A METHOD OF MEASURING A CABLE DELAY TIME

Abstract: In a method of measuring a cable delay time by using an oscilloscope, a pulse generator, and a frequency counter, a period of a pulse signal is adjusted so that a leading edge of the pulse signal coincides with that of a reflected wave of the pulse signal, and the cable delay time is calculated by the period of the pulse signal. By this method it is possible to achieve greater accuracy of measurement more simply than can be achieved with conventional methods.

Description: The present invention relates to a method of measuring a cable delay time. In data processing systems for high-speed processing, a central processing unit (CPU), in particular, is constituted by high-density semiconductor IC elements. Signals between these elements are weak, and coaxial cables are used to prevent noise mixing. The delay time of the signal transmission through the connection cables greatly influences the processing times or phases of various signals, and since cables having the same length may have different delay times, the delay times of the respective cables must be measured. A demand has arisen for a method of easily and accurately measuring the delay times of connection cables when the connection cables are prepared. One conventional method of measuring a cable delay time is practised as follows. One end of a cable to be measured is connected to a pulse generator, probes of a conventional signal sampling means, such as an oscilloscope, are connected respectively to both ends of the cable, and a time difference between the pulse waves from the probes is directly read on the screen of the oscilloscope. Another conventional method of measuring a cable delay time is practised as follows. One end of a cable to be measured is connected to a pulse generator, and the other end of the cable is kept open. The probes are connected to the pulse generator to read a time difference between the transmitted waveform of the pulse signal and the waveform reflected by the cable end, and half of that difference is measured to obtain the cable delay time. Versions of these conventional methods are discussed in the article "Simplified Instrumentation Setup Measures Prop Delays in pSecs" by W. Wilke, published in Electrical Design News, Vol. 28 (1983), Sept., No. 18, Boston, Mass., USA. According to these conventional methods, the two waveforms on the screen of the oscilloscope must be read with reference to the scale thereon, and large reading errors tend to occur. In particular, if the delay time is increased, the measuring range must be changed, thus further increasing the possibility of errors. According to the first method described above, the cable must be measured after a characteristic impedance is connected to the end of the cable, and it is cumbersome to connect the characteristic impedance element to or disconnect it from the cable for every measurement. The article "Measurement of Delay in Coaxial Cables" by D. K. Ryazanov, published in Instruments and Experimental Techniques, Vol. 20, no. 4/2, July/August 1977, pages 1118 - 1120, discloses a method of determining the delay time of a cable to be tested, which method includes the steps of transmitting a signal from a pulse generator, via an auxiliary cable, into one end of the cable to be tested, varying the period of that pulsed signal so as to achieve a coincidence between a transmitted pulse of that pulsed signal, viewed on an oscilloscope, and a pulse reflected along the cables from the other end of the cable to be tested, and recording the period of the pulsed signal at which such coincidence is achieved. However, this prior art method has been found to be in need of some improvement. According to the present invention there is provided a method of determining the delay time ℓ of a cable to be tested, which method includes the steps of:- transmitting a signal from a pulse generator, via an auxiliary cable, into one end of the cable to be tested, varying the period of that pulsed signal so as to achieve a coincidence between a transmitted pulse of that pulsed signal, viewed on an oscilloscope, and a pulse reflected along the cables from the other end of the cable to be tested, and recording the period T2 of the pulsed signal at which such coincidence is achieved; characterised in that the said oscilloscope has first and second inputs connected respectively, one of them via a delay element, to first and second probes located at different positions along the auxiliary cable, the said coincidence being achieved between a leading edge of the said transmitted pulse received at the said first probe and a leading edge of the reflected pulse received at the said second probe, and in that the method further includes the steps of transmitting the pulsed signal into the said auxiliary cable when it is not connected to the cable to be tested and varying the period of that pulsed signal so as to achieve a coincidence between a leading edge of a transmitted pulse of that pulsed signal which is received at the said first probe and a leading edge of a pulse reflected back along the said auxiliary cable from its free end and received at the said second probe, recording the period T1 of the pulsed signal at which such coincidence is achieved, and calculating, from the recorded periods T2 and T1, the value ℓ = (T2-T1)/2. Thus, an embodiment of the present invention can provide a method of measuring a cable delay time by utilizing a zero point measurement scheme wherein a period of a pulse signal from a pulse generator is changed in such a way that waveforms of a transmission pulse signal and a reflected signal of the transmitted pulse signal coincide with each other, thereby measuring the cable delay time. Reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a schematic diagram of circuitry for use in a method embodying the present invention; Figs. 2A and 2B, and Figs. 3A and 3B, show respective waveforms; Fig. 4 shows the circuitry of Fig. 1 in more detail; Fig. 5 shows a view of part of the circuitry of Fig. 4; Fig. 6 is a block diagram of another part of the circuitry of Fig. 4; and Fig. 7A is a schematic diagram of a device incorporated in the circuit of Fig. 6, and Figs. 7B and 7C show respective waveforms as they appear in the device shown in Fig. 7A. Figure 1 is a schematic diagram of measuring circuitry for use in a method of measuring a cable delay time embodying the present invention. The measuring circuitry includes an oscilloscope 1, having an A-channel probe 11 and a B-channel probe 12, and a delay element 3 connected between the oscilloscope 1 and the A-channel probe 11. A pulse generator 2 is connected for delivering pulsed signals, via an auxiliary cable 4, to a cable 5 whose delay time is to be determined. The A- and B- channel probes 11 and 12 are respectively connected to the auxiliary cable 4 from the side of the pulse generator 2. The cable 5 is connected to a connector 40 at the other end of the auxiliary cable 4. Initially, the cable to be tested is not connected to the measuring connector 40, and the measuring circuitry is set as described hereafter. Figures 2A and 2B are timing charts of waveforms displayed on the screen of the oscilloscope 1. Figure 2A shows the waveform from the A-channel probe 11, and Fig. 2B shows the waveform from the B-channel probe 12. The waveforms are stepwise waveforms obtained by superimposing the waveform reflected by the open end of the cable on the waveform of the pulse signal from the pulse generator 2. To set the measuring circuitry, a period T1 of the pulse generator 2 is changed in such a way that a leading edge (a) of the signal waveform of the A-channel waveform coincides with a leading edge (b) of the reflected waveform of the B-channel waveform. The period T1 when a coincidence is achieved between the leading edges (a) and (b) is recorded. Then, the cable 5 is connected to the connector 40, and a period T2 of the pulse generator 2 at the time when the leading edges (a) and (b) coincide with each other (Figs. 3A and 3B) is measured in the same way. A delay time of the cable under test is calculated from the resultant periods T1 and T2, as follows: ℓ = (T2 - T1)/2 The reason for this will now be described. With the delay element inserted in the measuring circuit, the waveform of the A-channel probe 11 is displayed with a delay time D1. Assume that a delay time of the auxiliary cable 4 between the A- and B-channel probes 11 and 12 is defined as D2, and that a delay time between the B-channel probe 12 and the measuring connector 40 is defined as ℓ0. If the time bases in Figs. 2A and 2B are matched, the actual A-channel waveform on the screen appears to advance by one wave from the B-channel waveform. The actual waveform coincident points are (c) and (b). The period T1 is given as T1 = D1 + D2 + 2ℓ0 Similarly, if a delay time of the measured cable is given as ℓ, the period T2 measured while the cable 5 is connected is defined as follows: T2 = D1 + D2 + 2ℓ0 + 2ℓ Therefore, the delay time ℓ of the measured cable 5 is calculated as follows: ℓ = (T2 - T1) /2 Figure 4 shows an arrangement of a measurement circuit for practising a method of measuring a cable delay time embodying the present invention. The same reference numerals denote the same parts throughout the drawings. In the embodiment shown in Fig. 4, a 7904A sampling oscilloscope manufactured by Tektronix Inc. may be used as the oscilloscope 1, a 7S11 type unit may be used for the vertical units A and B, a 7T11 type unit may be used for the horizontal (time base) unit, and an S6 unit may be used as the A-channel probe 11 and the B-channel probe 12. The period of the pulse generator 2 is adjusted by a frequency adjustment unit 13 in the oscilloscope 1 and is measured by a frequency counter 6 (period = 1/frequency). The function of the frequency adjustment unit 13 is to allow the two waveforms displayed on the screen of the oscilloscope to coincide, i.e. its function is to automatically adjust the period of the pulse generator 2. Reference numeral 7 denotes a power source for supplying a voltage of a DC measuring level, and 8 denotes a coaxial relay. The relay 8 sets the positions of the waveforms upon switching of the DC measuring level of the A and B channels of the oscilloscope 1 during calibration, and connects the pulse generator 2 and the auxiliary cable 4 during measurement. Reference numeral 9 denotes a measurement/ calibration state setting switch for the measuring. circuit, and 10 denotes a microcomputer for controlling the measuring circuit and acquiring the measured data. The delay element 3 delays a sampled signal of the A-channel probe. The length of the cable to be measured is doubled, and this cable is connected to the delay element 3 so as to allow a double delay time of the delay time to be measured. Insertion of the delay element 3 in the measuring circuit allows the A-channel waveform to advance relative to the B-channel waveform on the screen, as mentioned above. Initially, the cable 5 is not connected to the measuring circuit, and the setting switch 9 is switched to the "calibration" position to set the emission line positions of the A and B channels. The positional relationship between these waveforms is set as shown in Fig. 5. The setting switch 9 is then switched to the "measurement" position, and the coaxial relay 8 is set at the "A" position to set the measurement mode. The frequency adjustment unit 13 automatically adjusts the period of the pulse generator 2 such that both waveforms cross at the center of the screen. At this time, the period T1 of the frequency counter 6 is read by the microcomputer 10 and is stored in, e.g., a flexible disk in the microcomputer 10. Subsequently, the cable 5 is connected to the measuring connector 40 to read the period T2 of the frequency counter 6 in the same manner as described above. The period T2 is then stored in the flexible disk for the microcomputer 10. When the periods T1 and T2 are measured, the microcomputer 10 calculates the delay time ℓ of the measured cable according to the equation described above. At the same time, the calculated delay time ℓ is stored in the flexible disk, and the associated data is displayed on the display. The intersection between the waveforms displayed on the oscilloscope screen is adjusted to be at the central position, as shown in Fig. 5. Therefore, the screen scale units can be increased to improve measurement precision. Figure 6 is a block diagram of the frequency adjustment unit 13 shown in Fig. 4. The frequency adjustment unit has the following functions; i.e. to set an A channel measuring level, to set a B channel measuring level, to position an edge of an A channel waveform at a center position of a screen, and to adjust the period so that the edges of the A and B channel waveforms are crossed. (a) Setting of A channel measuring level When the DC source having an A channel measuring level (for example, - 300 mV) is connected to a measuring system, the signal corresponding to that level is supplied to the A channel terminal and is input to a comparator 41b (Fig. 6). Another input of the comparator 41b is connected to the output of a D/A converter 46d. When the output of the D/A converter 46d is lower than the A channel measuring level, the comparator 41b output is "1" level, a U/D terminal of a U/D counter 45d receives the "1" level output and a CL terminal of a U/D counter 45d receives a clock. Then, the U/D counter 45d is counted up so that the voltage of the D/A converter 46d rises. This operation is carried out until the output of the D/A converter 46d becomes equal to the A channel measuring level. When the output of the D/A converter 46d is higher than the A channel measuring level, the output of the comparator 41b becomes "0" level, the output of an inverter 42b becomes "1" level. The U/D terminal of the U/D counter 45d receives "0" level and CL terminal of the U/D counter 45d receives a clock. The U/D counter 45d is then counted down, and the output of the D/A converter 46d is lowered. This operation is carried out until this output becomes equal to the A channel measuring level. When the output voltage of the D/A converter 46d and the A channel measuring level become equal, the operation of the U/D counter 46d is stopped by an enable signal supplied thereto, and thus the voltage corresponding to the A channel measuring level (- 300 mV) can be stored with a high accuracy. (b) Setting of B channel measuring level This is carried out in the same way as for the A channel measuring level mentioned in item (a) above, except that + 300 mV is used as the voltage connected to the measuring system, and a comparator 41a, an U/D counter 45c, and a D/A converter 46c are employed. (c) Positioning an edge of the A-channel waveform at the center of the screen The coaxial relay 8 shown in Fig. 4 is switched to a position A, so that the waveform in the pulse generator 2 is passed through the A channel probe 11 and the B channel probe 12. A sawtooth wave (sweep wave) in a time base unit of the oscilloscope 1 is connected via a terminal H (Fig.6) to a comparator 41c. Another input of the comparator 41c is set to a voltage value V0 of a center portion of the sawtooth wave, that is, a center of the screen of the oscilloscope. An input of the comparator 41b receives a waveform corresponding to the pulse generator, and the other input thereof receives a measuring level set as in item (a) above. Therefore, the output of the comparator 41b is operated so that the measuring level (- 300 mV) is used as a threshold voltage, and the comparator 41c is operated so that the voltage V0 corresponding to a center of the screen is the same as the threshold voltage. The outputs of the comparator 41b and the comparator 41c are supplied to inputs of a phase comparator 43b. The phase comparator 43b operates as shown in Figs. 7A, 7B, and 7C. That is, the phase comparator 43b causes either an oscillator 44c or an oscillator 44d to oscillate during the time difference between the two phases A and B, and a U/D counter 45b is operated by using this oscillation as a clock. This U/D counter 45b distinguishes whether the clock is from the oscillator 44c or 44d by an oscillator 48 for U/D use. Accordingly, the output voltage of the D/A converter 46b is changed, and a voltage at the output terminal TP is changed, so that the A channel waveform is placed at the center of the screen. The T.P. signal at the output terminal is connected to a T.P. terminal of the H unit of the oscilloscope 1 so that the position on the screen varies linearly. (d) Adjusting the period so that the edges of A and B channel waveforms are crossed The coaxial relay 8 shown in Fig. 4 is switched to a position A, so that the waveform in the pulse generator 2 is passed through the A channel probe 11 and the B channel probe 12. An input of the comparator 41b receives the waveform corresponding to the pulse generator 2 and the other input is set as the measuring level set in item (a) mentioned above. An input of the comparator 41a receives the waveform corresponding to the pulse generator 2, and the other input thereof is set as the measuring level set in item (b) mentioned above. Therefore, the output of the comparator 41a is operated by using the measuring level (+ 300 mV) as a threshold voltage, and the output of the comparator 41b is operated by using the measuring level (- 300 mV) as a threshold voltage. The outputs of the comparator 41a and the comparator 41b are supplied to inputs of a phase comparator 43a. The phase comparator 43a operates as shown in Figs. 7A, 7B, and 7C. That is, the phase comparator causes either an oscillator 44a or an oscillator 44b to oscillate during the time difference between two phases. A U/D counter 45a is operated by using the above oscillation as a clock. This U/D counter 45a distinguishes whether the clock is from the oscillator 44a or 44b by an oscillator 48 for U/D use. Accordingly, the output voltage of the D/A converter 46a is changed. This voltage is connected to a voltage terminal of a voltage controlled oscillator 49 in which an oscillation voltage is changed. This voltage controlled oscillator shapes the waveform in the pulse generator 2, and the level thereof is determined and input to the A channel probe and the B channel probe. The frequency in the voltage controlled oscillator 49 changes until the time the waveforms of A channel and the B channel cross. According to an embodiment of the present invention, the delay time of the cable to be measured can be accurately measured with simple operations, thus providing practical advantages.

Claims: A method of determining the delay time ℓ of a cable (5) to be tested, which method includes the steps of transmitting a signal from a pulse generator, via an auxiliary cable (4), into one end of the cable (5) to be tested, varying the period of that pulsed signal so as to achieve a coincidence between a transmitted pulse of that pulsed signal, viewed on an oscilloscope (1), and a pulse reflected along the cables (4, 5) from the other end of the cable (5) to be tested, and recording the period T2 of the pulsed signal at which such coincidence is achieved; characterised in that the said oscilloscope (1) has first and second inputs connected respectively, one of them via a delay element (3), to first and second probes (11, 12) located at different positions along the auxiliary cable (4), the said coincidence being achieved between a leading edge of the said transmitted pulse received at the said first probe and a leading edge of the reflected pulse received at the said second probe, and in that the method further includes the steps of transmitting the pulsed signal into the said auxiliary cable (4) when it is not connected to the cable (5) to be tested and varying the period of that pulsed signal so as to achieve a coincidence between a leading edge of a transmitted pulse of that pulsed signal which is received at the said first probe and a leading edge of a pulse reflected back along the said auxiliary cable (4) from its free end and received at the said second probe, recording the period T1 of the pulsed signal at which such coincidence is achieved, and calculating, from the recorded periods T2 and T1, the value ℓ = (T2-T1)/2. A method as claimed in claim 1, wherein the said delay element (3) comprises a delay cable which is twice the length of the cable (5) whose delay time is to be determined.

Inventors: HIDEYUKI, OBARA
Applicants: FUJITSU LIMITED
Application year: 1986
Main classification: G01R31
IPC codes: H04B3, G01R31, G01R27, G04F10