Abstract:
A device for measuring the dielectric and/or magnetic properties of a sample by means of a microwave transmission measurement, comprising a transmitting antenna and a receiving antenna that define a transmission measuring section in which the sample to be measured can be positioned, at least one transmission-side synthesizer for generating a high-frequency signal with a frequency between 800 MHz and 30 GHz, a frequency standard that is connected via a transmission-side low-frequency synchronization signal line to the transmission-side synthesizer and to which the transmission-side synthesizer is coupled phase-locked reproducible, as well as an evaluation unit which is connected at least indirectly to the transmission-side synthesizer and the receiving antenna. At least one receiving-side synthesizer is provided which is connected to the frequency standard via a receiving-side low-frequency synchronization signal line and is coupled phase-locked reproducible thereto and is furthermore connected at least indirectly to the evaluation unit.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The invention relates to a device for measuring the dielectric and/or magnetic properties of a sample by means of a microwave transmission measurement, as defined in the preamble to claim  1 . 
       PRIOR ART 
       [0002]    Known from the prior art are numerous options for the contactless measuring of the dielectric properties of a sample, for example the moisture. For example, it is possible to transmit a microwave through the sample and to obtain the desired information by comparing the irradiated microwave, or a signal derived from it, to the transmitted microwave, or to a signal derived from it. The absorption as well as the phase shift can be determined in the process, so that the complete information on the complex epsilon of the sample can be obtained from the respective measurement. A suitable device comprises a transmitting module and a receiving module. The transmitting module is provided with at least one synthesis generator (also referred to as synthesizer) for generating a high-frequency signal and a transmitting antenna that is connected to the synthesizer. The synthesizer is clocked by a so-called frequency standard which emits a low-frequency signal, for example with a frequency of 10 MHz The high-frequency signal generated by the at least one synthesizer is furthermore transmitted to the receiving module, which is also provided with a receiving antenna, and is mixed therein with the microwave received at the receiving antenna. Also provided is an evaluation unit which can be embodied as separate module. The mixed signal is transmitted to this evaluation unit. 
         [0003]    A distinction is basically made between two types of measuring systems, namely so-called homodyne systems that operate with only a single frequency and have only one synthesizer, and so-called heterodyne systems which operate with two closely adjacent frequencies and two synthesizers. Both systems have in common that they operate by comparing two microwaves, wherein one microwave passes through the sample and thus experiences attenuation and/or a phase shift, while the other microwave does not pass through the sample and functions as a reference. A high-frequency reference line must therefore be provided between the transmitting module and the receiving module (this applies to homodyne as well as heterodyne systems). Under laboratory conditions, providing such a high-frequency line is generally not a problem since no long local distances must be overcome on the one hand and, on the other hand, constant conditions prevail in the laboratory, especially substantially constant temperatures. 
         [0004]    However, if a device of this type is used for industrial purposes, providing such a high-frequency reference line carries some problems and disadvantages, in particular since the temperature dependence on the wave propagation speed in a coaxial cable has an increasingly higher effect on the phase shift, the higher the frequency. It means that with non-constant environmental conditions, especially temperatures, considerable phase shifts can occur in the high-frequency reference line and the antenna feed lines which distort the measuring result. When used on an industrial scale, the transmitting module and the receiving module can furthermore be spaced very far apart, which makes this problem worse, especially in cases where such an arrangement is installed totally or partially in the open, so that it can be subject to irradiation from the sun. 
       OBJECT OF THE INVENTION 
       [0005]    Starting therefrom, it is the object of the present invention to improve a generic device in such a way that it is better suited for the use in industrial applications, in particular, and delivers constant and good measuring results even with fluctuating environmental conditions. 
         [0006]    This object is solved with a device having the features as disclosed in claim  1 . 
         [0007]    The core idea behind the invention must be seen in providing at least one synthesizer for generating a high-frequency signal on the transmitting side as well as the receiving side and in coupling these two synthesizers in a phase-locked reproducible manner, for which a joint frequency standard is provided which actuates the two synthesizers via respectively at least one low-frequency signal line, which is referred to as low-frequency synchronization signal line. The use of the aforementioned, problematic high-frequency reference line is thus rendered unnecessary when providing a synthesizer on the receiving side, according to the invention. Low-frequency signal lines of this type are nearly insensitive to the aforementioned environmental influences, even at long lengths, so that no re-calibration is required even for strongly fluctuating environmental influences, in particular considerable changes in the temperature. A device of this type according to the invention can in principle be embodied as a homodyne system as well as a heterodyne system, wherein the embodiment as a heterodyne system is generally preferred. 
         [0008]    Preferred embodiments of the invention follow from the dependent claims as well as from the exemplary embodiments which are explained in further detail in the following with reference to the Figures, which show in: 
     
    
     
       SHORT DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  The circuit diagram for a first exemplary embodiment of the invention; 
           [0010]      FIG. 2  A circuit diagram for a second exemplary embodiment of the invention, wherein the measuring principle used is the same as for the first exemplary embodiment; 
           [0011]      FIG. 3  A circuit diagram for a heterodyne system according to the prior art; 
           [0012]      FIG. 4  An alternative circuit diagram; 
           [0013]      FIG. 5  A further alternative circuit diagram; 
           [0014]      FIG. 6  A further alternative circuit diagram; 
           [0015]      FIG. 7  A first preferred use of the invention; and 
           [0016]      FIG. 8  A second preferred use of the invention. 
       
    
    
       [0017]    For a better understanding of the invention, we first want to discuss the prior art upon which the present invention is based in further detail and with reference to  FIG. 3 . 
         [0018]    As previously mentioned,  FIG. 3  shows a device for measuring the dielectric and/or magnetic properties of a sample P, wherein the device is embodied as a heterodyne measuring system. This system can be viewed as consisting of three modules, namely a transmitting module SM, a receiving module EM, and an evaluation unit. Usually, the transmitting module SM and the receiving module EM are spatially separated. The evaluation unit can be embodied as a physically separate evaluation module AM, but can also be integrated into one of the other two modules, for example the transmitting module SM. Functionally, however, these three elements can always be viewed as separate modules. A transmitting antenna  10  and a receiving antenna  20  define a transmission measuring section into which a sample P can be placed. In this application, “antenna” is understood to refer to each element which is suitable for transmitting and/or receiving a freely propagating microwave or a microwave conducted inside a waveguide, wherein the antenna can also be embodied integrally with another component. 
         [0019]    The following definitions and conventions apply for the text below: 
         [0020]    electromagnetic waves which propagate inside a conductor or which propagate freely and have a frequency between 800 MHz and 30 GHz are referred to as “high-frequency signal” or “microwave.” High-frequency signal lines (microwave conductors) suitable for these types of frequencies are known from the prior art. The high-frequency signal lines are shown as dash-dot lines in the Figures (this is true for the prior art  FIG. 3 , as well as for  FIGS. 1 and 2 ). The term “low-frequency” is understood to refer to all electro-magnetic waves or signals having a frequency below 200 MHz Signal lines used for the transmission of such low-frequency signals are here referred to as low-frequency signal lines and are shown in the drawings as solid lines. For reasons of clarity, not all signal lines (be it high-frequency signal lines or low-frequency signal lines) are given a separate name/reference symbol in the description and the drawings. The high-frequency signal lines as well as the low-frequency signal lines are generally physically embodied as coaxial cables, wherein for reasons of cost, coaxial cables of a higher quality are generally used for the high-frequency signal lines than for the low-frequency signal lines. However, this is not absolutely required, and sufficiently high-quality coaxial cables could be used for all signal lines. Insofar, the terms “high-frequency signal line” and “low-frequency signal line” should above all be understood to be functional terms. 
         [0021]    In addition to the transmitting antenna  10 , the transmitting module SM comprises two transmitting-side synthesizers  12  and  14 , two power dividers  18   a,    18   b,  one transmitting-side mixer  16  and and a frequency standard  32 . The receiving module EM comprises only a receiving-side mixer  26  in addition to the receiving antenna  20 . The evaluation module AM is composed of a central processor  30  as the evaluation unit. The transmitting module SM and the receiving module EM are connected via a high-frequency reference line  50 . The transmitting module SM and the receiving module EM are respectively connected to the evaluation module AM (meaning to the central processor  30 ) via a separate low-frequency signal line (IF 1 ; IF 2 ). The mode of operation is as follows: 
         [0022]    The frequency standard  32  clocks the two transmitting-side synthesizers  12 ,  14 , wherein the clocking frequency, for example, can be 10 MHz The first transmitting-side synthesizer  12  generates a first high-frequency signal F 1  with a first high frequency of 3 GHz, for example, and the second transmitting-side synthesizer  14  generates a second high-frequency signal F 2  with a slightly different high frequency, for example 3.001 GHz. The first high-frequency signal Fl of the first transmitting-side synthesizer  12  is supplied to a power divider  18   a,  the first output of which is connected to the transmitting antenna  10  while its second output is connected to the transmitting-side mixer  16 . The second transmitting-side synthesizer  14  is connected to the second power divider  18   b,  the outputs of which are connected to the transmitting-side mixer  16  and via the high-frequency reference line  50  to the receiving-side mixer  26 . The second input of the receiving-side mixer  22  is connected to the receiving antenna  20 . 
         [0023]    The transmitting-side mixer  16  thus generates a first intermediate-frequency signal IF 1  with a first intermediate frequency which represents the difference between the first high frequency (also the frequency of the transmitted microwave) and the second high frequency, meaning it amounts to 1 MHz for the selected example. The receiving-side mixer  26 , in turn, generates a second intermediate-frequency signal IF 2  that represents the difference between the received microwave (this signal is given the reference F 1 ′) and the second high frequency signal from the second transmitting-side synthesizer  14 . In this case, F 1  and F 1 ′ have identical frequencies since the transmission through the sample P changes the phase and the amplitude, but not the frequency. For that reason, the two intermediate frequency signals IF 1  and IF 2  also have the same intermediate frequency, in this case 1 MHz. From the comparison between the first intermediate-frequency signal IF 1  and the second intermediate frequency signal IF 2 , it is possible to deduce, in a manner known per se, the phase shift as well as the attenuation experienced by the microwave emitted by the transmitting antenna  10  when it passes through the sample P. In turn, it is possible to deduce from this the dielectric properties of the sample. The corresponding calculations are carried out by the evaluation module AM, namely by the central processor  30 . 
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    With reference to  FIG. 1 , a device according to the invention is now described, which is also embodied as a heterodyne system, as the above-described device according to the prior art. In the same way as the aforementioned device, the device according to the invention can be seen as being composed of three modules, namely a transmitting module SM, a receiving module EM, and a central module ZM which comprises the evaluation module AM—meaning the evaluation unit—as well as a synchronization module SYM which does not exist in this form in the prior art. The three modules here also do not absolutely have to be embodied as locally separated modules, but for the sake of clarity we will retain the above-used terminology. However, it should be taken into consideration that in particular the transmitting module SM and the receiving module EM are in practice frequently embodied as physically separate modules. The central module ZM can be integrated into one of the modules. 
         [0025]    In the same way as for the prior art, the transmitting module SM comprises two transmitting-side synthesizers  12  and  14  which respectively generate a high-frequency signal F 1  and F 2 , wherein these high frequencies differ slightly, for example the first high frequency can be 3 GHz and the second high frequency can be 3.001 GHz. As described above with reference to the prior art, the first transmitting-side synthesizer  12  also feeds its high-frequency signal F 1  into a power divider  18  which, in turn, is connected to a transmitting-side mixer  16  and the transmitting antenna  10 . The second transmitting-side synthesizer  14  feeds the second high-frequency signal F 2 , generated by it, directly into the transmitting-side mixer  16  which, in the same way as for the prior art, is connected to the evaluation module AM, namely to the central processor  30 . 
         [0026]    In contrast to the prior art, the transmitting module SM is not connected via a high-frequency reference line to the receiving module EM which is the reason why no second power divider is provided. Instead, the receiving module EM comprises a receiving-side synthesizer  22  which generates the same high frequency as the second transmitting-side synthesizer  14  which, for the selected example, is 3.001 GHz. This receiving-side synthesizer  22  feeds the third high frequency-signal F 3 , generated by it, into the receiving-side mixer  26 , wherein the second input of this mixer is connected to the receiving antenna  20 , so that it receives the first high-frequency signal F 1 ′ transmitted through the sample. 
         [0027]    As for the prior art, the transmitting side mixer  16  generates a first intermediate frequency signal IF 1  and the receiving-side mixer  26  also generates a second intermediate-frequency signal IF 2 , wherein the two intermediate frequencies are the same, namely 1 MHz for the herein described example. These two intermediate-frequency signals IF 1  and IF 2  are supplied, in the same was as for the prior art, to the evaluation module AM, meaning they are fed to the central processor  30 . In order to deduce from the phase shift between the first intermediate frequency signal IF 1  and the second intermediate-frequency signal IF 2  a relevant conclusion on the phase shift experienced by the first high-frequency signal F 1  when passing through the sample P, all synthesizers  12 ,  14  and  22  must be synchronized. The synchronization is ensured by the synchronization module SYM, meaning by the frequency standard  32 , which is connected via a transmitting-side low-frequency synchronization signal line  34   a  to the two transmitting-side synthesizers  12 ,  14  and via a receiving side low-frequency synchronization signal line  34   b  to the receiving side synthesizer  22  and which emits a clocking signal TS by means of which the synthesizers are coupled phase-locked reproducible. The “heart” of such a frequency standard is generally a quartz oscillator, the resonance frequency of which is used as normal frequency. Typically, this normal frequency ranges from 1 to 30 Mhz, in particular 10 MHz, as selected for this example. Both low-frequency synchronization signal lines  34   a,    34   b  are low-frequency signal lines that are preferably embodied physically identical, in particular having the same length and identical design. As a result, the use of a high-frequency reference line that connects the transmitting module SM and the receiving module EM can be omitted, thereby resulting in the improvement according to the invention. 
         [0028]    With the above-described exemplary embodiment, the high-frequency signals generated by the synthesizers  12 ,  14 ,  22  cannot be changed. Oftentimes, however, a measuring with different high frequencies is desired, wherein it always applies that the second high frequency of the second transmitting-side synthesizer  14  and the third high frequency of the receiving-side synthesizer  22  are identical, and these two second and third high frequencies are slightly different from the first high frequency of the first transmitting-side synthesizer  12 . In that case, it is necessary to ensure that the synthesizers  12 ,  14 ,  22  can be controlled by a controller. In principle, the central processor can take over this task, wherein it is preferable for long geometric distances if the controlling is not realized directly by the central processor, but occurs respectively via a transmitting-side controller  40  and a receiving-side controller  42  which, in turn, are controlled by the central processor  30  ( FIG. 2 ). 
         [0029]    Of course, since it is indispensable for the success of the invention that the synthesizers have a phasing, known to each other, their coupling to the frequency standard not only must be phase-locked, but also reproducible. It means that during the switch-on or for a change in the frequency, the same phase always adjusts for all synthesizers. However, a plurality of synthesizers known from the prior art exhibit this feature, so that no additional measures are required to achieve reproducibility. 
         [0030]    With the aid of the synthesizers, coupled phase-locked reproducible, both intermediate frequencies IF 1  and IF 2  can also be generated on the receiver side, so that the connecting cable for this signal between the transmitting module SM and the evaluation module AE can be omitted. A concrete exemplary embodiment is shown in  FIG. 4 . In that case, the transmitting module SM is provided with only one synthesizer  11 . All other components are integrated into the receiving module EM which thus has a first and a second synthesizer  23 ,  24 , wherein the second synthesizer  24  generates a second high-frequency signal F 2  that has the same high frequency as the first high-frequency signal F 1  from the transmitting-side synthesizer  11  (for example again 3 GHz), while the first synthesizer  23  (as for the above-described example) generates a third high-frequency signal F 3  with a slightly different high frequency (for example again 3.001 GHz). The first intermediate frequency signal IF 1  is generated by mixing the second high-frequency signal F 2  with the third high-frequency signal F 3 , using the first receiving-side mixer  27  of which one input is connected via a power divider  29  to the first receiving-side synthesizer  23 . The second intermediate frequency signal IF 2  is generated as described in the above by using the second receiving-side mixer  28  which corresponds to the receiving-side mixer  26  of the first exemplary embodiment. As for the above-described example, all synthesizers  11 ,  23 ,  24  are clocked phase-locked by the frequency standard  32 . 
         [0031]      FIG. 5  shows a further embodiment, provided with only one transmitting-side frequency generator  11  for generating a first high-frequency signal Fl and only one receiving-side frequency generator  22  for generating an additional high-frequency signal, which is referred to as third high-frequency signal F 3  for the sake of consistency. In this case, the clocking signal TS of the synchronization module functions directly as the reference signal (in the previous embodiments, it was the first intermediate frequency signal IF 1 ) or, if applicable, a signal derived directly therefrom. If the clocking signal TS is to be used directly as the reference signal, as shown in the exemplary embodiment according to  FIG. 5 , for which an additional low-frequency synchronization signal line  34   c  is provided to connect the frequency standard  32  with the central processor, then the frequency of the second intermediate frequency signal IF 2  (of the mixing signal from F 3  and F 1 ′) must be the same as the frequency of the clocking signal TS. If the clocking signal TS frequency in this case is also 10 MHz, then the frequency of the first high-frequency signal F 1  could be 3 GHz and the frequency of the third high-frequency signal F 3  could be 3.01 GHz. This exemplary embodiment leads to a simplified circuit. 
         [0032]    This course of action can be generalized: The frequency of the intermediate-frequency signal IF 2  coming from the receiving module need not be identical to the frequency of the clocking signal TS of the frequency standard  32 . It is only necessary that both signals are coupled phase-locked reproducible. As shown in  FIG. 6 , it is for example possible to provide a frequency converter, in particular a low-frequency synthesizer  44  which generates a reference signal RS, between the frequency standard  32  and the central processor  30  that functions as an evaluation unit. The frequency of this reference signal corresponds to the frequency of the second intermediate signal IF 2 , for example 1 MHz at f(F 1 )=3 GHz and f(F 3 )=3.001 GHz. The exemplary embodiment shown in  FIG. 6  thus corresponds essentially to the embodiment in  FIG. 4 , with the difference that the first intermediate reference signal IF 1 , which is obtained by mixing the two high-frequency signals F 2  and F 3  and which functions as reference, is replaced by the non-mixed reference signal RS that is generated directly by the low-frequency synthesizer  44 . 
         [0033]    The invention has been described with the aid of a heterodyne system which is also the preferred embodiment. However, the invention can also be used with a homodyne system. In that case, only one transmission-side synthesizer and one receiving-side synthesizer are provided, which generate the same high frequency. The phase-locked coupling of these two synthesizers via a joint frequency standard is identical to the example shown in the above. 
         [0034]    As previously mentioned, the advantages of the improvement according to the invention are particularly obvious when the device is used on an industrial scale, for example for the online measuring of bulk goods SG such as coal or iron ore conveyed on a conveying belt  60  ( FIG. 7 ), or for the online measuring of fluid streaming through a pipe ( 65 ) ( FIG. 8 ), wherein the measuring inside a container is possible as well. 
       LIST OF REFERENCES 
       [0000]    
       
           10  transmitting antenna 
           11  transmitting-side synthesizer 
           12  first transmitting-side synthesizer 
           14  second transmitting-side synthesizer 
           16  transmitting side mixer 
           18  power divider 
           18   a  first power divider 
           18   b  second power divider 
           20  receiving antenna 
           22  receiving-side synthesizer 
           23  first receiving-side synthesizer 
           24  second receiving-side synthesizer 
           26  receiving side mixer 
           27  first receiving side mixer 
           28  second receiving side mixer 
           29  power divider 
           30  central processor (evaluation unit) 
           32  frequency standard 
           34   a  transmitting-side low-frequency synchronization signal line 
           34   b  receiving-side low-frequency synchronization signal line 
           34   c  additional low-frequency synchronization signal line 
           40  transmitting-side controller 
           42  receiving-side controller 
           44  low-frequency synthesizer 
           50  high-frequency reference line 
           60  conveyor belt 
           65  pipe 
         F 1  first high-frequency signal with first frequency 
         F 2  second high-frequency signal with second frequency 
         F 3  third high-frequency signal with third frequency 
         IF 1  first intermediate-frequency signal with first intermediate frequency 
         IF 2  second intermediate-frequency signal with second intermediate frequency 
         TS clocking signal 
         RF reference signal 
         SM transmitting module 
         EM receiving module 
         AE evaluation unit 
         SYM synchronization module 
         ZM central module 
         SG bulk goods