Abstract:
The shape or amplitude of reference pulses in a filling level radar typically depend on the component quality of the circulator used, of a transmitting/receiving coupler, or else of the circuit temperature. In case of large echoes at close range, the quality of the measurement may deteriorate. A radar module for extracting a reference signal for a filling level radar comprises a tap extracting the reference signal and a delay line delaying the transmit signal. Delaying the transmit signal on its way to the antenna takes place after the extraction of the leakage pulse.

Description:
REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of the filing date of German Patent Application Serial No. 10 2005 057 053.4 filed Nov. 30, 2005 and United States Provisional Patent Application Ser. No. 60/741,190 filed Nov. 30, 2005, the disclosure of which applications is hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to level measuring. In particular, this invention relates to a high frequency module for extracting a reference signal for a filling level measuring instrument, a filling level measuring instrument for determining a filling level in a tank, the use of such a high frequency module for filling level measuring, and a method for extracting a reference signal for a filling level measuring instrument.  
       BACKGROUND OF THE INVENTION  
       [0003]     Known level measuring instruments have an antenna, which emits or receives radar or microwaves in order to determine the filling level of a medium in a filling material container. The antenna of such a level measuring instrument is then arranged for instance inside a container.  
         [0004]     The high frequency modules of such level measuring instruments are used for generating transmit signals, which are emitted via the antenna towards the filling material. The measuring signals received are picked up via the antenna, and evaluated using a reference signal. This reference signal can be configured as a reference pulse (leakage pulse), the shape and amplitude of which depend on the component quality of a circulator, a transmitting-/receiving coupler, or the temperature of the measuring device as well. In addition, a reflection of the antenna as such or the antenna coupling may have a negative impact on the leakage pulse, e.g. if the antenna is arranged close to the coupler (circulator), or in case of large echoes at close range.  
       SUMMARY OF THE INVENTION  
       [0005]     According to a sample embodiment of this invention, a high frequency module for extracting a reference signal for a level measuring instrument is provided, the high frequency module comprising a first transmission link for transmitting an electromagnetic transmit signal from a source to an antenna, or to a probe, a tap for extracting a reference signal from the transmission link, and a time-delay for delaying the transmit signal on its way to the antenna or probe, wherein the time-delay is arranged after the tap.  
         [0006]     I.e., the transmit signal is thus tapped on its way to the antenna (in case of a filling level radar) or to a probe (in case of a TDR level measuring unit). The reference signal thus extracted may be used for evaluating a measuring signal from the antenna. After extraction of the reference signal, the transmit signal is time delayed in relation to the reference signal, so that e.g. large echoes at close range, reflections of the antenna or insufficient isolation of the transmitting-receiving coupler cannot interfere with the evaluation of the reference signal.  
         [0007]     According to another sample embodiment of this invention, the tap comprises a first directional coupler.  
         [0008]     The directional coupler is configured e.g. as a symmetric or asymmetric hybrid coupler.  
         [0009]     Such a hybrid coupler may be integrated at low cost into a circuit of the high frequency module.  
         [0010]     According to another sample embodiment of this invention, the time-delay comprises a delay line.  
         [0011]     E.g., the delay line is configured as a strip line, which is integrated into a multilayer circuit board. Such a strip line has ground planes on either side, i.e. may be for example arranged in a different plane as the transmission link. Depending on the desired delay, the strip line may have a corresponding length.  
         [0012]     According to another sample embodiment of this invention, the delay line is configured as a microstrip.  
         [0013]     Also, the delay line may have a so-called “Low Temperature Cofired Ceramic” (a so-called LTCC component). This ceramic has a high dielectric constant, and may e.g. be SMD populated.  
         [0014]     According to another sample embodiment of this invention, the delay line is configured as a wound coaxial cable.  
         [0015]     The directional coupler for extracting the reference signal may have a variable coupling factor.  
         [0016]     According to another sample embodiment of this invention, a controller is provided, which is configured for setting the coupling factor of the directional coupler.  
         [0017]     Depending on environmental conditions and requirements, this controller may perform adequate settings at the coupling factor of the directional coupler. Thus, the amplitude of the reference pulse may be varied adequately.  
         [0018]     According to another sample embodiment of this invention, the high frequency module comprises in addition a second transmission link for transmitting a measuring signal from the antenna to an evaluation circuit, and a coupling for coupling a reference signal into the second transmission link.  
         [0019]     This may ensure that the extracted reference signal can be used for evaluating the measuring signal, wherein the measuring signal has been delayed in relation to the reference signal.  
         [0020]     According to another sample embodiment of this invention, the coupling is configured as a directional coupler, which may be e.g. a hybrid coupler.  
         [0021]     According to another sample embodiment of this invention, the high frequency module comprises in addition a pulse generator for generating a pulsed transmit signal, wherein the reference signal is a reference pulse.  
         [0022]     According to another sample embodiment of this invention, a level measuring instrument for determining a filling level in a tank is provided, the level measuring instrument comprising a high frequency module as described above.  
         [0023]     According to another sample embodiment of this invention, the level measuring instrument comprises an antenna for transmitting and/or receiving electromagnetic waves.  
         [0024]     According to another sample embodiment of this invention, the level measuring instrument comprises a probe for transmitting the electromagnetic waves to the filling material.  
         [0025]     In addition, the use of a high frequency module according to the invention for level measuring is provided.  
         [0026]     In addition, a method for extracting a reference signal for a filling level radar is provided, wherein an electromagnetic transmit signal is transmitted via a transmission link from a source to an antenna, a reference signal is extracted from the transmission link, the transmit signal is delayed on its way to the antenna, wherein the delay of the transmit signal takes place after the extraction of the reference signal.  
         [0027]     Thereby, a method is provided, by which a leakage pulse may be picked up, which is not biased by the antenna pulse, even if the antenna is arranged close to the coupler, or in the case of large echoes at close range. Thereby, the quality or accuracy of the measurement may be substantially increased.  
         [0028]     Further sample embodiments, objects, and advantages of the invention result from the subclaims.  
         [0029]     Hereafter, with reference to the figures, sample embodiments of this invention will be described. 
     
    
     SHORT DESCRIPTION OF THE FIGURES  
       [0030]      FIG. 1  shows a schematic view of a high frequency module.  
         [0031]      FIG. 2   a  shows a schematic view of a high frequency module according to a sample embodiment of this invention.  
         [0032]      FIG. 2   b  shows a schematic view of a high frequency module according to another sample embodiment of this invention.  
         [0033]      FIG. 3  shows a schematic view of a level measuring instrument according to a sample embodiment of this invention. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0034]     The views in the figures are schematic and not to scale.  
         [0035]     In the following description of the figures, the same reference symbols may be used for identical or similar items.  
         [0036]      FIG. 1  shows a schematic view of a radar module. The radar module has a pulse generator  101 , which is used for generating a transmit pulse. This transmit pulse is then filtered via the band-pass filter  102  and supplied to a circulator  105  via the transmission link  115 .  
         [0037]     The pulse travels through the circulator with an attenuation of a 1 =1 dB, and is transmitted via line  107  to the antenna  117 . Next, the antenna  117  sends the pulse towards the filling material, and receives thereupon a corresponding measuring pulse. The measuring pulse is transmitted via line  107  to the circulator  105 , and then delivered to line  118 , also with an attenuation of a 2 =1 dB.  
         [0038]     In addition, during the transmission of the transmit signal from line  115  to line  107  towards the antenna, the circulator  105  derives a reference pulse from line  115  to line  118  at an attenuation of a 3 =20 dB. The reference pulse and the measuring signal are fed into the sampling mixer  108 .  
         [0039]     A second pulse generator  109  is provided, which generates a pulsed signal, which travels through a band-pass filter  110 , and is then also entered into the sampling mixer  108 .  
         [0040]     Next, the sampling mixer  108  generates a retarded or time elongated signal, which is amplified by the amplifier  11 , and then forwarded as an intermediate frequency  112  to an evaluation circuit  122 .  
         [0041]      FIG. 2   a  shows a schematic view of a radar module according to a sample embodiment of this invention. As may be seen in  FIG. 2   a , the radar module  100  comprises substantially a transmission link  115  for transmitting an electromagnetic transmit signal from a pulse generator  101  to an antenna  117 , a tap  103  for extracting a reference signal from the transmission link  115 , and a time-delay  104  for delaying the transmit signal on its way to the antenna  117 . Herein, the time-delay  104  is arranged after the tap  103 .  
         [0042]     Instead of an antenna  117 , it may also be possible to provide a line for conducting the transmit signal to the filling material in order to perform a non contactless measurement.  
         [0043]     The pulse generator  101  is fed by a PRF(Tx) signal  113  with the pulse repetition frequency (PRF) for the transmit path (Tx).  
         [0044]     The output signal  123  generated by the pulse generator  101  is thereafter filtered in a band-pass filter  102 , and then travels through the transmission link  115  to the directional coupler  103  (tap).  
         [0045]     The directional coupler  103  is configured e.g. as an asymmetric hybrid coupler, the coupling factor of which is variable. From the transmit signal, a reference pulse  116  is then derived or extracted via the directional coupler and supplied to a second directional coupler  106 .  
         [0046]     After tapping of the reference pulse, the transmit signal travels through a delay line  104  so as to be time delayed. This delay line  104  is configured e.g. as a strip line in a multilayer circuit board, as a wound coaxial cable, or as a LTCC component. After the time delay of the transmit signal, the transmit signal travels through a circulator  105 , which forwards the transmit signal with low attenuation via the transmission link  107  to the antenna  117 .  
         [0047]     By the antenna  117 , the transmit signal is then emitted, and transmitted as a signal  119  towards the filling material. Therefrom, it is then reflected as the receive signal  120  and picked up by the antenna  117  and supplied to the circulator  105 . Therein, the receive signal is then conducted with low attenuation to line  118 , and then to the directional coupler  106 , where it merges with the reference pulse.  
         [0048]     The reference pulse and the receive signal (measuring signal) are forwarded to the sampling mixer  108 . The sampling mixer  108  is driven by the second pulse generator  109 .  
         [0049]     The second pulse generator  109  (which by the way may be either a stand-alone unit or be the same as the first pulse generator  101 ) is supplied by a PRF (LO) Signal  114  with the pulse repetition frequency (PRF) of the local oscillator (LO) and generates a pulsed output signal  124 , which thereafter travels through the band-pass filter  110  before being supplied to the sampling mixer  108 .  
         [0050]     When the sampling mixer  108  has retarded the measuring signal, amplification of the output signal by the amplifier  111  takes place. The resulting signal  112  is then supplied as an intermediate frequency to the evaluation circuit  122 .  
         [0051]     The amplitude of the reference pulse may be set via the coupling factor of the directional couplers  103 ,  106 . The directional couplers  103 ,  106  may be driven via a controller  121 , and thus the coupling factors thereof may be set (individually or in common).  
         [0052]     The improved pulse form of the reference pulse provides for higher accuracy of the measurement. Due to tapping the reference pulse in combination with the delay line, biasing of the reference pulse by the antenna or the coupling or due to echoes at close range may be avoided.  
         [0053]      FIG. 2   b  shows a similar arrangement as  FIG. 2   a , however, herein an attenuation member  125  is inserted between the directional couplers  103  and  106 , which may allow to set the magnitude of the reference signal  116 . This attenuation member  125  may be configured as a fixed attenuation member or as a variable attenuation member, which may then be set individually via a controller  126 .  
         [0054]      FIG. 3  shows a schematic view of a filling level radar according to a sample embodiment of this invention. As may be seen in  FIG. 3 , the filling level radar  300  has a radar module  100 , an evaluation circuit  122 , and an antenna  117 . Herein, the antenna  117  is used for transmitting and receiving electromagnetic signals  119 ,  120 , which are reflected by a filling material surface  302 .  
         [0055]     The level measuring instrument  300  may be configured as a filling level radar or else for non contactless measuring of filling levels, e.g. as a TDR unit.  
         [0056]     Additionally, it is to be noted that “comprising” does not exclude any other items or steps, and that “a” or “an” do not exclude a plurality. Furthermore, it is to be noted that features or steps having been described with reference to one of the above sample embodiments can also be used in combination with other features or steps of other embodiments described above. Reference numerals in the claims are not to be construed as limitations.