PULSED RADAR LEVEL GAUGE SYSTEM AND METHOD FOR REDUCED RELATIVE BANDWIDTH

Abstract

A radar level gauge system comprising: pulse generating circuitry for generating an electromagnetic transmit signal in the form of a first pulse train formed by a time-sequence of substantially identical transmit pulses, each exhibiting a full period waveform; and an electromagnetic reference signal in the form of a second pulse train formed by a time-sequence of substantially identical reference pulses, each exhibiting a half period waveform; a propagation device arranged to propagate the transmit signal towards a product in a tank, and to return a surface reflection signal resulting from reflection of the transmit signal at a surface of the product; measurement circuitry for forming a measurement signal based on a time-correlation between the surface reflection signal and the reference signal; and processing circuitry connected to the measurement circuitry for determining the filling level based on the measurement signal.

Claims

1. A radar level gauge system for determining a filling level of a product in a tank, said radar level gauge system comprising: pulse generating circuitry for generating: an electromagnetic transmit signal in the form of a first pulse train having a first pulse repetition frequency, said first pulse train being formed by a time-sequence of substantially identical transmit pulses, each transmit pulse in said time-sequence of transmit pulses exhibiting a full period waveform; and an electromagnetic reference signal in the form of a second pulse train having a second pulse repetition frequency, said second pulse repetition frequency differing from said first pulse repetition frequency by a predetermined frequency difference, said second pulse train being formed by a time-sequence of substantially identical reference pulses, each reference pulse in said time-sequence of reference pulses exhibiting a half period waveform; a propagation device connected to said pulse generating circuitry and arranged to propagate said transmit signal towards a surface of said product in the tank, and to return a surface reflection signal resulting from reflection of said transmit signal at said surface; measurement circuitry connected to said propagation device and to said pulse generating circuitry for forming a measurement signal based on a time-correlation between said surface reflection signal and said reference signal; and processing circuitry connected to said measurement circuitry for determining said filling level based on said measurement signal.

2. The radar level gauge system according to claim 1, wherein a pulse width of each transmit pulse in said time-sequence of transmit pulses is at least approximately twice a pulse width of each reference pulse in said time-sequence of reference pulses.

3. The radar level gauge system according to claim 1, wherein said half period waveform is substantially identical to one half of said full period waveform.

4. The radar level gauge system according to claim 1, wherein: each transmit pulse in said time-sequence of transmit pulses is sinusoidal; and each reference pulse in said time-sequence of reference pulses is sinusoidal.

5. The radar level gauge system according to claim 1, wherein said pulse generating circuitry comprises: a first pulse generator for generating an intermediate signal in the form of an intermediate pulse train having said first pulse repetition frequency, said intermediate pulse train being formed by a time-sequence of substantially identical intermediate pulses, each intermediate pulse in said time-sequence of intermediate pulses exhibiting a half period waveform; and a waveform converter connected to said first pulse generator for receiving said time-sequence of intermediate pulses and providing said time-sequence of transmit pulses.

6. The radar level gauge system according to claim 5, wherein said waveform converter comprises differentiator circuitry.

7. The radar level gauge system according to claim 6, wherein said differentiator circuitry comprises a passive differentiator.

8. The radar level gauge system according to claim 7, wherein said passive differentiator comprises a coupling capacitor connected in series between said first pulse generator and said propagation device.

9. The radar level gauge system according to claim 5, wherein said waveform converter comprises: delay circuitry connected to said first pulse generator for providing a first intermediate signal with a first delay, and a second intermediate signal with a second delay different from said first delay; and a differential amplifier connected to said delay circuitry to receive said first intermediate signal and said second intermediate signal, and to provide said transmit signal as a difference signal between said first intermediate signal and said second intermediate signal.

10. The radar level gauge system according to claim 5, wherein said pulse generating circuitry further comprises: a second pulse generator for generating said reference signal; and timing circuitry for controlling said first pulse generator and said second pulse generator to provide said predetermined frequency difference.

11. The radar level gauge system according to claim 1, wherein said measurement circuitry comprises correlating circuitry for time-correlating said surface reflection signal and said reference signal to form a correlation signal.

12. The radar level gauge system according to claim 11, wherein said measurement circuitry further comprises integrating circuitry for integrating said correlation signal to form said measurement signal.

13. The radar level gauge system according to claim 1, wherein said propagation device is a probe.

14. The radar level gauge system according to claim 13, further comprising a non-conducting signal coupling arrangement connected between said probe, and said pulse generating circuitry and said measurement circuitry.

15. The radar level gauge system according to claim 13, wherein said probe is grounded.

16. A method of determining a filling level of a product in a tank using a radar level gauge system comprising pulse generating circuitry, a propagation device, measurement circuitry, and processing circuitry, said method comprising the steps of: generating, by said pulse generating circuitry, an electromagnetic transmit signal in the form of a first pulse train having a first pulse repetition frequency, said first pulse train being formed by a time-sequence of substantially identical transmit pulses, each transmit pulse in said time-sequence of transmit pulses exhibiting a full period waveform; generating, by said pulse generating circuitry, an electromagnetic reference signal in the form of a second pulse train having a second pulse repetition frequency, said second pulse repetition frequency differing from said first pulse repetition frequency by a predetermined frequency difference, said second pulse train being formed by a time-sequence of substantially identical reference pulses, each reference pulse in said time-sequence of reference pulses exhibiting a half period waveform; propagating, by said propagation device, said transmit signal towards a surface of said product in the tank; propagating, by said propagation device, a surface reflection signal resulting from reflection of said transmit signal at said surface; receiving, by said measurement circuitry, said surface reflection signal; time-correlating, by said measurement circuitry, said surface reflection signal and said reference signal to form a measurement signal; and determining, by said processing circuitry, said filling level based on said measurement signal.

17. The method according to claim 16, wherein said propagation device is a probe, and wherein said method further comprises the steps of: non-conductively coupling said transmit signal between said pulse generating circuitry and said probe; and non-conductively coupling said surface reflection signal between said probe and said measurement circuitry.

101-117. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0051] These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing example embodiments of the invention, wherein:

[0052] FIG. 1 schematically illustrates an exemplary tank arrangement comprising a radar level gauge system according to an embodiment of the present invention;

[0053] FIG. 2 is schematic illustration of the measurement unit comprised in the radar level gauge system in FIG. 1;

[0054] FIG. 3 is a schematic block diagram of the transceiver comprised in a radar level gauge system according to an embodiment of the present invention;

[0055] FIG. 4A schematically illustrates examples of the transmit signal, the surface reflection signal and the reference signal;

[0056] FIG. 4B is a partial enlarged view of a portion of the transmit signal and the reference signal in FIG. 4A;

[0057] FIG. 4C is an illustration of the bandwidth of the transmit signal as compared to conventional DC-pulses;

[0058] FIG. 4D schematically illustrates the measurement signal resulting from time-correlation of the surface reflection signal and the reference signal in FIG. 4A;

[0059] FIG. 5A is a schematic block diagram of a first example configuration of a pulse forming circuit for generating the transmit signal;

[0060] FIG. 5B is a schematic block diagram of a second example configuration of a pulse forming circuit for generating the transmit signal;

[0061] FIG. 6A schematically shows a first example configuration of the connection arrangement comprised in the radar level gauge system in FIG. 1;

[0062] FIG. 6B is a schematic circuit diagram of the connection arrangement in FIG. 6A;

[0063] FIG. 6C is a graph schematically illustrating signal attenuation simulated for an example configuration of the connection arrangement comprised in the radar level gauge system according to embodiments of the invention as a function of frequency; and

[0064] FIG. 7 is a flow-chart schematically illustrating an example embodiment of the method according to the present invention.

DETAILED DESCRIPTION OF AN EXAMPLE EMBODIMENT OF THE INVENTION

[0065] In the present detailed description, various embodiments of the present invention are mainly discussed with reference to a pulsed radar level gauge system with a non-conductive coupling between transceiver and probe.

[0066] It should be noted that this by no means limits the scope of the present invention, which also covers a pulsed radar level gauge system with other couplings between transceiver and probe, such as a conventional conductive coupling between transceiver and probe.

[0067] FIG. 1 schematically shows a level measuring system 1 comprising a radar level gauge system 2 according to an example embodiment of the present invention, and a host system 10 illustrated as a control room.

[0068] The radar level gauge system 2 of GWR (Guided Wave Radar) type is installed at a tank 4 having a tubular mounting structure 13 (often referred to as a nozzle) extending substantially vertically from the roof of the tank 4.

[0069] The radar level gauge system 2 is installed to measure the filling level of a product 3 in the tank 4. The radar level gauge system 2 comprises a measuring unit 6 and a propagation device in the form of a single conductor probe 7 extending from the measuring unit 6, through the tubular mounting structure 13, towards and into the product 3. In the example embodiment in FIG. 1, the single conductor probe 7 is a wire probe, that has a weight 8 attached at the end thereof to keep the wire straight and vertical. The probe 7 is grounded through conductive electric connection to a metallic structure, here the tubular mounting structure 13, of the tank 4, and the radar level gauge system 2 comprises a connection arrangement 15 for non-conductive transmission of electromagnetic signals between the measurement unit 6 and the probe 7. The connection arrangement 15 will be described in greater detail further below.

[0070] By analyzing transmitted signals S.sub.T being guided by the probe 7 towards the surface 11 of the product 3, and reflected signals S.sub.R traveling back from the surface 11, the measurement unit 6 can determine the filling level of the product 3 in the tank 4. It should be noted that, although a tank 4 containing a single product 3 is discussed herein, the distance to any material interface along the probe can be measured in a similar manner.

[0071] The radar level gauge system in FIG. 1 will now be described in more detail with reference to the schematic block diagram in FIG. 2.

[0072] Referring to the schematic block diagram in FIG. 2, the measurement unit 6 of the radar level gauge system 2 in FIG. 1 comprises a transceiver 17, a measurement control unit (MCU) 19, a wireless communication control unit (WCU) 21, a communication antenna 23, an energy store, such as a battery 25, and the connection arrangement 15.

[0073] As is schematically illustrated in FIG. 2, the MCU 19 controls the transceiver 17 to generate, transmit and receive electromagnetic signals. The transmitted signals pass through the tank connection arrangement 15 to the probe 7, and the received signals pass from the probe 7 through the tank connection arrangement 15 to the transceiver 17.

[0074] The MCU 19 determines the filling level of the product 3 in the tank 4 and provides a value indicative of the filling level to an external device, such as a control center, from the MCU 19 via the WCU 21 through the communication antenna 23. The radar level gauge system 1 may advantageously be configured according to the so-called WirelessHART communication protocol (IEC 62591).

[0075] Although the measurement unit 6 is shown to comprise an energy store 25 and to comprise devices (such as the WCU 21 and the communication antenna 23) for allowing wireless communication, it should be understood that power supply and communication may be provided in a different way, such as through communication lines (for example 4-20 mA lines).

[0076] The local energy store need not (only) comprise a battery, but may alternatively, or in combination, comprise a capacitor or super-capacitor.

[0077] The radar level gauge system 2 in FIG. 1 will now be described in greater detail with reference to the schematic block diagram in FIG. 3.

[0078] Referring now to FIG. 3, there is shown a more detailed block diagram of the transceiver 17 in FIG. 2.

[0079] As is schematically shown in FIG. 3, the transceiver 17 comprises a transmitter branch for generating and transmitting a transmit signal S.sub.T towards the surface 11 of the product 3 in the tank, and a receiver branch for receiving and operating on the reflected signal S.sub.R resulting from reflection of the transmit signal S.sub.T at the surface 11 of the product 3. As is indicated in FIG. 3, the transmitter branch and the receiver branch are both connected to a directional coupler 27 to direct signals from the transmitter branch to the probe 7 and to direct reflected signals being returned by the probe 7 to the receiver branch.

[0080] As is schematically indicated in FIG. 3, the transceiver 17 comprises pulse generating circuitry, here in the form of a first pulse forming circuit 29, a second pulse forming circuit 31, and a timing control unit 35 for controlling the timing relationship between the transmit signal output by the first pulse forming circuit 29 and the frequency shifted reference signal S.sub.REF output by the second pulse forming circuit 31.

[0081] The transmitter branch comprises the first pulse forming circuit 29, and the receiver branch comprises the second pulse forming circuit 31 and measurement circuitry 33.

[0082] As is schematically indicated in FIG. 3, the measurement circuitry 33 comprises a time-correlator, here in the form of a mixer 37, a sample-and-hold circuit 39 and amplifier circuitry 41. In embodiments of the present invention, the measurement circuitry 33 may further comprise an integrator 43.

[0083] Additionally, as was briefly described above with reference to FIG. 2, the radar level gauge system 1 comprises processing circuitry 19 (not shown in FIG. 3) that is connected to the measurement circuitry 33 for determining the filling level of the product 3.

[0084] When the radar level gauge system 1 in FIG. 3 is in operation to perform a filling level determination, a time correlation is performed in the mixer 37 between the surface reflection signal S.sub.R and the reference signal S.sub.REF that is output by the second pulse forming circuit 31. The reference signal S.sub.REF is a pulse train with a pulse repetition frequency that controlled to differ from the pulse repetition frequency of the transmit signal S.sub.T, by a predetermined frequency difference f. When a measurement sweep starts, the reference signal S.sub.REF and the transmit signal S.sub.T are in phase, and then the time until the reference signal catches up with the reflected signal S.sub.R is determined. From this time and the frequency difference f, the distance to the surface 3 can be determined.

[0085] The time-expansion technique that was briefly described in the previous paragraph is well known to the person skilled in the art, and is widely used in pulsed radar level gauge systems.

[0086] As is clear from the above discussion, the output from the mixer 37 will be a sequence of values, where each value represents a time correlation between a pulse of the reference signal S.sub.REF and the surface reflection signal S.sub.R. The values in this sequence of values are tied together to form a continuous signal using the sample-and-hold circuit 39.

[0087] In this context it should be noted that the sample-and-hold amplifier 39 is simply an illustrative example of a device capable of maintaining a voltage level over a given time, and that there are various other devices that can provide the desired functionality, as is well known to the person skilled in the art.

[0088] In the example embodiment of FIG. 3, the time-correlated signalthe correlation signal S.sub.coutput from the sample-and-hold circuit 39 is provided to an integrator to form a measurement signal S.sub.M from which the filling level is determined by the MCU 19, following amplification of the measurement signal S.sub.M by the low noise amplifier LNA 41.

[0089] FIG. 4A is a simplified timing diagram schematically showing the relative timing of the transmit signal S.sub.T, the reflected signal S.sub.R, and the reference signal S.sub.REF.

[0090] As is schematically indicated in FIG. 4A, the transmit signal S.sub.T, formed by transmit pulses 45, and the reference signal S.sub.REF, formed by reference pulses 47, are controlled by the timing circuitry 21 to be in phase at the start of the measurement sweep. A full measurement sweep may typically be defined by the difference frequency f, since the transmit signal S.sub.T and the reference signal S.sub.REF, in this particular example, need to be in phase at the start of a new measurement sweep. As is also schematically indicated in FIG. 4A, the surface reflection signal S.sub.R has the same pulse repetition frequency as the transmit signal S.sub.T, but lags behind the transmit signal S.sub.T with a time corresponding to the time-of-flight indicative of the distance to the surface 11 of the product 3.

[0091] The reference signal S.sub.REF is initially in phase with the transmit signal, but will, due to its lower pulse repetition frequency run away from the transmit signal S.sub.T and catch up with the surface reflection signal S.sub.R.

[0092] When the time-varying phase difference between the transmit signal S.sub.T and the reference signal S.sub.REF corresponds to the time-of-flight of the reflected signal S.sub.R, there will be a time-correlation between pulses of the reference signal S.sub.REF and pulses of the surface reflection signal S.sub.R. This time-correlation, results in a time-expanded correlation signal S.sub.c, which can, in turn, be converted to a measurement signal S.sub.M as will be described further below with reference to FIG. 4D.

[0093] First, however, example waveforms of the transmit pulses 45 and the reference pulses 47 will be described with reference to the schematic magnified view in FIG. 4B. As is shown in FIG. 4B, each transmit pulse 45 exhibits a full period waveform having a crest 49 and a trough 51, and each reference pulse 47 exhibits a half period waveform (here the half period with a crest 53). Further, the pulse time T.sub.T of each transmit pulse 45 is at least approximately twice the pulse time T.sub.REF of each reference pulse 47.

[0094] As was explained in the Summary section, the full period waveform of the transmit pulses 45 considerably reduces the relative bandwidth of the transmit signal S.sub.T as compared to conventional DC-pulses (such as the reference pulses 47 shown in FIG. 4B).

[0095] FIG. 4C is a diagram showing simulations of the power spectrum 55 of the transmit pulse 45, and the power spectrum 57 of the reference pulse 47. It is immediately clear from FIG. 4C that the relative bandwidth of the transmit signal S.sub.T (and the surface reflection signal S.sub.R) is considerably smaller than the relative bandwidth of the reference signal S.sub.REF (conventional DC-pulses).

[0096] Referring now to FIG. 4D, the above-mentioned correlation signal S.sub.c and measurement signal S.sub.M are schematically illustrated in a diagram. The correlation signal S.sub.c results from direct time-correlation between the surface reflection signal S.sub.R (surface reflection pulses each exhibiting a full period waveform) and the reference signal S.sub.REF (reference pulses each exhibiting a half period waveform). As is schematically shown in FIG. 4D, the correlation signal S.sub.c is a time-expanded full waveform signal 59.

[0097] Following integration by integrator 43 and amplification by LNA 41, the measurement signal S.sub.M in FIG. 4D is obtained. As can be seen in FIG. 4D, the measurement signal S.sub.M is a time-expanded half waveform signal 61, which can be subjected to conventional signal processing, implemented in pulsed radar level gauge systems in which DC-pulses are transmitted towards the product in the tank.

[0098] It should be noted that the present invention is equally applicable to pulsed level gauge systems in which the time-varying phase difference between the transmit signal S.sub.T and the reference signal S.sub.REF is achieved by providing the reference signal as the transmit signal being delayed by a time varying delay, or vice-versa.

[0099] Different example configurations of the first pulse generator 29 in FIG. 3 for generating the transmit pulses 45 exhibiting the full period waveform shown in FIG. 4B will now be described with reference to FIG. 5A and FIG. 5B.

[0100] Referring first to FIG. 5A, the first pulse forming circuit 29 comprises a first pulse generator 63 and a waveform converter, in the form of differentiating circuitry, here a series coupling capacitor 65. As is schematically indicated in FIG. 5A, the first pulse generator 63 generates an intermediate signal S.sub.I with half period waveform pulses. Due to the time derivation function of the series capacitor 65, the half period waveform pulses of the intermediate signal S.sub.I are converted to the full period waveform pulses of the transmit signal S.sub.T.

[0101] Turning to FIG. 5B, the first pulse forming circuit 29 comprises a first pulse generator 63 and a waveform converter including a delay circuit 67 and a differential amplifier 69. As is schematically indicated in FIG. 5B, an undelayed version of the intermediate signal S.sub.I is provided to the positive input of the differential amplifier 69, and a delayed version of the intermediate signal S.sub.I is provided to the negative input of the differential amplifier. The delay circuit 67 is configured to provide a delay corresponding to the pulse width of the pulses of the intermediate signal S.sub.I. As is indicated, the differential amplifier will output the transmit signal S.sub.T as a pulse train of full period waveform transmit pulses.

[0102] A first example of the connection arrangement 15 comprised in the radar level gauge system 2 in FIG. 1 will now be described with reference to FIG. 6A. As is schematically shown in FIG. 6A, the connection arrangement 15 comprises an electrically conductive feed-through member 71, a signal conductor 73, a dielectric 75, and a tank coupling arrangement 76.

[0103] The feed-through member 71 extends from a first end 77 on an outside of the tank 4 to a second end 79 on an inside of the tank 4. The probe 7 is conductively connected to the feed-through member 71, and extends towards the product in the tank 4 from the second end 79 of the feed-through member 71. In the example configuration of the connection arrangement 15 in FIG. 6A, the probe 7 comprises an upper probe part 10a with a first probe diameter D.sub.a, and a lower probe part 10b with a second probe diameter D.sub.b. As is schematically indicated in FIG. 6A, the first probe diameter D.sub.a is greater than the second probe diameter D.sub.b. The upper probe part 10a, which is here shown to be screwed into the feed-through member 71, acts as an impedance transformer to contribute to the bandwidth that is obtained by the connection arrangement 15.

[0104] The feed-through member 71 is in conductive contact with a conductive lid 81 at a grounding position 83. As is indicated in FIG. 6A, the grounding position 83 is spaced apart from the second end 79 of the feed-through member 71 by a distance L substantially corresponding to a quarter of the wavelength of the transmit signal S.sub.T at a center frequency of the transmit signal.

[0105] In the example configuration of the connection arrangement 15 shown in FIG. 6A, the feed-through member 71 is in conductive contact with the tubular mounting structure 13 via a welded connection between the feed-through member 51 and the lid 81, a threaded connection between the lid 81 and a tubular member 82 fixed to the tubular mounting structure (nozzle) 13 by bolts (not shown in FIG. 6A). It should be noted that this is only one exemplary way of achieving an electrically conductive contact between the feed-through member 71 and a conductive structure (here the tubular mounting structure 13) of the tank 4, and that there are many other ways of achieving the desired conductive contact.

[0106] The signal conductor 73 extends through the feed-through member 71 from the outside of the tank 4 to the inside of the tank 4. In the example configuration schematically shown in FIG. 6A, the signal conductor 73 is connected to a connector 84 at the outside of the tank 4. When the measurement unit 6 has been connected to the connection arrangement 15, the transceiver 17 will be connected to the connector 84 to provide the transmit signal S.sub.T to the signal conductor 73.

[0107] As is schematically indicated in FIG. 6A, the dielectric 75 is arranged between the signal conductor 73 and the feed-through member 71 to prevent conductive contact between the signal conductor 73 and the feed-through member 71. The signal conductor 73, the dielectric 75, and the feed-through member 71 together form a coaxial line having a first coaxial line portion 85 having a first thickness of the dielectric 75, and a second coaxial line portion 87 having a second, greater, thickness of the dielectric 75. The second coaxial line portion 87 acts as an impedance transformer contributing to the bandwidth of the connection arrangement 15.

[0108] The tank coupling arrangement 76 is connected to the signal conductor 73 on the inside of the tank, and is configured to provide inductive and capacitive coupling in series between the signal conductor 73 and the inner wall of the tubular member 82. In the example configuration of the connection arrangement in FIG. 6A, the tank coupling arrangement 76 comprises radially extending electrically conductive coupling member, here provided in the form of a bent metal ribbon 89 encircling the upper probe portion 8a of the probe 7. The ribbon 89 is arranged and configured to form a parallel plate capacitor together with the inner wall of the tubular member 82. In particular, the dimensions (vertical extension and radius of curvature) of the metal ribbon 89, and the distance between the metal ribbon 89 and the inner wall of the tubular member 82 are selected to achieve a desired capacitance of the capacitor formed by the ribbon 89, the tubular member 82, and the dielectric between the ribbon 89 and the tubular member 82. The desired capacitance may be in the range of 0.1 pF to 10 pF. It will be straight-forward for one of ordinary skill in the relevant art to dimension the coupling member (ribbon 89), and/or to position the coupling arrangement 15 in relation to the tubular member 82 to achieve a capacitance that is desired for a particular frequency range of the transmit signal S.sub.T.

[0109] In the example embodiment in FIG. 6A, the tubular member 82 is delivered as a part of the radar level gauge system 2. This allows the supplier of the radar level gauge system 2 to precisely control critical dimensions (in particular the distance between the ribbon 89 and the inner wall of the tubular member 82). It should be noted, however, that this distance, and other dimensions, can be set by the customer and/or controlled by the supplier in other ways.

[0110] FIG. 6B is a simplified circuit schematic illustrating the electrical filter properties of the connection arrangement 15 in FIG. 6A. With the dimensions indicated in FIG. 6A and described above, the coaxial line formed by the tubular member 82 and the feed-through member 71 between the grounding position 83 and the second end 79 (which electrically corresponds to the uppermost portion of the probe 7) forms a parallel resonant circuit 91. The tank coupling arrangement 76 forms, together with the tubular member 82, a series resonant circuit 93 with substantially the same resonance frequency as the parallel resonant circuit 91.

[0111] As is schematically indicated in FIG. 6B, the series resonant circuit 93 is characterized by a series inductance L.sub.s and a series capacitance C.sub.s. Simulations give that the series inductance should preferably be in the range of 0.1 nH to 10 nH, and that the series capacitance should preferably be in the range of 0.1 pF to 10 pF.

[0112] A simulation performed for dimensions such as those shown in FIG. 6A resulted in the signal transmission (from the connector 64 to the lower probe portion 8b of the probe 7) in dB as a function of frequency is shown in FIG. 6C.

[0113] An example embodiment of the method according to the present invention will now be described with reference to the flow-chart in FIG. 7.

[0114] In step 100, the transmit signal S.sub.T is generated as a pulse train of transmit pulses 45, each exhibiting a full period waveform, and thus having a relatively small relative bandwidth.

[0115] In step 101, taking place at the same time as step 100, the reference signal S.sub.REF is generated as a pulse train of reference pulses 47, each exhibiting a half period waveform.

[0116] In step 102, the transmit signal S.sub.T is propagated towards the surface 11 of the product 3 in the tank 4, and in step 103, the surface reflection signal S.sub.R resulting from reflection at the surface 11 of the transmit signal S.sub.T is received by the transceiver 17.

[0117] In step 104, the surface reflection signal S.sub.R and the reference signal S.sub.REF are time-correlated to form the time-expanded measurement signal S.sub.M, and in step 105, the filling level is determined based on the measurement signal S.sub.M and the frequency difference f between the pulse repetition frequency of the transmit signal S.sub.T and the pulse repetition frequency of the reference signal S.sub.REF.

[0118] The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. For example, many other configurations of the connection arrangement 15 may be feasible. In particular, many other configurations of the tank coupling arrangement 76 and the connection of the feed-through member 71 to the tank 4 will be possible. Moreover, many other pulse shapes of the transmit signal S.sub.T and the reference signal S.sub.REF may be beneficial.