a coplanar waveguide. Slot 54 functions in this configuration as a slotline waveguide having two branches, one terminating at sampling gate 24 and the other terminating at sampling gate 26. When a driving pulse from the step recovery diode 60 is generated on conductor 58, a pair of pulses are induced in each of the branches of slot 54. In accordance with the characteristic behavior of slotline waveguides, a positive pulse is created which travels near the inner edges 61 and 65 of slot line 54. Within the same ground plane and on the inner edges 63 and 67, there is generated a negative going pulse which is symmetrical with the positive going pulse generated along the edges 61 and 65. These pulses are generated inductively by the driving pulse traveling along conductor 58. Thus, the intersection of signal-carrying line 58 and slotline 54 functions in a manner similar to a very high efficiency transformer in which a single-ended driving pulse is transformed into pairs of oppositely phased but time coherent pulses which are then supplied to the traveling wave sampling gates 24 and 26 to synchronously actuate these gates. When the traveling wave sampling gates 24 and 26 are turned off, test pulses may be transmitted to a device under test (not shown) which may be connected to input jacks 34 and 36. These test pulses are generateed by flat pulse current generators 30 and 32. Since generators 30 and 32 are connected directly to the respective nodes 20 and 22 without an intervening coaxial cable or connectors, degradation of the wave shape of the generators 30 and 32 is minimized. Furthermore, the flat current pulse generators 30 and 32 are oriented across from each of the traveling wave sampling gates 24 and 26 on opposite sides of internal input lines 16 and 18, respectively. This orientation minimizes any capacitive reactance that might otherwise be caused by orienting the input lines for the pulse generators 30 and 32 at other than right angles to respective internal input lines 16 and 18. In order to utilize the advantages of the traveling wave sampling gates 24 and 26 as disclosed in the aforementioned U.S. Patent No. 4647795, it is necessary to generate a high amplitude voltage pulse having an extremely fast rise time. This capability is provided by the circuit in FIG. 4 which includes drive circuit 62, inductor L1 and step recovery diode 60. The strobe pulse generator (not shown) provides driver circuit 62 with a strobe trigger input applied to coupling capacitor C1. Electronic switches comprising transistors Q1 and Q2 function like a thyristor in which Q1 is permanently biased in an "on" state by a voltage divider comprising R1 and R2. A voltage Ec is stored in capacitor C2 through resistor R3. Resistors R6 and R4 provide a path for negative voltage -V to bias the step recovery diode 60 through inductor L1. Q1 is on permanently because of the voltage divider formed by R1 and R2, which in turn provides a bias signal to the base of Q2. When Q1 receives a negative pulse through C1, it develops a high voltage at the base of Q2 which turns Q2 on. Since Q2 is pre-biased, the switching action is very fast. When Q2 turns on it increases the base current of Q1, which in turn increases the voltage level at the base of Q2. This creates a cascading effect which quickly drives both transistors Q1 and Q2 into saturation and allows the voltage Ec which is stored in C2 to flow through inductor L1. Inductor L1 is, in the preferred embodiment, a bond wire connection between the cathode of step recovery diode 60 and driver circuit 62. The extremely fast switching action of transistors Q1 and Q2 provides a high amplitude voltage Ec to step recovery diode 60 which in turn provides a high amplitude pulse (greater than 15 volts) having an extremely fast rise time (shorter than 100ps). Thus, a dual channel TDR has been provided capable of generating flat test pulses having a very sharp rise time with little signal degradation to a device under test which may be sampled at both its input and its output synchronously. This is made possible by the internal connection geometry of the front end of the TDR and by a balanced strobe generator incorporating a waveguide coupler which can control with precision the time at which signals from the device under test are sampled. Since the internal input lines for either channel are located immediately adjacent each other and close to the front panel of the TDR, the effects of signal and wave form degradation and distortion caused by internal reflection or capacitive reactance are minimized. The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.

Claims: A time domain reflectometer comprising: a pair of input channels (12, 14); sampling gate means (24, 26) connected to each of said input channels for sampling signals thereon; balanced strobe generator means (28) for providing sampling control pulses for controlling the time at which said sampling gate means (24, 26) are actuated; whereby said balanced strobe generator means (28) includes waveguide coupling means (48) transforming a single-ended driving pulse into a pair of time coherent sampling control pulses to synchronously activate said sampling gate means (24, 26) so that data is sampled from the signals on both input channels (12, 14) simultaneously. A time domain reflectometer as recited in claim 1 further comprising current pulse generator means (30, 32) coupled to each of the input channels for generating test pulses thereon. A time domain reflectometer as recited in claim 1 wherein the balanced strobe pulse generator means (28) comprises: a step recovery diode circuit coupled to the sampling gate means (24, 26) via the waveguide means; and a driver circuit responsive to trigger pulses for providing high amplitude fast risetime pulses to the step recovery diode, the step recovery diode in response thereto providing the sample control pulses to the sampling gate means (24, 26). A time domain reflectometer as recited in claims 1 or 2 wherein the sample control pulses comprise, for each of said sampling gate means, a pair of opposite polarity sample control pulses generated simultaneously. A time domain reflectometer as recited in claims 1 or 2 wherein each input channel (12, 14) comprises: an internal input line (16, 18) to which the sampling gate means (24, 26) is coupled; and a termination on an outside panel (38) of the time domain reflectometer for coupling the internal input line to an external device (4). A time domain reflectometer as recited in claim 5 wherein each internal input line (16, 18) comprises a node (20, 22), the nodes being located adjacent each other behind the outside panel and the sampling gate means (24, 26) being coupled to the nodes. A time domain reflectometer as recited in claim 6 wherein the waveguide means (48) comprises: a driving waveguide (56) coupled to receive the single ended driving pulse; and a pair of output waveguides (54) inductively coupled to the driving waveguide (56) and coupled to the sampling gate means (24, 26). A time domain reflectometer as recited in claim 7 wherein the driving waveguide (56) comprises a coplanar waveguide and the output waveguides each comprises a slotline waveguide. A time domain reflectometer as recited in claim 8 wherein the slotline waveguides are collinear with each other and disposed on opposite sides of the coplanar waveguide. A time domain reflectometer as recited in claim 7 wherein the output waveguides are orthogonally disposed with respect to the driving waveguide. A time domain reflectometer as recited in claim 5 wherein each internal input line (16, 18) comprises: a signal line (42) coupled to the termination at one end; and a coaxial ground line (40) coupled to the termination at one end and surrounding the signal line (42), the coaxial ground line (40) being separated from the signal line by air. A time domain reflectometer as recited in claim 11 wherein each internal input line (16, 18) comprises a cutaway portion (44, 46) of the coaxial ground line (40) to expose the signal line (42), the exposed signal line (42) forming the node to which the sampling gate means (24, 26) is coupled. A time domain reflectometer as recited in claim 12 wherein the sampling gate means (24, 26) comprises a sampling gate for each channel, the current pulse generator means (30, 32) comprises a current pulse generator for each channel, and for each channel the sampling gate and current pulse generator are coupled to the nodes on opposite sides of the signal lines. A time domain reflectometer as recited in claim 6 wherein the waveguide means (48) comprises: a ground plane conductor (52) having two orthogonal slots (54, 56) therein; and a signal carrying conductor (58) disposed within one of the slots to form a coplanar waveguide, the other of the slots having opposite ends thereof coupled to the sampling gate means (24, 26) for each of the input channels (12, 14). A time domain reflectometer as recited in claim 14 wherein the balance strobe pulse generating means (28) further comprises: a step recovery diode driver means (62) responsive to trigger pulses for generating the single-ended driving pulse. A time domain reflectometer as recited in claim 15 wherein the step recovery diode driver means (62) comprises: a pair of electronic switches (Q1, Q2) with one being biased permanently on and the other being responsive to the trigger pulse; and a step recovery diode (60) coupled between the pair of electronic switches (Q1, Q2) and the waveguide means (48) to generate the sample control pulses from the high amplitude fast risetime pulses. A time domain reflectometer as recited in claim 16 wherein the step recovery diode driver means (62) further comprises an inductor coupling the pair of electronic switches (Q1, Q2) to the step recovery diode. A time domain reflectometer as recited in claim 17 wherein the inductor comprises a bond wire connection. A time domain reflectometer as recited in claim 18 wherein the step recovery diode is coupled between the ground plane and the signal carrying conductor.

Inventors: AGOSTON AGOSTON, CARLSON JOHN E, FINKBEINER ANDREW E, KAVECKIS STANLEY P, RETTIG JOHN B, AGOSTON, AGOSTON, CARLSON, JOHN E., FINKBEINER, ANDREW E., KAVECKIS, STANLEY P., RETTIG, JOHN B.
Applicants: TEKTRONIX INC, TEKTRONIX, INC.
Application year: 1987
Main classification: G01R27
IPC codes: G01R27, G01R31