GB2031248A - Method and apparatus for measuring telephone line characteristics - Google Patents

Abstract

A test set for measuring line characteristics for a telephone line having tip and ring leads to enable calculation of the leakage resistances, the line capacitances and the spurious voltages between tip and ground, ring and ground, and tip and ring directly from the measurements, comprises means for applying three successive d-c voltage states from tip to ground, and ring to ground respectively; and means for separately measuring the steady state current flowing to the tip and ring leads during each of the three states, each of the three successive voltage states being applied from tip to ground and ring to ground through first and second reference resistors respectively; and means for separately measuring the transient charge flowing through the first and second resistors to the tip and ring leads respectively. These measurements are used by a computer, controlling the set, to calculate said characteristics.

Description

SPECIFICATION Method and apparatus for measuring telephone line characteristics This invention relates to a method and apparatus for measuring a telephone line and more particularly to one for deriving thirteen distinct parameters from the resultant tip and ring current and charge flow monitored during three successive states of applied voltage.

Background of the invention During the operation of a telephone system, it is often necessary to test the conditions of a telephone line, particularly those referred to as subscriber loops which connect terminal sets to a central office. In the past, this has been accomplished by connecting various configurations of test equipment to the line to separately determine each of the wanted parameters. An extensive analysis would involve measuring at least thirteen separate parameters including any spurious a-c voltage, d-cvoltage, leakage resistance and line capacitance between the tip and ground, ring and ground, and tip and ring ofthe subscriber's loop, as well as the frequency of the spurious a-c voltage.It will be readily apparent that determining each of these parameters separately in order to evaluate the state of the subscriber loop was a long and tedious operation.

United States Patent No. 4,028,507 invented by Richard Scott Hoppough, and issued June 7, 1977, discloses an improved apparatus for measuring the various parameters, which entails forcing the tip and ring to several different sets of potentials within relation to ground, and then measuring the resultant currents flowing therethrough. From these measurements, the various parameters can then be calculated.

However with this arrangement separate sets of d-c and a-c voltages must be applied to measure the respective leakage resistances and the line reactances (primarily capacitive). In addition, filters are required to reject any spurious a-c signals on the line during the a-c measurements.

Statement of the invention It has been found that by connecting only three successive d-c voltage states to the line through known tip and ring resistors (rather than directly to the line) and monitoring the resultant transient charge and steady state current therethrough, that sufficient information can be obtained to derive the thirteen different parameters of the subscriber loop-detailed above, thereby eliminating the separate application of a-c signals for the reactance tests. In a typical application this can be achieved in a period less than three seconds, the majority of the time being required for the line to settle (in order to measure the transient charge) when the voltage state on the line is altered. Thus the measurement time is directly related to the leakage resistance and the capacitance of the loop.

In accordance with the present invention there is provided a test set for measuring a telephone line having tip and ring leads, which comprises means for separately measuring during a first state, spurious currents flowing from tip to ground through a first reference resistor, and ring to ground through a second reference resistor. During a second state, the test set includes means for measuring the transient charge and the steady state current flowing through the first and second resistors when connected in series with first sources of d-e voltage between tip and ground, and ring and ground respectively.During a third state, the test set includes means for measuring the transient charge and the steady state current flowing through the first and second resistors when connected in series with second sources of d-c voltage between tip and ground, and ring and ground respectively, the second sources having a different voltage ratio than the first sources.

In a particular embodiment both sides of the loop are grounded through the reference resistors for measurement of the spurious currents. A low d-c voltage is applied to both sides of the loop through the resistors during the second state, and in the third state the low voltage is removed from one side. Each state results in two values: a transient charge flow as a result of the change of state, and a steady state current flow after the transient. the currents give the spurious voltages and leakage resistance which may be present, while the transient charges give the capacitance values.

The invention also encompasses a method of measuring these parameters which includes individually measuring and recording the spurious currents flowing through individual reference resistors connected from each side of the line to ground, then individually measuring and recording the transient charge and the steady state current flowing through the individual reference resistors connected in series with first sources of d-c voltage from each side of the line to ground, and then individually measuring and recording the transient charge and the steady state current flowing through the individual reference resistors connected in series with second sources of voltage from each side of the line to ground, the second sources having a different voltage ratio than the first sources; and thereafter calculating the various parameters directly from the recorded measurement.

Brief description of the drawings An example embodiment of the invention will now be described with reference to the accompanying drawings in which: Figure lisa block and schematic circuit diagram of a telephone line connected to a test set for measuring the line; Figure 2 illustrates typical current waveforms developed across the reference resistors during the three applied states, when no spurious a-c signals are present; Figure 3 illustrates a typical steady-state current on either the tip or ring lead when a spurious a-c component is present, and the a-c component of that current after having been rectified; and Figure 4 illustrates the transient current flow when a step function d-c voltage is applied to one side of the line during the presence of a spurious a-c component.

Description of the preferred embodiment Figure 1 illustrates the test set connected to the tip (t) and ring (r) of a telephone line 10 (often referred to as a subscriber loop) which is terminated in a conventional telephone set 11. When the set 11 is on-hook, it generally appears as about 0.45if capacitor Cp across the line 10, which is primarily contributed by a d-c isolating capacitor in series with a ringer (not shown) in the set 11.Shown in dotted form is the leakage resistance Rtg and line capacitance Ctg in series with spurious a-c and d-c voltages Vactg and Vdotg from tip (t) to ground (g); leakage resistance Rrg and line capacitance Crg in series with spurious a-c and d-c voltages Vacrg and Vdorg from ring (r) to ground (g); and leakage resistance Rtr and line capacitance Ctrfrom tip (t) and ring (r) of the line 10. Since the capacitor Cp is in shunt with the capacitance Ctr the total tip to ring capacitance will equal Ctr= Cp + Ctr. Illustrated in lumped form is the series resistance Rand Rrofthe line 10.

Normally this resistance is small ( < 3000Q) compared to any leakage resistance ( > > 50000Q) of the line 10 and consequently has little effect on the resistance measurements. Also the line resistance does not affect the accuracy of the capacitance measurements, since these calculations which take place only after the transient voltages have subsided, are based on the total charge flow and not on the charge flow for a preselected period of time.

In the following description of the test set, corresponding elements and waveforms for both t and r will be identified by the same reference numeral or character followed by the distinguishing reference character t or r. However, only the reference numeral will be referred to except where it is necessary to distinguish between the two elements. Also, the location in Figure 1 of the waveforms illustrated in Figures 2,3 and 4 is indicated by corresponding reference characters. In addition, all switches which are illustrative only, are connected concurrently to their respective contacts bearing identical contact numbers, under control of timing control circuitry, as will be explained in greater detail hereinafter.

Both t and r of the line 10 are coupled through input reference resistors Rit and Rir (each typically 100KQ) to switches 13 which control the three different voltages states applied to the line 10. During each of the three measurement stages, the current and transient charge flow through the reference resistors Rit and Rir is obtained indirectly by monitoring the voltage thereacross. The balance of the test set will manifest itself from the following description of its function and operation.

During the initial stage, the resistors Rit and Rir are both connected to ground g to enable measurement of any spurious a-cord-c voltages Vac or Vdc on the line 10. Initially, with all switches in position 1, spurious a-c voltages developed across the resistors Rit and Rir respectively are coupled through capacitors 14, a-c/d-c converters 15 and switches 16to respective voltage/frequency converters 17. The use of v/f's 17 permits simply counting (integration techniques for obtaining capacitance measurements). As explained hereinafter, the d-c measurements (yielding spurious d-c voltages Vdctg and Vdcrg, leakage resistances and capacitances) are made with the v/f converters 17 coupled directly to the line 10.The a-c measurements (spurious a-c voltages Vactg and Vacrg) are made by coupling the v/f converters 17 through the d-c blocked a-c/d-c converters 15 to the line 10.

The outputs from the capacitors 14 are also fed to positive zero crossing detectors 20. If there is a detectable spurious a-c component present on the line 10 then the sample period twill be that between two positive-going zero crossings of the a-c component. All d-c measurements are synchronized with integral multiples of cycles of the spurious a-c component to give a-c rejection.

The following simplifying assumptions are made about spurious a-c sources: (1) That any spurious a-c voltage is periodic. If this assumption is not valid, for example if the a-c voltage is broadband white noise, then the meter will still make an attempt to produce a mean a-c voltage reading but will not be able to make any other measurements since synchronization of the sample period will be impossible.

(2) That any spurious a-c voltages on both the tip and ring are due to a single source. Separate sources on tip and ring of similar amplitudes but of differing frequencies will result in a situation similar to the above.

As a result of assumption 2, the loop 10 may be reduced to one of two equivalents for a-c measurement: one where the spurious a-c sources on the tip and ring are in-phase and the other where they are out-of-phase. The phase 0 is detected by comparing the outputs of the positive zero crossing detectors 20 in a phase detector 21. The tip to ring a-c potentials may be calculated:

The outputs from the v/f converters 17 are fed to counters 22 which provide at their output, a frequency count n over a preselected interval as explained hereinafter. In addition an internal 1MHz clock 23 provides a reference output frequency fref which is fed to a further counter 24.The counter 24 provides two outputs: (1) a measure of the sample period T which is the time between successive zero crossings of the spurious a-c componsnt Vac as detected by one or other of the positive "O" crossing detectors 20, which indirectly controis the resetting of the counter 24; or (2) if no a-c component is present the maximum sample period #max which is provided each time the counter 24 is filied. #max is selected to be longer than the period of the lowest spurious a-c signal expected to be encountered. Typically #max = 100msec is used.

Either sample period pulses Tt, Tr or Tmax are coupled by a switch 30 and/or an OR gate 25 to a computer 26 which also receives the counter and phase outputs nt, n, and 0 as shown in order to perform thevarious calculations as determined by the formulas given in TABLES I and II at the end of this description.

The switch 30 is initially connected to the output of detector 20t. However, if no spurious a-c signal is detected the information is conveyed to the computer 26 which controls the connection of the switch 30 to the output of detector 20r. This permits a spurious signal on either the tip andlor ring to control the resetting of the counters 22 and 24.

In addition the computer 26 provides the reset signals for the counters 22 and 24 each time -a control signal is received from the OR gate 25. The computer 26 also outputs the calculated values to a record- and display unit 27, as well as provides control signals to the various switches. Once the measurements for switch positions 1'are completed the switches 13 and 16 are moved to positions 2 under control of the computer 26 for measurements of any spurious d-c voltages. The switches 13 and 16 are then moved to position 3.Here an internal voltage source Vs (typically Vs = 50v) is applied to both t and r through.the resistors Riband Rip 3 respectively, and another set of d-c voltage measurements is taken as well as the magnitude of the transient - charge flowing through the resistors Rit and Rjr.

When a steady state condition is reached (as determined by two sucessive measurements of equal magnitude) the ring r is again connected to-ground through resistor Rip as indicated by switch positions 4 and a further set of d-c and charge flow measurements is taken. It is to be noted that resistor Rlr need not necessarily be connected to ground but may be connected to a voltage source of a different magnitude. The important thing is that the ratio of the two voltages applied to the resistors Rit and Rir be different for positions 3 and 4.

Once this is completed sufficient information has been fed to the computer 26 to initially obtain the derived measurements shown in the TABLES and ultimately the 13 calculated parameters. However only three different voltages state have been applied to the resistors Rit and Rir In positions 1 and 2, both resistors are connected to ground during position 3, both resistors are connected to Vs; during position 4 Rir is again connected to ground while Rit continues to be connected to source Vs.

The instantaneous output frequency f of the voltage/frequency converters 17 is proportional to the instantaneous current i through the resistors Ri. The transfer function of the converters 17 is therefore: f = K.i + f0 where K is a gain term and f0 is the output frequency of the converters 17 when i = 0. Also ift be a measure of time and n be a count of cycles off, the instantaneous frequency f is then given by f = dn/dt.By rearrangement, the current i and the charge flow q can be calculated in terms of n and t as follows : i = 1/k (dn/dt - fO) q = #idt = 1/k[jdn - f,Sdt] For a finite measurementsample period T the charge flow is given by:

The mean valuei of the current is given by:

where

i.e. the count accumulated during the period #.

The frequency f5 of the spurious a-c voltage is then f5 = lir.

Figure 2 illustrates typical charge flow and steady state currents which flow through the reference resistors Rit and Rir during the various measurement intervals when a spurious d-c voltage of different magnitude is present on both the tip and ring. If this voitage were not present then the currents itl and irl would be zero during the initial interval when the switches were in positions 1 and 2. The transient charge flow Qt and Qr during states 2 and 3 results from the change in voltage applied to both sides of the line 10. This also results in a different steady state currentit and ir during states 2 and 3.

An a-c component resulting from the presence of a spurious a-c voltage on the line 10 is shown superimposed on a steady-state currenti in Figure 3. The charge flow 9 for one cycle of the spurious signal is also indicated. The equivalent current output from the a-c/d-c converter 15 is also illustrated in the bottom portion of Figure 3.

Figure 4 illustrates the transient charge that flows following a change of state p-l to p (e.g. from state 2 to state 3). The transient is considered to be complete when consecutive samples show no detectable mean current change thus: ism.1 = tm.

The transient charge flow is as follows:

This is not necessarily the total charge in the capacitance but is related to it. Hence the capacitance values can be determined directly from the measurements by applying corrections for leakage resistance as shown in the accompanying TABLES. It should be noted that the change of state is synchronized with the period t under control of the computer 26. The leakage resistance values are calculated from the stable, post-transient current values which can be readily derived from the formulas given above.

Initially, the derived measurements shown in TABLES I and II are determined from the information fed to the computer 26 during the respective switch positions. These measurements can then be used to derive the intermediate calculations as shown, as well as the desired 13 calculated parameters which are subsequently fed to the record and display unit 27.

TABLE I

Switch Derived Intermediate Calculated positions measurements calculations parameters 1 1 at ti Vac= i,, ti Rit i as rl Vacrg = ≈ tw rl-Rir T \/actr=vact2g+vacr2g-2 yaCtg vacrg cos0 8 = 1/T 2 Tn VdCtg = - Rit Vdcrg = - iriRlr Iri Vdctr = Vdctg - Vac,, c 3 It2 I t2 = t2 - ti Vtg2 Vtg3y -. . r2 - irl Rts E ~~~~~~~~~~~ 1r2 r2 r2 1r1 tg 1tr - 1t3 Vtr2 lt3 C~ V6 Vtg2 = V6 - lt2.Rit (known) Vrg2 = V9 ~ IR"Rir Vrg2 - Vtr2 Vrg2~ Vrg3y Vtr2 = Vtg2 - Vrg2 Rrg = Vtr2 1'3 - 1r3 Vtrs 4 It3 1t3 = It3 - ti 1r3 1r3 = Ir3 - Iri Vtg3 = V9 - lt3Rit Vtg3 - Vtg2y t2Vg3 Vrg3 = 0 - Ir3-Ri, Rtr - l?3 + X Vrg2 Vr3 Vtr3 = Vtg3 - Vrg3 TABLE 11 Switch Derived Intermediate Calculated parameters position measurements calculations 3 Qr2 Gi=1/Rit=1/Rit Ctg=1/VsGi2[Ot2(Gi+Gtg+Gtr)(Gi+Gtg)-Qr2(Gi+Grg)Gtr+Qt3 (Gi+Gtg+2.Gtr)(Gtg - Grg)] Qr2 Gtg=1/Rtg Crg=1/VsGi2[Qr2(Gi+Grg+Gtr)(Gi+Grg)-Qt2(Gi+Gtg)Gtr+Qt3 (Gi+Grg+2.Gtr)(Gtg-Grg)] 4 Qt3 Grg=1/Rrg Qr3 Gtr=1/Rtr Ctr=1/VsGi2[Qt2(Gi+Gtg+Gtr)Gtr+Qr2(Gi+Grg+Gtr)Gtr+Qt3 (Gi+Gtg+2.Gtr)(Gi+Grg+2.Gtr]

Claims (6)

1. A test set for measuring a telephone line having tip and ring leads to enable calculation of the leakage resistances, the line capacitances and the spurious voltages between tip and ground, ring and ground, and tip and ring directly from the measurements, the test set comprising: means for applying three successive d-e voltage states from tip to ground, and ring to ground respectively, the two applied voltages in one state differing from the two applied voltages in the second and third states in at least magnitude and ratio respectively; and means for separately measuring the steady current flowing to the tip and ring leads during each of the three states, characterized by each of the three successive voltage states is applied from tip to ground and ring to ground through first and second reference resistors respectively; and means for separately measuring the transient charge flowing through the first and second resistors to the tip and ring loads respectively.

2. Atest set as defined in claim 1 which is additionally characterized by: means for separately measuring any spurious current which flows from tip to ground through the first reference resistor, and ring to ground through the second reference resistor; means for applying first and second d-c voltages through the first and second resistors from tip to ground and ring to ground respectively, and then for separately measuring the transient charge and the steady state currentthroughsaid resistors; and means for applying third and fourth d-c voltages, having a different ratio than the first and second voltages, through the first and second resistors from tip to ground and ring to ground respectively, and then for separately measuring the transient charge and the steady state current through said resistors.

3. Atest set as defined in claim 2 in which three of the four d-c voltages are of the same magnitude.

4. A test set as defined in claim 2 which is additionally characterized by: means for measuring any a-c components in the spurious currents; means for detecting zero crossings of the a-c component in the spurious currents; and means responsive to no detectable change in the mean current between adjacent zero crossings of the a-c component to terminate measurement of the transient charge and to initiate measurement of the steady state current.

5. A test set for measuring a telephone line, substantially as hereinbefore described with reference to the accompanying drawings.

6. A method of taking measurements on a telephone line substantially as hereinbefore described.