WO1996029572A1

WO1996029572A1 - Amplifying signals

Info

Publication number: WO1996029572A1
Application number: PCT/GB1996/000679
Authority: WO
Inventors: Duncan Smith
Original assignee: Radiodetection Limited
Priority date: 1995-03-21
Filing date: 1996-03-21
Publication date: 1996-09-26
Also published as: GB9505636D0

Abstract

In order to amplify a plurality of input signals, the input signals received on a plurality of input channels (2, 3) are passed to a corresponding plurality of amplifiers (20, 30) via first switches (10, 12) which permit each input channel (2, 3) to be connected to each amplifier. Similarly, the outputs of the amplifiers (20, 30) are passed to a corresponding plurality of output channels (17, 18) via second switches (11, 13) which permit each amplifier (20, 30) to be connected to each output channel (17, 18). By controlling the switching of the first and second switches (10, 12; 11, 13), the input on any one input channel (2, 3) is output via a corresponding one output channel (17, 18), but is amplified subsequently by the plurality of amplifiers (20, 30). This prevents variations in the amplifiers affecting comparison of the output, particularly if the outputs on the output channels (17, 18) are time averaged.

Description

AMPLIFYING SIGNALS

The present invention relates to a receiver for amplifying a plurality of input signals. Many conventional electronic instruments used in measuring applications receive input signals via more than one input channel, and require the outputs derived from such multiple input channels to be compared during signal processing. Here, the term "input channel" refers to a specified path for the transmission and reception of signals, rather than a frequency band.

If the signal to be sensed is weak, it normally requires amplification before being processed, and this amplification is performed separately for each input channel. Ideally the input channels will be connected to respective amplifiers each with identical responses, but in practical situations small variations in the amplifier components cause the amplifiers to have different gains. Such differences in the gains of the amplifiers will cause errors in the comparison of the outputs. A calibration process is then required to offset these differences so that a comparison of the amplified outputs gives the true relationship of the input signals. Such a calibration process typically involves the use of variable resistors or inductors which add cost to the product and the associated quality control system. An example of a device which is to sense weak signals, which are likely to require amplification, is a locator for locating a buried conductor cable in such as a pipe. Such a locator has aerials which detect alternating magnetic fields caused by electrical currents in the underground pipe or cable. In this way the location of the underground pipes or cables can be determined. It is disclosed in GB-A-1509914 and GB-B- 2075199 that the performance of such locators can be improved by using two aerials which are parallel to each other at differing but known heights above the ground and comparing the signals in the two aerials. It may, in certain circumstances, also be advantageous to use more than two aerials. Such systems are subject to the disadvantages described above, namely that the amplifiers which amplify the signals corresponding to the detected magnetic fields must be calibrated.

The present invention proposes that each input signal is fed to each of the amplifiers in turn, and the output corresponding to the input signal is then averaged over time. If all input signals are processed in this way, so that they are all amplified by all the amplifiers, then the time averaged outputs will then be independent of variations in the amplifiers.

Thus, a receiver for amplifying a plurality of input signals received on a plurality of input channels may have amplifiers equal in number to the number of input channels, and means for switching the input channels among the amplifiers, so that each input channel can be connected to each amplifier. The outputs of the amplifiers may then also be switched in sequence with the switching of the input channels, so that output signals are generated corresponding to the input signals. Over a suitable period of time, those output signals will then correspond to the input signals are amplified by each of the amplifiers sequentially. Then, a time averaging is applied to the output signals, which is such as to ensure that each time averaged signal is independent of any particular amplifier. As a result, the time averaged signals are then directly comparable, if comparison of the input signals is needed.

As described above, the switching may be performed on both the input and output sides of each amplifier. Alternatively, the switching on the output side of the amplifiers may be dispensed with by providing a computer to recognise which particular aerial/amplifier combination is represented by the output at any time in the cycle.

At its simplest, the switching of the input channels to the amplifiers may be such that each channel is connected to each amplifier for a common predetermined time. The time average of the output is then a direct one. However, the present invention is not limited to such common predetermined time switching, and it is possible for one or more of the input channels to be connected to the amplifiers for different lengths of time. In this case, however, the output signals must be weighted, in dependence on the relative lengths of time the corresponding input channel is connected to each amplifier, in order to get a satisfactory time averaged signal.

When using the receiver of the present invention the amplifiers need not be calibrated because each channel is connected to each amplifiers successively. Thus, the averaging process which takes place for each output can be performed so as to result in the same percentage error being present on each output, so that the amplified outputs reflect the true input channel signal levels. Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 is a circuit diagram of a receiver having two input channels, being a first embodiment of the invention.

Fig. 2a is a diagram of one amplified output corresponding to the first input channel, before being time-averaged when the input signal is alternating; Fig. 2b is a diagram of another amplified output corresponding to the second input channel, before being time-averaged when the input signal is alternating; and

Fig. 3 is a circuit diagram of a receiver having two input channels, being a second embodiment of the present invention.

In the first embodiment shown in the drawings, a receiver 1 has two input channels 2,3 by which it receives suitable input signals. Those signals may, for example be generated by aerials (not shown) which detect varying electromagnetic fields generated by alternating currents in an underground conductor, such as a cable or a pipe. Each input channel 2,3 is connected to amplifiers 20,30 via switches 10,12 which enable each input channel 2,3 to be connected to each amplifier 20,30. The switches 10,12 are such that when one input channel 2 is connected to one of the amplifiers 20 the other input channel 3 is connected to the other amplifier 30, and vice versa. The amplifiers 20,30 have nominally identical gains but slight variations in their components will normally make their gains slightly different. On the output side of the amplifier 20,30 are further switches 11,13 which connect the amplifiers 20,30 to averaging circuits 15,16. Again, these switches 11,13 are arranged such that each amplifier 20,30 may be connected to each averaging circuit 15,16, with the switching being such that when one amplifier 20 is connected to one of the averaging circuits 15 the other amplifier 30 is connected to the other averaging circuit 16, and vice versa. Moreover, the switching of the further switches 11,13 is synchronised with the switches 10,12, e.g. by a suitable pulse train 39. Hence, the signal on line 17 to the averaging means 15 always corresponds to the input to the input channel 2, and the signal on line 18 to the other averaging means 16 always corresponds to the input to the second input channel 3. Thus, the signal on line 17 will correspond to the input signal of the input channel 2 amplified successively by the two amplifiers 20,30. Similarly, the signal on line 18 will correspond to the input signal to the input channel 3, amplified successively by the amplifiers 30,20. Hence, over a suitably long period of time, each input signal is amplified by each amplifier 20,30.

As stated above, a square-wave pulse 39 is used to trigger each switch 10,11,12,13 in synchronisation between its two states. The mark-to-space ratio of the square wave pulse 39 is 1:1 so that the switching occurs at constant time intervals.

Suppose now that the first input channel 2 is connected via the first amplifier 20 to the first averaging means 15 and the second input channel 3 is connected via the second amplifier 30 to the second averaging means 16. When the square-wave pulse 40 triggers the switches 10,11,12,13 the first input channel 2 becomes connected via the second amplifier 30 to the first averaging circuit 15 and the second input channel 3 becomes connected via the first amplifier 20 to the second averaging circuit 16. Further triggering by the square-wave pulse 40 causes the receiver 1 to alternate regularly between these two switching states. Thus, a given input channel 2,3 always provides an input to the same averaging means 15,16 but via alternating amplifiers 20,30.

Fig. 2a shows the signal on line 17 from the switch 11 before the averaging process is carried out by the averaging circuit 15 in the case when the first input channel 2 is first connected to whichever of the amplifiers 20,30 has the lower gain. Fig. 2b shows the signal on line 18 from the switch 13 before the averaging process is carried out by the averaging circuit 16, the second input channel 3 then being connected first to whichever of the amplifiers has the higher gain. In both cases the amplitude of the alternating signal is shown on the y-axis, with time along the x-axis. The difference in amplifier gain introduces an amplitude modulation in the signals on lines 17 and 18. Thus in Fig. 2a there is a periodically varying signal, the first half 17a of which has a relatively low amplitude and a duration equal to the time between switching, and the second half 17b of which has a relatively high amplitude and the same duration. In contrast, Fig. 2b shows a signal with a relatively high amplitude in the first half 18a of the cycle and a relatively low amplitude in the second half 18b. The signal on each line 17,18 then enters the corresponding averaging circuit 15,16. These include conventional rectifying circuits 22, 24 and filtering circuits 26, 28 to remove the high frequency components of the signal. The averaging circuits 15,16 then derive a d.c. value equivalent to the average of the signals on lines 17 and 18. Although the errors in each signal are not individually eliminated, each signal is subject to the same percentage error because both signals have been amplified by both amplifiers for the same period of time. Thus the outputs of the averaging circuits 15,16 and hence the signals to the input channels 2,3 can be compared without being subject to errors due to the amplifiers and the amplifiers need not be calibrated.

In the embodiment of Fig. 1, the averaging circuits 15, 16 may be formed by suitable discrete components. Fig. 3 shows a second embodiment, in which the averaging operation is carried out by a suitable microprocessor 40. Components for this embodiment which are similar to that of Fig. 1 are indicated by the same reference numerals. In the embodiment of Fig. 3, the microprocessor 40 carries out series of functions on the signals from the amplifiers 20, 30 equivalent to components now to be described. The first operation is a switching operation by switch 41 which passes either the signal from the amplifier 20 or the signal from the amplifier 30 to an analog to digital convertor 42. The digital signals then generated are passed via a further switching function 43 to one of two paths. Switches 41 and 43 thus carry out switching of signal paths in a way similar to the switches 11 and 13. Each signal path contains a rectifying function 44, 45 and a filtering function 46, 47 which are similar to the rectifiers and filters 22, 24, 26 and 28. However, in the embodiment of Fig. 3, the switch 43, the rectifiers 44, 45 and the filters 46, 47 are functions implemented by suitable software programming of the microprocessor 40. As before, a d.c. value is derived which is equivalent to the average of the signals. This derived d.c. value could also be made equivalent to the RMS value of the signal. While the above embodiments of the invention have been described in detail for alternating electromagnetic field sensing receivers using two channels, the principle of cyclic switching of amplifiers can be extended to other types of sensor, and/or to any number of channels.

Claims

1. An amplifier system having: a plurality of input channels for receiving a plurality of input signals; a corresponding plurality of amplifiers (20, 30) for amplifying said input signals; a corresponding plurality of output channels (17, 18); first switch means (10, 12) for switchably interconnecting said plurality of input channels (2, 3) to said plurality of amplifiers (20, 30) such that each input channel (2, 3) is connectable to each amplifier (20, 30) whereby said input signals are successively amplifiable by different ones of said plurality of amplifiers (20, 30); second switch means (11, 13; 41, 43)for switchably interconnecting said plurality of amplifiers (20, 30) to said plurality of output channels (17, 18) such that each amplifier (20, 30) is connectable to each output channel (17, 18); and means (39, 40) for synchronising the first and second switch means (10, 12; 11, 13) to switch in sequence such that each input channel (2, 3) is connectable by said first and second switch means (10, 12; 11, 13; 41, 43) to a corresponding one of said plurality of output channels (17, 18).

2. An apparatus according to claim 1, wherein said first switch means (10, 12) comprises a plurality of first switches (20, 30) corresponding to the plurality of amplifiers, (20, 30) each of the first switches having an output connected to a respective one of the amplifiers (20, 30) and a plurality of inputs respectively connected to the plurality of input channels (2, 3) and means for switchably interconnecting said output and any one of the inputs of the switch.

3. An apparatus according to claim 1 or claim 2, wherein said second switch means (11, 12) comprises a plurality of second switches corresponding to the plurality of output channels (17, 18), each of the second switches having an output connected to a respective one of the output channels (17, 18), a plurality of inputs respectively connected to the plurality of amplifiers (20, 30), and means for switchably interconnecting said output and any one of the inputs of the switch.

4. An apparatus according to claim 1 or claim 2, wherein said second control means (41, 43) comprises second and third switches arranged in series, the second switch having a plurality of inputs, an output and means for switchably interconnecting said output and any one of said inputs, and the third switch has an input connected to the output of the second switch, a plurality of outputs respectively connected to the plurality of output channels (17, 18), and means for switchably interconnecting the input to one of the outputs.

5. An apparatus according to claim 3 or claim 4, having a processing device (40), said processing device being programmed so as to form said second switch means (41, 43).

6. An apparatus according to claim 5, wherein said processing device (40) is programmed so as to form said synchronising means.

7. An amplifier system having: a plurality of input channels for receiving a plurality of input signals; a corresponding plurality of amplifiers (20, 30) for amplifying said input signals; a corresponding plurality of output channels (17,

18); first switch means (10, 12) for switchably interconnecting said plurality of input channels (2, 3) to said plurality of amplifiers (20, 30) such that each input channel (2, 3) is connectable to each amplifier

(20, 30) whereby said input signals are successively a plifiable by different ones of said plurality of amplifiers (20, 30); and processing means for analysing the outputs of the respective output channels in dependence on the action of said first switch means, to determine which of said plurality of output channels is connected to each output channel.

8. An apparatus according to any one of the preceding claims, having means (15) for averaging output signals output from said output channels.