GB2238201A

GB2238201A - Ground probing radar

Info

Publication number: GB2238201A
Application number: GB9023454A
Authority: GB
Inventors: Paul Beaumont Cordes
Original assignee: British Gas PLC
Current assignee: British Gas PLC
Priority date: 1989-11-17
Filing date: 1990-10-29
Publication date: 1991-05-22
Anticipated expiration: 2010-10-29
Also published as: GB9023454D0; GB2238201B

Abstract

Radio transmission especially ground probing radar for locating objects buried in the ground such as pipes and cables varies the frequency of a controlled oscillator (16) by feeding it with random or pseudo-random voltages or currents. A feed for a transmitter is derived from the oscillator. Pseudo-random voltages may be produced by driving a digital to analogue convertor (14) by three outputs from a 16-stage shift register (51 ... 516). The oscillator (16) has a voltage variable capacitor (20) and its output is divided by 64 internally before being passed to a monostable (28) to drive an impulse generator (30). The transmission is received by a sampling gate (34) triggered by a voltage controlled delay 32 receiving the output from the monostable (29). The voltage controlled delay is set by a ramp voltage produced by a digital to analogue convertor (44) driven by a counter (42) receiving the output from the monostable (28). The oscillator may itself contain a noise source (126, Fig 9) connected into a feedback loop controlling the oscillator. <IMAGE>

Description

METHOD & APPARATUS FOR RADIO TRANSMISSION This invention relates to methods and apparatus for radio transmission, particularly though not exclusively for use as part of ground probing radar in locating objects buried in the ground.

Safety considerations require spurious emissions of power from transmitters of radio frequencies to be limited. For example, in the United Kingdom the Department of Trade and Industry has issued regulations requiring such emissions to be below 4 nanowatts (nW) at any frequency in certain bands used for Instrument Landing Systems (ILS) applications and below 250nW elsewhere in the range 0 to 2000MHz. Such considerations apply to radar transmitters, for example, and the invention provides a method and apparatus applicable particularly, though not exclusively, to ground-probing radar transmitters used for locating pipes and cables buried underground.

In a typical ground-probing radar system, for example, the transmitter emits pulses of electromagnetic energy at intervals of 12ns, the pulse width being lns. The energy radiated is distributed among frequencies spaced apart by 82.5 kHz and ranging in a spectrum from zero to over 1GHZ. Each such frequency may be considered to be a line in the spectrum, the height of each line representing the energy associated with that particular frequency (see Figure 1).

The maximum power in watts in a line is approximately (ftv)2/50 when the transmitter in a line is approximately a load of 50 ohms, where f is the pulse repetition frequency, t is the pulse width in seconds and V is the amplitude of the pulses in volts.

Typically, for example, f = 80 kHz, t = 1 ns and V = 100 volts.

In that case the maximum spectral line power is 1300nW, which exceeds the desired maximum of 4nW by a factor of 325. The maximum spectral line power could be reduced by, for example, reducing the pulse amplitude by a factor of (325)0.5 to 5.6 volts or by reducing f to 4.5 kHz. Neither of those solutions is acceptable.

The object of the invention is to provide a method and apparatus by which the maximum spectral line power can be reduced by distribution of the transmitted power among a relatively larger number of spectral lines.

In general the output waveform of the impulse generator is of the form shown in Figure 2(a). It consists of a regularly spaced (in time) series of voltage spikes, which are fed to the antenna.

The spectral analysis of this signal is a fundamental sine wave and a series of harmonics diminishing in intensity with increasing order extending to infinity, as in Figure 2(b).

The basic relationship between pulse repetition rate (prf) of the pulse train, fA, and frequency spacing TA is f = l/tA. From this relationship it may be seen that a change in the prf of the pulse train results in a corresponding shift in the frequencies of the resulting fundamental and harmonics. The ability to use prf variation depends on the sampling system operation being tolerant of small changes in prf.

In Figure 3a the top line shows the pulse repetition rate of the pulses is decreased compared with Figure 2a and the bottom line shows the rate increased compared with Figure 2a.

Figure 3b shows the corresponding spectra. The upper line shows the frequencies closer together and the lower line shows them spaced further apart.

Consider a situation where the prf is varied in a repetitive sinusoidal manner, from a prf of 0.9 fA up to 1.1 fA and back down to 0.9 f over a period of Tc and then repeating Figures 4 (a). A corresponding sweep of fundamental and harmonic frequencies will occur, as in Figure 4(b). Considering the fundamental frequency of the impulse train prf, its frequency will vary over a range f plus or minus 0.1fA. 1A Regarding the fifth harmonic, its frequency will vary over the range 5fA plus or minus 0.1(5fA), the upper frequency limit being thus 5.5f.

Consider the sweep limits of the sixth harmonic, the lower being 6 x 0.9fA = 5.4fA. 4A It can be seen therefore that in this case the lower frequency limit of the sixth harmonic is lower than the upper limit of the fifth harmonic. Therefore, in this instance, there is the potential for the generation of all the spectral frequencies above 5fA. If the frequency deviation is increased, overlap could occur for lower order harmonics. In practice, the range of frequencies in which energy may be present is further constrained by the rate at which the prf is varied. In practice, energy will be distributed in sidebands of the harmonics, the spacing of which is equal to the reciprocal of the sweep cycle period.

Thus, a condition of being able to spread spectral energy over a continuous spectrum is that the reciprocal of the sweep cycle must approach zero frequency. This is clearly impractical in that, at any instant in time, energy will exist in a number of harmonically related frequencies, the values of which will be varying in time.

It is therefore necessary to employ a frequency sweeping function containing energy at relatively high frequencies. A suitable frequency sweeping function would be one in which energy is spread equally among not less than 325 frequencies within the bandwidth of the modulating signal (since the energy per line is required to be reduced by this factor in this instance). A better solution is to employ a signal with true random or pseudo random properties containing energy at all frequencies from (ideally) zero to some upper limit. In using this signal to control the prf, these properties will be imposed upon the fundamental and harmonics of the spectrum of the impulse generator (Figures 5(a) and 5(b)).

According to the invention, a method of effecting radio transmission comprises varying the frequency of a controlled oscillator of the transmitter by controlling it with random or pseudo-random voltages or currents and deriving from the oscillator output a feed for the impulse generator of the transmitter.

Apparatus for effecting radio transmission according to the invention comprises a transmitter in which a controlled oscillator generates a feed for an impulse generator of the transmitter, the oscillator being controlled by voltages from a digital-to-analogue convertor which is driven by at least some of the voltage levels available from a multi-stage shift register.

Apparatus for effecting radio transmission according to the invention comprises a transmitter in which a controlled oscillator generates a feed for an impulse generator of the transmitter, the oscillator being controlled by an amplifier receiving the output from a semi-conductor noise generating device.

Apparatus for effecting radio transmission according to the invention comprises a transmitter in which a controlled oscillator generates a feed for an impulse generator of the transmitter, the oscillator including a source of noise the output of which controls the oscillator to vary the frequency of the oscillator.

The invention will be described by way of example with reference to the following drawings, in which Figure 1 is a spectrum showing energy plotted against frequency for a typical known type of radio signal; Figures 2(a) and 2(b) are typical trains of impulses used to drive a transmitter and a spectrum, respectively, of the same train of impulses; Figures 3(a),3(b); 4(a),4(b); and 5(a),5(b); are figures corresponding to figures 2(a) and 2(b) showing different variations in the rates of the source impulse trains and their spectra; Figure 6 is a block diagram of one form of circuit according to the invention showing how the method can be performed; Figures 7 and 8 correspond to Figure 6 and show variations on the circuit; and Figure 9 shows a further variation of the circuit.

Figure 6 shows one form of apparatus, in this example it is particularly suitable for use in detecting objects buried underground, such as cables and pipes using ground-probing radar.

Figure 6 shows a system for detecting objects buried in the ground by ground-probing radar, including a transmitter and a sampling receiver. The signal to be fed to the transmitter antenna at 10 is generated using a pseudo-random sequence generator 12. This consists of a sixteen stage shift register and the stages are numbered S1 ------ S16. Each stage is a digital integrated circuit. Of course, a number of stages other than sixteen can be used e.g. 4,8,12 etc. Three 'bits' of information are used as the output of the register, each 'bit' being a number and the numbers are fed to a digital-to-analogue convertor 14. Of course, more than three 'bits' of information may be used but three is quite sufficient.

Instead of using a generator based on a shift register, a counter and memory circuit may be employed, whereby the counter generates addresses for the memory and the data contained in and read out from the memory consists of pseudo-random binary numbers.

The output of the convertor 14 is a series of voltage levels which may take any of 23 = 8 discrete voltage levels. The output is fed to a voltage-controlled oscillator 16. In a modification this could be replaced by a current-controlled oscillator. In this example, the frequency of the oscillator 16 is controlled by an inductor 18 and a voltage-variable capacitor 20 or an array of such elements, across which the output from the convertor 14 is imposed. In modifications the oscillator frequency could be determined by a resistor and a capacitor, or an array of such elements, rather than an inductor and a capacitor, or by an inductor and a resistor, or an array of such elements. Fixed capacitors 22,24 are also present in the oscillator 16. The oscillator 16 includes an amplifier 25 and a frequency-divider 26 which divides the frequency of the output by sixty-four.

In another modification (not shown) the divider 26 can be used to modify the frequency of the oscillator 16 in response to the output from the convertor 14.

The output of the oscillator 16 is fed to a monostable multivibrator 28 and to an impulse generator 30. The output from the generator 30 drives the transmit antenna. The monostable output is also fed back to the sixteen stages S1 ----- S16. The same output is fed to a voltage controlled delay 32 and thence to a sampling gate 34 in the line from a receive antenna at 36. The output from the sampling gate 34 passes to an analogue-to-digital convertor 38 and thence to the receive computer 40.

Finally, the monostable output is fed to a counter 42 and a digital to analogue convertor 44. The output from the convertor 44 sets the delay in the voltage controlled delay 32. For example, the delay may be set by a sequential ramp voltage generated by the counter 42 and the convertor 44.

With this circuit, sampling occurs in timed order sequence.

The oscillator operates at about 11 megahertz and the frequency of the output after division at 26 is about 180 kilohertz. The output from the monostable is a short fast pulse suitable for triggering the impulse generator of the transmitter and the receiver.

This pulse also drives the pseudo-random sequence generator so that each time a new impulse is generated the sequence generator advances and a new oscillator frequency, and hence a new interval between impulses, occurs in a random manner.

Figure 7 corresponds to Figure 6 but the random sequence generator is replaced by a true random noise generator. In this case it is an avalanche diode 50, the output of which is amplified at 52 and fed to the oscillator 16.

Figure 8 shows a slightly different arrangement for the circuit.

The output from the monostable 28 is fed to the impulse generator 30 via the voltage controlled delay 32. This is timed in random manner by the output from the digital-to-analogue converter 14.

The output from the monostable 28 is also fed, via a fixed delay 50, to trigger the sampling gate 34. Thus, the variable delay is imposed on the transmit line. For this reason the output of the pseudo-random sequence generator 12 is passed to the computer 40 so that it has the information necessary to re-constitute the received signal in the correct order. Instead of passing the output from the pseudo random sequence generator 12 to the computer 40 it would be possible for the computer to generate the same information. That would necessitate providing the same pseudo-random sequence generator at the computer or, instead, providing suitable software at the computer having the capability of generating the same sequence as the generator 12.

A power supply unit 60 is indicated by broken lines in Figure 6 and acts as a spurious transmitter which, if its output were not controlled, would have energies exceeding the level set by the Department of Trade and Industry. Furthermore, the power supply unit 60 would produce a signal which would 'beat' with the signal produced by the system otherwise shown in Figure 6.

In the systems described so far, the separate functional units of the circuit have provided the source of noise and the controlled oscillator. However, it is possible for the oscillator to provide the source of noise. Figure 9 shows, by way of example, how this could be done. The noise is internally generated within the input stage of the amplifier 125, within the oscillator 16.

This is indicated at 126, which is to be taken as representative of a diode, a transistor or some other semi-conductor element for example. Whatever the source 126, its output is added, by a summing function 128, to the input. The combined input passes to a gain function 130, and then to a non-linear function 132.

The non-linearity represented by 132 within the amplifier 125 causes the period of oscillation of the controlled oscillator to be dependent upon the amplitude of the noise output from the source 126.

Usually, of course, the spreading of energy from a single frequency, commonly called "phase noise", is present in all oscillators to some extent and is normally minimised by design.

In the present case, the noise is maximised to give as much spread of the signal radiated by the antenna as possible.

In Figure 9 the frequency-determining network 134 is shown arranged, for example, in a voltage feedback relationship with the amplifier 125. The network 134 would, for example, comprise a resistor and a capacitor, or an inductor and a capacitor, or an inductor and a resistor, or an array, in each case, of such elements.

The power supply employs switching techniques to achieve voltage conversion, and the regular nature of this switching generates radiated signals comprising a fundamental and harmonics similar in nature to the signal produced by the transmitter.

The output of the voltage controlled oscillator is used to control the switching circuits of the power supply thereby causing the instants at which switching occurs to be varied in a true random or pseudo-random manner. The effect of this is to spread the spectrum of the spurious radiation of the power supply in the same manner as is the radiation from the impulse generator of the transmitter. The invention is therefore useful in controlling this spurious transmission in the same way as it controls the ground probing radar transmission.

Claims

1. A method of effecting radio transmission comprising varying the frequency of a controlled oscillator of the transmitter by controlling it with random or pseudo-random voltages or currents and deriving from the oscillator output a feed for the impulse generator of the transmitter.

2. A method according to claim 1 in which voltages or currents are derived using digital techniques.

3. A method according to claim 1 in which the voltages are derived directly using analogue techniques.

4. A method according to any preceding claim, said transmission being received by a sampling receive system.

5. Apparatus for effecting radio transmission comprising a transmitter in which a controlled oscillator generates a feed for an impulse generator of the transmitter, the oscillator being controlled by voltages from a digital-to-analogue convertor which is driven by at least some of the voltage levels available from a multi-stage shift register.

6. Apparatus for effecting radio transmission comprising a transmitter in which a controlled oscillator generates a feed for an impulse generator of the transmitter, the oscillator being controlled by an amplifier receiving the output of a semi conductor noise generating device.

7. Apparatus according to claim 6 the device being an avalanche diode.

8. Apparatus for effecting radio transmission comprising a transmitter in which a controlled oscillator generates a feed for an impulse generator of the transmitter and said transmission being received by a sampling receive system and the sequence of sampling being in timed order both in transmit and receive mode.

9. Apparatus for effecting radio transmission comprising a transmitter in which a controlled oscillator generates a feed for an impulse generator of the transmitter and said transmission being received by a sampling receive system and the sequence of sampling not being in timed order.

10. Apparatus for effecting radio transmission comprising a transmitter in which a controlled oscillator generates a feed for an impulse generator of the transmitter, the oscillator including a source of noise the output of which controls the oscillator to vary the frequency of the oscillator.

11. Apparatus according to claim 5 substantially as herein described with reference to Figure 6 of the accompanying drawings.

12. Apparatus according to claim 6 substantially as herein described with reference to Figure 7 of the accompanying drawings.

13. Apparatus according to claim 5 substantially as herein described with reference to Figure 8 of the accompanying drawings.