GB2397973A

GB2397973A - Comparing sideband signal quality metrics

Info

Publication number: GB2397973A
Application number: GB0302197A
Authority: GB
Inventors: Richard William Fling; Peter Michael Hopwood
Original assignee: RADIODETECTION Ltd; Radiodetection Ltd
Current assignee: RADIODETECTION Ltd; Radiodetection Ltd
Priority date: 2003-01-30
Filing date: 2003-01-30
Publication date: 2004-08-04
Also published as: GB0302197D0

Abstract

A device for reducing the effects of magnetic interference in an electromagnetic field received from a buried, underground or otherwise inaccessible object, e.g. an underground boring or locating tool sonde. The device generates and compares quality metrics for a plurality of sideband signals of one or more input signals and selects at least one sideband signal dependent on the comparison. The quality metric may include bit error rate (BER), bit width modulation and crest factor measurement, i.e. peak-to-RMS amplitude ratio and the input signals may come from one or more detectors at or separate from the device, e.g. three perpendicular antennae. Interference may come from ambient noise, power cables, etc. and the best sideband signal, (e.g. upper, lower or double sideband) will depend upon the magnitude of the noise and whether it is random or periodic. The device (fig 1) includes band-pass filter (106), ADC (107), transformer (108), signal splitter (110), Hilbert transformer demodulators (112, 114), phase locked looped demodulator (116), quality metric generation means (120), comparison logic unit (130) and multiplexer (140).

Description

DEVICE AND METHOD FOR IMPROVED DATA TRANSFER

Field of the invention

The present invention relates to a device for reducing the effects of magnetic interference on electromagnetic signals from a buried of inaccessible object and a method of performing the same.

Background of the invention

Guided and unguided underground boring or locating tools are generally monitored in order to determine their location and orientation, in order to allow steering of and/or allowing instructions to be given to the tool.

It is known for tools to emit signals in order to be monitored, rather than using a physical connection between the tool and the monitor. The emitting device is commonly known as a "sonde". It is known for sondes to emit electromagnetic fields as signals. Such wireless emission allows greater versatility of the sonde and monitor, for example by allowing the sonde to be located and tracked while otherwise inaccessible.

However, use of such electromagnetic field signals means that other electromagnetic fields, such as those generated by power cables, interfere with the signals from the sonde by also being detected by the monitor together with the signals from the sonde.

It is also known to modulate the signal emitted from the sonde. The modulation carries additional data, such as the orientation (yaw, pitch, roll) of the sonde, and therefore the tool, as well as the basic electromagnetic field allowing tracking of the location of the tool. Ambient electromagnetic fields can corrupt both the overall electromagnetic field and the modulation applied thereto. Such corruption can therefore reduce the effectiveness of the wireless sonde and monitor, especially in areas with high ambient electromagnetic "noise". There is therefore a need to reduce the effects of such ambient radiation, in order improve the efficiency of the communication between sonde and monitor, for example in order to increase the data transmission rate.

Summary of the invention

According to a first aspect of the invention, there is provided a device that can reduce the effects of interference on an electromagnetic field emitted from a buried, underground or otherwise inaccessible object. The device generates quality metrics relating to a plurality of sideband signals from one or more input signals, compares the quality metrics and selects one or more sideband signals, dependent on the respective quality metrics.

The input signals may be electronic signals from one or more detectors, which detect the electromagnetic field at one or more specified locations. The device may generate quality metrics for sideband signals from more than one such input signal, or from a single input signal. The device may select which sideband signals are to have quality metrics generated, and/or which quality metrics are to be compared. For example, if a sideband signal does not have a sufficient signal amplitude, it may be ignored, either before or after a quality metric is determined, and, if determined, the quality metric in such a case may not be compared with other determined quality metrics.

In an embodiment, three detectors of electromagnetic fields provide the input signals.

In a further embodiment, the three detectors are arranged such that two are horizontal, relative to the ground, and one is vertical. The two horizontal sensors may be orthogonal. The device may additionally comprise such detectors, or the detectors may be separate to the device.

In an embodiment, each input signal is split into an upper sideband signal (i.e. the signal carried by the upper sideband of the input signal) , a lower sideband signal (i.e. the signal carried by the lower sideband of the input signal) and a double sideband signal (i.e. the signal carried by both sidebands when combined). Each of these three signals is a sideband signal. These sideband signals each carry the same modulation, apart from noise added to the input signal before and after detection. There is therefore redundancy in the sideband signals of the input signal. The quality metric of each signal is then generated. The quality metrics of each signal are compared. In an embodiment, the or each input signal is split into more than three sideband signals.

In an embodiment, the sideband signal corresponding to the highest quality metric, i.e. the quality metric showing the highest quality of sideband signal, is selected, and in an embodiment that sideband signal is output from the device. In an embodiment, the quality metric for at least one of the sideband signals is determined by measuring the ratio of peak voltage to RMS (root mean square) voltage of the signal. In an embodiment, the quality metric for at least one of the sideband signals is determined by measuring the bit error rate of a respective sideband signal. The bit error rate is determined by measuring the ratio between the number of bits that are received that are erroneous in value with the total number of bits received.

In an embodiment of the invention, the quality metric for at least one of the sideband signals is determined by measuring modulation of the bit width of a respective sideband signal. The modulation of the bit width is determined by analysing the received bit widths against the known correct bit width. The lower the level of modulation, and/or the closer the actual bit width to the expected bit width, the higher the quality metric.

The output sideband signal(s) may be output to a demodulator, and the signal from the sonde interpreted. The output may be to a switch or multiplexer, to connect the selected sideband signal(s) directly with a demodulator or other device. In an embodiment the sideband signals are converted from analogue to digital before quality metrics are determined, and the sideband signals demodulated before quality metrics are determined. In an embodiment, the input signals are amplified, filtered and digitised using an ADC (analogue to digital converter). In an embodiment, the data in each input signal is recovered and the amplitude and phase of each incoming signal is measured.

In an embodiment a phase locked loop double sideband demodulator is used to demodulate the double sideband signal. In an embodiment, two Hilbert transformer demodulators are used to demodulate the single sideband signals.

Brief description of the drawings

Embodiments of the invention will now be described, purely by way of example, in reference to the following drawings, in which: Figure 1 shows a device according to a first embodiment of the invention; Figure 2 shows a flow diagram showing a method of operation of the device of Figure 1; Figure 3 shows a device according to a second embodiment of the invention; and Figure 4 shows a flow diagram showing a method of operation of the device of Figure 3.

Detailed description of embodiments of the invention A device according to a first embodiment of the invention is shown in Figure 1. The device comprises an initial processing stage 100, a signal splitter 110 to split the detected signals and route them to first, second and third demodulators 112, 114 and 116, which demodulate the detected signals after splitting. The device also comprises a, quality metric generation stage 120 connected to each of the outputs of the demodulators 112, 114, 116 to generate quality metrics for the demodulated signals, a comparison logic unit 130 for comparing the quality metrics, selecting a preferred demodulated signal and controlling a multiplexer to output signal corresponding to the chosen quality metric. A main stage 150 comprises all parts of the device apart from the comparison logic unit 130.

The initial processing stage 100 comprises an antenna 102, which detects an electromagnetic field generated by an underground transmitter (sonde). A switched gain amplifier 104, a bandpass filter 106, an analogue to digital converter (ADC) 107 and a transformer 108 are connected in series to the antenna 102. The output from the transformer 108 is connected to the signal splitter 110. The first, second and third demodulators 112, 114, 116 respectively demodulate the upper, lower and double sideband signals of the input signal. In the present embodiment, the signal emitted from the sonde is an amplitude modulated (AM) signal, the amplitude modulation producing the sideband signals. The upper and lower sideband signals each carry the same modulated signal. The double sideband signal is the combination of both upper and lower single sideband signals. In this embodiment, the first and second demodulators, demodulating the upper and lower single sideband signals respectively, are Hilbert transformer demodulators. The third demodulator 116 is a phase locked loop double sideband demodulator.

The quality metric generation stage 120 comprises first, second and third quality metric generators 122, 124, 126, which receive demodulated signals from and generate quality metrics for the first, second and third demodulators 112, 124, 126 respectively. The quality metrics are output to the comparison logic unit 130, which then controls the multiplexer 140 to output one of the sideband signals.

Figure 2 shows a method of operation of the device of the first embodiment for use in detecting an underground transmitting sonde. An input signal is received at the antenna 102 at S100. The signal is subject to electromagnetic noise, especially that caused by mains power cables, which produce fields at multiples of for example, 50Hz in the United Kingdom, and 60Hz in the United States of America. It is convenient to assume in the system that the mains interference will be at multiples of 300Hz as this encompasses both 50Hz and 60Hz noise.

The signal is amplified by the amplifier 104 at S102, before being passed through the filter 106 at S104. The filter 106 removes components at low frequencies, e.g. mains frequencies, and so has a high pass to reject signals below 60Hz. The filter also has low pass to reject frequencies above 9kHz. The low pass is included to avoid the ADC 107 failing to reject multiples of its sampling rate. The ADC 107 converts the analogue signal into a digital signal at S106 and the digital signal is output to the transformer 108.

The transformer 108 converts the digitised signal into amplitude and phase elements, which can then subsequently be analysed.

The transformed signal is then split by the signal splitter 110 and a signal is passed to each of the first, second and third demodulators 112, 114, 116. The demodulators 112, 114, 116 demodulate each sideband signal at S110. The Hilbert transformer of the first demodulator 112 removes frequency components at and below the carrier frequency.

The Hilbert modulator of the second demodulator 114 removes frequency components at and above the carrier frequency. The phase locked loop double sideband demodulator 116 demodulates the combined signal of both upper and lower sideband signals. Each demodulated signal is passed to the multiplexer 140 for selective output at S112.

The quality metric generation stage 120 generates quality metrics for each of the demodulated signals at S 114. The quality metric generators 122, 124, 126 make use of "Crest factor measurement" to generate the quality metrics. This measurement finds the ratio of the peak amplitude of the demodulated signal to the RMS value of the signal.

The crest factor is the quality metric in this embodiment and gives an indication of the relative noise in a signal, the higher the value, the lower the noise level. The quality metrics are compared by the comparison logic unit 130 and the highest quality metric is selected at S 116. The comparison logic unit 130 then controls the multiplexer 140 to output the demodulated sideband signal having the highest quality metric at S 118.

The quality metric generators 122, 124, 126 may also calculate other measures of the quality of the signals. For example, the bit error rate may be calculated for each demodulated signal. The bit width modulation of each signal may also be calculated.

The quality metric generators 122, 124, 126 may produce some or all ofthese three signal quality indicators and the comparison logic unit 130 may choose the signal to be output by a combination of these metrics with any suitable weighting being used. Other suitable signal quality indicators may also be used.

If the signals are processed with substantially random noise at a low level, then the double sideband signal will generally have the highest quality metric because only common noise between the upper and lower sideband signals will be in the combined signal. This should lead to a 3dB improvement in signal to noise ratio for the double sideband signal. However, if there is a noise signal of periodic nature, this will generally appear in one or other of the sidebands only. The sideband which does not contain the periodic noise signal will therefore have a lower noise level and higher quality metric than the other sideband signal. Once the periodic noise reaches a certain level, the non- interfered sideband signal will have a higher quality metric than the double sideband signal, and this single sideband signal will be output from the further multiplexer 240.

Figure 3 shows an apparatus according to a second embodiment of the invention. The apparatus comprises three main stages 200A, 200B, 200C, comprising the parts shown in the first embodiment of the invention within system 150. A comparison logic unit 230 receives quality metrics from each of the main stages and sends control signals to the multiplexer in each main stage 200A, 200B, 200C. Each main stage 200A, 200B, 200C outputs a selected demodulated signal to a further multiplexer 240. The comparison logic unit 230 also sends control signals to the further multiplexer 240, and the further multiplexer 240 outputs one of the sideband signals it receives.

Figure 4 shows a method of operation of the apparatus of the second embodiment. At S200, S202 and S204, the selected sideband signals from each main stage 200A, 200B, 200C are output on the basis of the quality matrices, as described in the first embodiment. Each selected signal is input into the further multiplexer 240 at S206.

The quality metrics for each selected signal are compared by the comparison logic unit 230 at S208, and the comparison logic unit 230 controls the further multiplexer 240 to output a sideband signal selected by the comparison logic unit 230 at S210.

Alternatively, the dotted line in figure 4 shows a variation in the method. The quality metrics can be determined from the signals from each main stage 200A, 200B, 200C, rather than using the already generated quality metrics. In this case, comparison logic unit 230 can be two independent comparison logic units, one of which selects a sideband signal for each main stage 200A, 200B, 200C, and one of which selects a signal from the three main stage 200A, 200B, 200C outputs.

In implementation, the comparison logic unit 130, 230 and the quality matrix generation stages 110 of either of the above embodiments may be combined in a single processor.

The multiplexers 140, 240 of either embodiment may also be combined in the single processor. The signal splitters 110 and/or the demodulators 112, 114, 116 may also be combined in a single processor. In fact, any suitable combination of the above parts may be placed in one or more processors. The one or more processors may be software controlled, and any software used, in order for the device of the invention to function correctly, is also a part of the invention.

The chosen signal output from the multiplexer 140 in the first embodiment, or further multiplexer 240 in the second embodiment is then processed to decode the signals encoded into the electromagnetic field emitted by the sonde, in order to determine the pitch, yaw or orientation, for example, of the sonde.

The second embodiment is particularly useful in detection of underground sondes, because the three main stages can comprise three antennae, wherein two of the antennae are horizontally mounted, perpendicular to one another, and the third is vertical. This configuration allows the antennae used to locate a sonde in three dimensions to also be used to demodulate the additional data carried by the AM carrier sideband signals.

Therefore, a total of nine sidebands can be analysed and the strongest signal used for the subsequent decoding of the signals to obtain the data sent from the sonde to the monitor.

The apparatus also requires only the three antenna for all of the location and decoding processes. This allows the apparatus to be of a relatively compact size.

Any discussion of the prior art throughout the specification is not an admission that such prior art is widely known or forms part of the common general knowledge in the

field.

It should be appreciated that further modifications and variations will suggest themselves to those versed in the art upon making reference to the foregoing description, which is given by way of example only and which is not intended to limit the scope of the invention. In particular, although the invention has been particularly described in relation to AM signals, it could equally apply to frequency modulation (FM) signals and phase modulation (PM) signals, and the invention encompasses the use of any such modulation system. The invention also encompasses systems for analysing input systems comprising more than two different sidebands, for example in FM signal demodulation, and more than three sideband signals may be demodulated from a single input signal, in the same/similar ways as described above.

The present invention has been described above purely by way of example, and modifications can be made within the spirit of the invention. The invention also consists in any individual features described or implicit herein or shown or implicit in the drawings or any combination of any such features or any generalization of any such features or combination, which extends to equivalents thereof. Each feature disclosed in the specification, including the claims, abstract and drawings may be replaced by alternative features serving the same, equivalent or similar purposes, unless expressly stated otherwise.

Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise", "comprising", and the like, are to be construed in an inclusive as opposed to an exclusive or exhaustive sense; that is to say, in the sense of"including, but not limited to".

Claims

CLAIMS: 1. A signal interference reduction device for reducing the effects

of magnetic interference in an electromagnetic field from a buried or inaccessible object, the device comprising: at least one receiving means for receiving at least one input signal, each input signal comprising a plurality of sideband signals; determining means for determining a plurality of quality metrics for the plurality of sideband signals; comparison means for comparing the determined quality metrics; and output means for selectively outputting at least one sideband signal dependent on the compared quality metrics.
2. A device according to claim 1, further comprising demodulating means for demodulating the plurality of sideband signals of said at least one input signal.
3. A device according to claim 2, wherein the demodulating means for demodulating the double sideband signal comprises a phase locked loop demodulator.
4. A device according to claim 2, wherein the demodulating means for demodulating each single sideband signal are Hilbert transformer demodulators.
5. A device according to any of the preceding claims, further comprising selecting means for selecting a number of the plurality of sideband signals of said at least one input signal for comparison.
6. A device according to claim 5, wherein the selecting means is arranged to select sideband signals from a group containing upper, lower and double sideband signals of a single input signal.
7. A method according to claim 5, wherein the selecting means is arranged to select sideband signals from a group containing upper, lower and double sideband signals of a plurality of input signals.
8. A device according to any one of the preceding claims, wherein the output means is arranged to output said at least one sideband signal with a highest quality metric.
9. A device according to any one of the preceding claims wherein the determining means is arranged to determine at least one quality metric by measuring the ratio between the peak and RMS voltages of a respective sideband signal of said at least one input signal.
10. A device according to any one of the preceding claims wherein the determining means is arranged to determine at least one quality metric by measuring the bit error rate of a respective sideband signal.
11. A device according to any one of the preceding claims wherein the determining means is arranged to determine at least one quality metric by measuring modulation of the bit width of a respective sideband signal.
12. A device according to any one of the preceding claims, wherein the determining means and the comparison means comprise a processor.
13. A method of reducing effects of magnetic interference in an electromagnetic field from a buried or inaccessible object, the method comprising: receiving at least one input signal, each input signal comprising a plurality of sideband signals; determining a plurality of quality metrics for the plurality of sidebands signals; comparing the determined quality metrics; and selectively outputting a sideband signal dependent on the comparison of the quality metrics.
14. A method according to claim 13, further comprising selecting a number of the plurality of sideband signals of said at least one input signal to be compared.
15. A method according to claim 13, wherein the sideband signals of the at least one input signal are chosen from a group containing upper, lower and double sideband signals of a single input signal.
16. A method according to claim 13, wherein the sideband signals of the at least one input signal are chosen from a group containing upper, lower and double sideband signals of a plurality of input signals.
17. A method according to any one of claims 13 to 16, wherein the sideband signal with a highest quality metric is output.
18. A method according to any one of claims 13 to 17, wherein at least one quality metric is determined by measuring the ratio between the peak and RMS values of a respective sideband signal.
19. A method according to any one of claims 13 to 18, wherein at least one quality metric is determined by measuring the bit error rate of a respective sideband signal.
20. A method according to any one of claims 13 to 19, wherein at least one quality metric is determined by measuring the bit width modulation of a respective sideband signal.
21. A method according to any one of claims 13 to 20, wherein the method is carried out by a processor.
22. A data carrier medium carrying processor readable code to instruct a processor to carry out the method of claim 21.
23. A signal interference reduction device for reducing the effects of magnetic interference in an electromagnetic field from a buried or inaccessible object, the device comprlsmg: a receiver to receive at least one input signal, each input signal comprising a plurality of sideband signals; a processor connected to the receiver to determine a plurality of quality metrics for the plurality of sideband signals, and to compare the determined quality metrics; and an output device connected to the processor to selectively output one or more sideband signals dependent on the compared quality metrics.
24. A device according to claim 23, further comprising a plurality of demodulators connected to the receiver to demodulate the plurality of sideband signals.
25. A device according to claim 23 or claim 24, wherein one of the demodulators is a phase locked loop demodulator.
26. A device according to any one of claims 23 to 25, wherein one of the demodulators is a Hilbert transformer demodulator to demodulate a single sideband signal.
27. A device according to claim 26, further comprising a further Hilbert transformer demodulator to demodulate a further single sideband signal.
28. A device according to any one of the preceding claims, wherein the processor is adapted to select a number of the plurality of sideband signals in said at least one input signal for comparison.
29. A device according to claim 28, wherein the processor is adapted to select sideband signals from a group containing upper, lower and double sideband signals of a single input signal.
30. A method according to claim 28, wherein the processor is arranged to select sideband signals from a group containing upper, lower and double sideband signals of a plurality of input signals.
31. A device according to any one of claims 23 to 30, wherein the output device is arranged to output the sideband signal of said at least one input signal with a highest quality metric.
32. A device according to any one of claims 23 to 31, wherein the processor is adapted to determine at least one quality metric by measuring the ratio between the peak and RMS voltages of a respective sideband signal.
33. A device according to any one of claims 23 to 32, wherein the processor is adapted to determine at least one quality metric by measuring the bit error rate of a respective sideband signal.
34. A device according to any one of claims 24 to 33, wherein the processor is adapted to determine at least one quality metric by measuring modulation of the bit width of a respective sideband signal.