GB2329255A

GB2329255A - Cable pair tracing

Info

Publication number: GB2329255A
Application number: GB9714700A
Authority: GB
Inventors: Mark Samuel Govier; Kevin Round
Original assignee: Chesilvale Electronics Ltd
Current assignee: Tempo Europe Ltd
Priority date: 1997-07-11
Filing date: 1997-07-11
Publication date: 1999-03-17
Anticipated expiration: 2017-07-11
Also published as: GB2329255B; GB9714700D0

Abstract

An apparatus 10 for cable pair tracing comprises two sensor plates 12, 14 which are coupled to a differential amplifier 20 which provides a differential output signal to detection means. The differential amplifier 20 may have independently adjustable gains 22, 24 for each of the input signals derived from a respective sensor plate 12, 14. A high-pass filter may be placed between each respective sensor plate 12, 14 and the differential amplifier 20. The differential amplifier 20 may have a low input impedance. Each sensor plate 12, 14 may be formed by a foil member of adjustable area. The detection means may provide an audible output. The sensor plates 12, 14 may be acoustically damped. A method of using the apparatus such that far field signals (eg mains hum signals and associated harmonics) are substantially cancelled, when tracing cable pairs, is also disclosed. The apparatus senses a signal carried by a cable pair when the sensor plates are near the said cable pair and then provides an audible signal to indicate the detection of the said cable pair.

Description

CABLE PAIR TRACING The present invention relates to equipment and a method for use in the tracing of cable pairs, particularly telecommunications cable pairs.

Communication and control wiring cable pair tracing was originally effected by contact methods, where physical contact was made with the target pair and a signal was detected. The signal may have been AC or DC but either way this signal would have been tedious to find. Later systems have used an audio frequency oscillator to apply a signal to the target cable pair. A very high gain electrostatic field detecting probe is used to locate the signal field emanating from the target pair. This pair tracing method is much faster than contact methods.

A disadvantage of non-contact methods is that they are prone to power hum pick-up. It is possible to provide hum rejection within the circuit of the probe but this is not totally effective. Power hum is fundamentally 50 Hz in most of the world and 60Hz in North America but it also contains many harmonics. The pair tracing probe typically operates at 1000Hz or a similar frequency. At such frequencies there are often strong residual harmonics. While it is possible to virtually eliminate the 50/60Hz element by filtering, some of the harmonics are within the filter acceptance band. The 50/60Hz element is strong enough to cause the probe to saturate and has to be eliminated. Some harmonic hum can be tolerated because the signals are relatively weak and unlikely to cause saturation.

However, users of the probe may still find the buzz irritating because it is superimposed on the signal.

One attempt to eliminate the problem uses a series of precision notch filters to eliminate the power-hum harmonics. Typically they would be set to suppress the dominant odd harmonics but they would have to deal with the fifth harmonic or higher to be effective. This can be relatively expensive.

Another solution uses a clocked delay line set to run at 50 or 60Hz as appropriate to delay the input signal by one half cycle period along a separated path and then to re-combine that signal with the original equal in level but in anti-phase causing complete cancellation. This has the advantage of cancelling harmonics across the frequency spectrum. Its disadvantage is that it will cancel the power harmonics so effectively that proximity to power lines may not be recognised. It further has the disadvantage of cancelling any multiple of 50 or 60Hz as appropriate, which may cause strange reverberance or step frequency effects to be heard from the oscillator signal.

According to the present invention there is provided apparatus for use in tracing cable pairs comprising a pick-up assembly comprising a pair of pick-up plates; differential amplifier means coupled to said pick-up plates to provide a differential output; and means for detecting said output.

In another aspect the invention provides a method of tracing cable pairs using such apparatus wherein the pick-up assembly and/or the differential amplifier means is constructed and arranged so that far field signals are substantially cancelled.

An embodiment of the invention will now be described in greater detail with reference to the accompanying drawings in which: Fig 1 is a schematic diagram of a probe embodying the invention; Fig 2A is a plan view of part of a pick-up of the probe; and Fig 2B is a section on B-B in Fig 2A The illustrated probe 10 employs a pair of differential foil pick-ups 12,14 of generally known type (c.f. the T-Bug of Industrial Technology, Inc.) That is, they are a pair of similar thin conductive (foil) plates, mounted in parallel with a small spacing. Each is connected via a respective capacitor 16,18 (for blocking low frequencies) to either the inverting or the noninverting input of a differential operational amplifier 20. The inverting and the non-inverting gain paths are balanced (being adjustable by means of respective controls 22,24) The output is passed through a high pass filter comprising a pair of capacitors 26,28 in series, and a resistor 30 having one terminal connected between the capacitor 26,28 and the other earthed. Means 32 are provided for adjusting the gain, in this example under the control of a logic block 34, though manual adjustment e.g. by means of a potentiometer would also be possible.

The output passes through high or band pass filters with gain. There will generally be more than one stage of filtering, two (36,38) being shown. Finally the signal is amplified by a power amplifier 40 and fed to a loudspeaker 42.

The system as a whole has a very high gain. Thus there is a risk that vibrations from the speaker would be fed along the probe to the pick-ups, creating audio feedback and instability. To minimise this risk the pick-ups 12,14 may be mechanically damped, e.g. provided with sheaths (44) of rubber or other resilient material (Figs 2A, 2B) In use, an AC signal (e.g. at 1000 Hz) is applied to a conductor pair, e.g. at a telephone exchange. The illustrated probe is then used to identify the signalbearing pair at a remote location where there are, in general, many conductors. The differential pick-ups (12,14) and the gain paths in the differential amplifier 20 are selected and/or adjusted so that when both pick ups sense the same field, the amplifier's output is zero or close to zero. Since signal sources remote from the two pick-ups 12,14 affect them more-or-less equally, their effects can be cancelled out. Such sources may include sources bearing mains frequency currents, which would otherwise be responsible for mains hum.

In fact it is generally not desirable for the effect of mains-bearing conductors to be cancelled out completely, since an operator may need to be warned of their presence, e.g. on grounds of safety. Therefore the system may be operated so that the cancellation is not complete, e.g. by having slightly unequal pick-ups 12,14 and/or by using the controls 22,24 to prevent perfect balancing by the differential amplifier 20. A signal from a nearby main conductor tends to be very strong, so a system with a small imbalance for permitting its detection can still substantially eliminate other signals from remote sources. Indeed the probe is typically insensitive to sources more than a few centimetres away.

Thus as the pick-ups are moved over and among conductors, the ones carrying the test signal are readily located.

The strongest signal will generally be detected when one (only) of the sought-for pair passes between the pick-ups 12, 14.

Claims

CLAIMS:

1. Apparatus for use in tracing cable pairs comprising a pick-up assembly comprising a pair of pick-up plates; differential amplifier means coupled to said pick-up plates to provide a differential output; and means for detecting said output.

2. Apparatus as claimed in claim 1 wherein the differential amplifier means includes means for independently adjusting the gain of at least one path associated with a respective plate.

3. Apparatus according to claim 1 or claim 2 wherein a respective high pass filter is provided between each pick-up plate and the differential amplifier means.

4. Apparatus according to any preceding claim wherein the differential amplifier means is arranged to operate with a low input impedance.

5. Apparatus according to any preceding claim wherein the pick-up plates are foils whose areas can be adjusted.

6. Apparatus according to any preceding claim wherein the means for detecting said output comprises audible output means.

7. Apparatus according to any preceding claim wherein said pick-up plates are acoustically damped.

8. Apparatus for use in tracing cable pairs substantially as herein described with reference to and as illustrated in the accompanying drawings.

9. A method of tracing cable pairs using apparatus according to any preceding claim wherein the pick-up assembly and/or the differential amplifier means is constructed and arranged so that far field signals are substantially cancelled.