Abstract:
Described is a method and an apparatus of directly digitizing microwave signals reflected at a filling product surface of a filling product present in a receptacle, in which method the microwave signals reflected at the filling product surface are sampled unmodified in their frequency, and the therefrom resulting analog values are converted into digital values, with successive digital values being stored in various intermediate memories, and the intermediately stored digital values are read out from the intermediate memories and are stored in a terminal memory unit, to which accesses an evaluation arrangement which determines the filling level height.

Description:
PRIORITY CLAIM 
   This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/315,272 filed on Aug. 28, 2001 and German Patent Application Serial No. 101 40 821.8 filed on Aug. 20, 2001 which are expressly incorporated herein, in their entirety, by reference. 

   TECHNICAL FIELD 
   The present invention relates to a method and an apparatus for directly digitizing microwave signals reflected from a filling product surface of a filling product present in a receptacle, as well as to a filling level measuring device working on the transit time principle and emitting microwave signals. Furthermore, the present invention relates to a filling level measurement device working on the transit time principle and emitting microwave signals. 
   Reduced to the core features and hence expressed in a simplified manner, microwave signals are generated (on a pulse basis or continuously) by a transmitting means in a filling level measuring device of the mentioned kind, and are radiated via an antenna, such as, for example, a horn, rod or microstrip antenna into the direction of the filling product surface, or are guided to the filling product surface by means of a waveguide (rod, cable or also rope). The microwave signals reflected at the filling product surface—in general also designated as echo signals—are fed to a reception circuit via a reception means, which in the most cases is identical to a transmission antenna, or by means of the above-mentioned waveguide, and are sampled. The analog values resulting from the sampling process may be converted into digital values, and then are made available to an evaluation means. In the evaluation means, the filling level height is determined from the digital values. 
   BACKGROUND INFORMATION 
   For example, DE 42 40 491 A1 appears to disclose a filling level measuring device of the initially mentioned kind that work on the transit time principle and emit for example microwave signals. For processing the reflected and received microwave signals, various analog circuits or components are nowadays available so as to generate a so-called envelope curve, which may then be digitized for assessing the filling level height here from. The generation of an envelope is sufficiently known, and accordingly, there is no need for further explaining this technology (e.g. cf.: DE 44 07 396 C2; M. Scholnick, “Introduction to Radar Systems”, 2 nd  ed. 1980, McGraw-Hill; Peter Devine, “Radar Level Measurement—The User&#39;s Guide” VEGA Controlls Ltd., 2000, ISBN 0-9538920-0-X). 
   In the present technical field, one works with microwave signals having a frequency of for example 1 up to 30 GHz or more. Due to these high frequencies, various analog circuits or components have been necessary to date for determining the exact filling level from the received microwave signals. In particular, so-called “mixers” are inter alia used, by means of which a temporal “lengthening” of the envelope is performed, so that the sampling of the envelope curve with usual A/D converters (pulse frequencies of, for example, about 25 microseconds) with a sufficiently fine sampling lattice (for achieving a sufficient accuracy of the determined filling level height) becomes possible. This means that in particular microwave pulses are transformed by means of a mixer and an associated oscillator into another frequency and time domain. Hereby, pulses are, for example, lengthened in time after the time transformation by about the factor 160,000, whereby a pulse duration of 1-2 nanoseconds prior to the transformation corresponds to a pulse duration of about 160-320 microseconds after the transformation. These time-lengthened pulses are then supplied to the A/D converter, which samples this signal in a sampling lattice of about 25 microseconds. Thus, with a pulse length of 160 microseconds, about six sampling values are available per pulse. With these values, accuracies of about 5 mm may be achieved in the filling level height determination. It has to be pointed out that the accuracy of the filling level height determination depends also on the magnitude of the sampling values per echo pulse, on the steepness of the pulse slope and on the accuracy of the A/D converter. 
   In summary, it has to be noted that devices for determining the filling level height on the basis of reflected microwaves, in particular microwave pulses, perform an analog processing of the received signals so as to be able to achieve the desired accuracy in the determination of the filling level height. The use of analog components or analog circuit technology, however, can be subject to disadvantages. Thus, in the dimensional configuration of the circuit, component tolerances and manufacturing divergences of the components have to be taken into account, and also have to be monitored during the production. Analog components moreover exhibit temperature dependencies, which have to be considered and cause a rather significant test effort. Temperature dependencies and component tolerances, in addition, can result in a decrease of the reproducibility and accuracy of the output signals, and have to be minimized or compensated by an additional effort. An disadvantage may also to be seen in that modifications in the functional mode of such analog circuits can only be performed at an important effort. Finally, analog circuits may be also sensitive for interfering influences. 
   SUMMARY OF THE INVENTION 
   An object of the present invention consists in providing an apparatus, by means of which the number of analog circuit components in devices of the mentioned kind may be reduced, and the aforementioned disadvantages thus are at least reduced. 
   This object may be solved according to a first aspect of the present invention by means of a method of directly digitizing microwave signals reflected at a filling product surface of a filling product present in a receptacle, wherein the method comprises the following method steps: sampling the microwave signals reflected at the filling product surface unmodified in their frequency, converting the analog values resulting from the sampling step into digital values, storing successive digital values of the converting step in various intermediate memories, reading out the intermediately stored digital values from said intermediate memories and storing the read out digital values in a terminal memory unit, to which accesses an evaluation means for determining the filling level height. 
   An corresponding inventive apparatus comprises, according to the first aspect of the invention, a reception circuit into which the reflected microwave signal is to be fed, and which is configured for the amplification of same, but leaves the frequency thereof unmodified, an A/D converter connected with said reception circuit, which is configured for sampling the amplified microwave signal and converts the therefrom resulting analog values into digital values, several intermediate memory means in each case connected with the A/D converter, a terminal memory means connected with the intermediate memory means, and a control means providing for intermediately storing successive digital values in various intermediate memory means, and the intermediately stored digital values are restored in the terminal memory unit, which are then available to the evaluation means for determining the filling level height. 
   Furthermore, the above-mentioned object may be solved according to a second aspect of the present invention by means of a method of directly digitizing microwave signals reflected at a filling product surface of a filling product present in a receptacle, wherein the method comprises the following method steps: sampling the microwave signals reflected at the filling product surface unmodified in their frequency, converting the therefrom resulting analog values into digital values by means of a plurality of A/D converters, with in each case successive analog values being converted into digital values from various A/D converters, and storing the digital values in a terminal memory unit, to which accesses an evaluation means for determining the filling level height. 
   A corresponding the inventive apparatus for directly digitizing microwave signals reflected from a filling product surface of a filling product present in a receptacle, comprises a reception circuit into which the reflected microwave signal is to be fed, and which is configured for the amplification of same, but leaves the frequency thereof unmodified. Moreover, several A/D converters each connected with the reception circuit are provided, which are each configured for sampling the amplified microwave signals and convert the therefrom resulting analog values into digital values. Furthermore, a terminal memory unit is provided connected with the A/D converters for storing the individual digital values. Finally, a control means is present providing for the fact that the individual A/D converters sample alternatingly (with more than two A/D converters in turns or one after the other) the amplified microwave signal and convert in each case an analog value into a digital value, and providing for the fact that the individual digital values are stored in the terminal memory unit, which are then available to an evaluation means for determining the filling level height. 
   According to a third aspect of the present invention, the above-mentioned object may be solved by means of a method of directly digitizing microwave signals reflected at a filling product surface of a filling product present in a receptacle, wherein the method comprises the following method steps: a first sampling step for sampling microwave signals reflected at the filling product surface unmodified in their frequency, a first converting step for converting the analog values resulting from the first sampling step into digital values by means of an A/D converter, a first storing step for storing the digital values resulting from the first converting step in a terminal memory unit, a second sampling step for sampling at least one further microwave signal reflected later in time at the filling product surface unmodified in its frequency but offset from the first sampling, and a second converting step for converting analog values resulting from the second sampling step into digital values by means of an A/D converter, a second storing step for storing the digital values of the second converting step in the terminal memory means, wherein the entirety of the stored digital values of the first and second converting steps represent an envelope and are available to an evaluation means for determining the filling level height. 
   An exemplary embodiment of the present invention according to the third aspect provides that for an envelope, sampling values from temporally successively received echo signals are used. Since for determining a filling level, the time between the emission of the microwave pulse and the reception of the echo signal reflected at the filling product surface is measured, a maximum value is predetermined for this time defining the maximum measurement range of the device. During this time, the echo signal is normally sampled by the A/D converter in equal intervals of time. When the filling level has been determined from this envelope, the next pulse is emitted, the echo signals are sampled, and the filling level is again determined here from. If the sampling values are now so generated that the moments of the samplings are at the same place in the echo signal from cycle to cycle, than a doubling of the sampling rate may be achieved in that the sampling of the second envelope is offset by half a sampling time. So as to obtain an envelope having a doubled sampling rate, two envelopes having the lower sampling rate hence have to be generated. For a determination of the filling level, two cycles are therewith necessary. This method may be applied when the echo signals scarcely change from cycle to cycle; each modification tampers the composed envelope and hence result in measurement errors. With this method, a triplication or quadruplication, etc., may also be achieved. By means of the mentioned variant, a direct digitization of the envelope without the interconnection of intermediate memories or the use of several A/D converters would likewise be possible. In other words: only by means of one A/D converter, a high sampling rate may be achieved in toto by sampling several envelopes with in each case a low sampling rate, and at the same time, without the interconnection of intermediate memories, a storage of the digital values originating from the A/D converter in an available terminal memory unit may ensue. The envelope serving the purpose of evaluation, which in the end is available in the form of digital values in the terminal memory unit, hence is composed here of several envelopes, which are generated temporally successively, and which have been sampled by the A/D converter with a “low” frequency. 
   According to a forth aspect of the present invention, a method of directly digitizing microwave signals reflected at a filling product surface of a filling product present in a receptacle comprises the following method steps: sampling microwave signals reflected at a filling product surface unmodified in their frequency with a first rate, storing analog values resulting from the sampling step in an analog memory unit, reading out the stored analog values with a second rate less than the first rate, converting the read out analog values into digital values by means of an A/D converter, storing the digital values in a terminal memory unit, and providing the entirety of the stored digital values representing an envelope curve for an evaluation means for determining the filling level height. 
   An exemplary embodiment of an apparatus for directly digitizing microwave signals reflected at a filling product surface of a filling product present in a receptacle according to the forth aspect comprises a reception circuit, into which the reflected microwave signal has to be fed, and which is configured for amplifying same, but which leaves the frequency thereof unmodified, an analog memory unit connected with the reception circuit for storing the amplified analog values of the reception circuit, an A/D converter connected with the analog memory unit, which is configured for reading out the analog values stored in the analog memory unit, and wherein the A/D converter samples the analog values and converts them into digital values, a terminal memory unit connected with the A/D converter for storing the individual digital values, and a control means providing for the fact that the amplified reflected signal is stored into the analog memory unit with a first rate, and providing for the fact that the amplified reflected signal stored in the analog memory unit is read out with a second rate less than the first rate and converted into digital values by means of the A/D converter, and providing for the fact that the digital values are stored in the terminal memory unit, which are then available to an evaluation means for determining the filling level height. 
   In general, the invention is, for the first time, based on the idea of no longer temporally “lengthening” the received microwave signals by means of a mixer, but to directly digitize the received, temporally “unlengthened” microwave signal by means of one or several A/D converters. So as to be able to achieve the sampling rates required for a sufficient measurement accuracy in spite of the non-performed temporal lengthening and to be nevertheless able to use memory components having commercial access times, said intermediate memories are present and/or several A/D converters are used. By interconnecting intermediate memories or by using several A/D converters, despite the very high frequencies of the “unlengthened” microwave signals, sampling rates may be achieved as in the prior art with temporal lengthening (by means of analog components). As soon as the respective digital values are stored in the terminal memory unit, the processing of the digital values, and hence the evaluation ensues in the usual manner. This means that now, due to the mixer no longer present, very high frequencies of the signals to be digitized and therewith the required, very high sampling rates are compensated for by the use of intermediate memories and/or several A/D converters or combination of these two alternatives. 
   If only one A/D converter is used for digitizing the microwave signals and/or the therefrom determined envelope, then it may be better to provide at least two intermediate memories between the A/D converter and the terminal memory unit, into which intermediate memories digital values are alternatingly written. Thus, the high frequency of the A/D converter may also be used with usual memory chips such as SRAM or DDR-SRAM that have cycle times of, for example, 2.5-5 ns. The more intermediate memories are provided, the slower the used memory chips are able to work. 
   In that several A/D converters are provided, an A/D converter may work alternatingly for each pulse, and the corresponding value may be stored in the terminal memory unit, so that the sampling rate of the individual A/D converter may be reduced. 
   Thus, it may be possible according to an exemplary embodiment of the present invention to perform digitizing with usual components in spite of the immensely high frequencies, a fact which eliminates the disadvantages of analog components and in particular the disadvantages of the previous mixer. 
   According to a further exemplary embodiment of the present invention each A/D converter is assigned one or several intermediate memories, so that a converter cycle and a storage cycle of one A/D converter may ensue independent of those of the other A/D converter. 
   In that several intermediate memories connected in series are provided, it is possible to use slower-working memory chips, which hence are more cost-efficient. 
   The A/D converters and/or individual intermediate memory means may also be connected with a single terminal memory unit, it is of course also possible to provide single terminal memory chips which in turn are all connected with the evaluation unit, hence are available for an access by the evaluation means. 
   In principle, it is the question of maintaining the required high sampling rate of the A/D converter with the high frequencies of the “unlengthened” received microwave signals, and to be nevertheless able to use commercial components. By means of the alternating storage and various intermediate memory means, this may be performed for the first time, alternatively hereto, the use of several A/D converters is just possible. 
   A/D converters able to perform the high pulse time requested for sampling the microwave signals having a very high frequency, it is true, require in part a power of up to five watt; however, by the operation of the A/D converter for a space of time of 1 ns up to 0.5 microseconds, and in particular 10 ns up to 0.3 microseconds, an operation may also be carried out on a 4-20 mA two-wire loop. The mentioned spaces of time suffice for performing the required sampling of a microwave signal. An inventive apparatus for directly digitizing according to the aforementioned explanations may be arranged as a single constructional unit spaced apart from the remaining components of a filling level measurement device. In particular, it is, however, purposeful to provide such apparatuses or such a circuit directly inside of the filling level measurement device. Same is then preferably accommodated in the housing of the filling level measurement device. 
   In conclusion, it has to be stated that for directly digitizing, preferably one or several A/D converters are used in combination with intermediate memories. The use of intermediate memories may also be omitted in certain circumstances. In this case, the digital output values of the at least one A/D converter would be directly written into the terminal memory unit, to which an evaluation means accesses. The possible sampling frequency with which the envelope is digitized, essentially depends on the speed of the terminal memory unit. The speed of the available A/D converters suffices for achieving the requested sampling frequency, but the corresponding number of digital values must also be allowed to be written into the memory unit with the corresponding speed. The faster the terminal memory unit is able to store, the higher may then be the sampling frequency or sampling rate without any intermediate memories being necessary. 
   As already mentioned above, according to a forth aspect of the invention, an analog memory unit is connected between a reception circuit and an A/D converter. In this case the design is based on the principle of FISO (Fast In Slow Out). The analog memory unit may be designed as, for example, shown in DE 30 13 256 A1, U.S. Pat. No. 5,200,981, or U.S. Pat. No. 5,144,525. The analog memory unit may alternatively be designed basically like a circuit sold under the name “CompuScope 85G” of the company Tectronix. The analog memory unit makes it possible to store the received microwave signal (analog signal) without modifying its frequency. Furthermore, an A/D converter can be used which samples the read out analog values with a rate less than the rate with which the analog values are stored in the analog memory unit. By means of such an design, it is possible to use an A/D converter less fast than an A/D converter used in the other exemplary embodiments of the invention. 
   Finally, one skilled in the art may derive from the description of the various exemplary embodiments of the invention that the terms “unmodified frequency”, “leaves the frequency unmodified”, and similar terms include methods wherein a frequency is slightly modified but not as modified as it is done in combination with “mixers” mentioned before by means of which a temporal “lengthening” of the envelope is performed. With other words, the terms mentioned above mean that in particular according to the present invention microwave pulses are not transformed by means of a mixer and an associated oscillator into another frequency and time domain. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For further explanation and better understanding, several embodiments of the invention will be described in detail in the following: 
       FIG. 1  shows an exemplary embodiment of a receptacle including a filling level measurement device according to the present invention; 
       FIG. 2  shows electronic components of a filling level measurement device according to a first exemplary embodiment of the present invention; 
       FIG. 3  shows details of the embodiment as per FIG.  2  and the associated sampling and storage scheme; 
       FIG. 4  shows a further exemplary embodiment of an apparatus according to the present invention; 
       FIG. 5  shows details of the embodiment as per FIG.  4  and the associated sampling and storage scheme; 
       FIG. 6  shows electronic components of an apparatus comprising a mixer and an oscillator according to the present invention; 
       FIG. 7  shows a representation of a microwave pulse; 
       FIG. 8  shows a representation of an envelope; 
       FIG. 9  shows a representation of a monopulse; 
       FIG. 10  shows a representation of a variant of a method according to the present invention for directly digitizing several envelopes for generating a digital envelope to which an evaluation means has access for determining the filling level height; and 
       FIG. 11  shows a diagrammatic representation of electronic components of an apparatus according a further exemplary embodiment of the present invention comprising an analog memory unit. 
   

   DETAILED DESCRIPTION 
   With reference to FIG.  1  and  FIGS. 6-9 , the principle working method and the principle structure of a filling level measurement device will initially be explained. As can be seen from  FIG. 1 , in the cover zone of a receptacle  1 , a filling level measurement device  2  is mounted, that is composed of an electronic unit  3  and a horn antenna  4 . The filling level measurement device  2  is connected with a remote control panel or such like via a two-wire loop  9 . Via said two-wire loop  9 , which is in particular configured as a 4-20 mA two-wire loop, ensues the communication, as well as the energy supply of the filling level measurement device  2 . In the electronic unit  3 , microwave pulses are generated and radiated via the horn antenna  4  in the direction of the bulk product  7  present in the receptacle  1 . The microwave pulses  5  are reflected at the bulk product surface  8 , and are again received by the horn antenna  4  as an echo signal  6 . 
   The generation of microwave pulses  5  or pulse packets, and the processing of reflected pulses  6  will be described in more detail by means of the  FIGS. 6-9 . The microwave pulse is generated in a pulse generator  10  and is transmitted via a circulator  11  or a directional coupler to the horn antenna  4 , and is radiated from there. The received echo signal in turn, is again further transmitted via the circulator  11  or directional coupler to a pre-amplifier  12 , serving the purpose of amplifying the received microwave signal. Downstream of the pre-amplifier  12 , a mixer  19  is connected, which in turn is in connection with a local oscillator  20 . In the mixer  19 , the received echo signal is mixed with the fixed-frequency signal of the local oscillator  20  for transforming the echo signal into a lower frequency range (in particular kHz range). During this transformation, by an appropriate tuning of the oscillators  10  and  20 , a temporal lengthening of the echo signal is achieved; for more details hereto, reference is, for example, made to DE 31 07 444 C2. 
   The output signal of the mixer  19  that corresponds to the microwave pulse packet  21  as per  FIG. 7  corresponding to the monopulse  23  as per  FIG. 9 , is then filtered by a low pass filter  13 , and is then digitized by an A/D converter  14 . By the low pass filter  13 , an envelope  22  as per  FIG. 8  is formed. So as to make a larger dynamic range available to the A/D converter, a logarithmic or controllable amplifier may be provided upstream of the A/D converter  14 . 
   The digitized values from the A/D converter  14  are stored in a memory  17  for further processing. A signal processing or evaluation means  18 , which is not explained in more detail here, since it corresponds to the prior art, accesses said memory. A control unit  15  performs the start of the A/D converter and the transfer of the converted values into the memory  16 . Alternatively hereto, the control unit  15  or the pulse generator  10  may predefine the start moment of the pulse generation, and hence the start of the measurement process. 
   It has still to be noted here that with current realizations, the components  14 ,  15 ,  17  and  18  are in the most cases parts of a microprocessor or are realized by same. If pulse packets are used as transmission pulses, then a rectifier has to be provided upstream of a possible logarithm processing. 
   In contrast to the apparatus mentioned above, it is now inventively provided according to a first exemplary embodiment of the present invention, to omit the mixer  19  and the associated local oscillator  20 ; cf. FIG.  2 . For this purpose, the A/D converter  14  and the associated memory  17  are configured as per  FIG. 3 , i.e. the A/D converter  14  accesses the “unlengthened” echo signal. So as to achieve the sampling rate necessary for the desired measurement accuracy, and to be able to use usual memory chips, the A/D converter  14 , however, is connected with several intermediate memories  16   a ,  16   b , which in turn are connected with the terminal memory unit  17  composed of several memory chips  17   a ,  17   b . Of course, it is also possible to provide the terminal memory unit  17  for each of the single intermediate memories  16   a ,  16   a , as it is illustrated in  FIG. 3   
   When the first data value is now stored in the intermediate memory  16   a , then the intermediate memory  16   b  is available for storing the next data value. At the same time, the stored value has already been written into the terminal memory  17  by the intermediate memory  16   a , so that the intermediate memory  16   a  is now again available for the third data value. Thus, all data values are in the end present in the terminal memory unit  17  or  17   a ,  17   b . When single terminal memory units  17   a ,  17   b  are present, then the first, third and fifth data value, etc., is stored in the memory unit  17   a , and the second, fourth and sixth is stored in the memory unit  17   b . Both terminal memory units  17   a ,  17   b  are in turn available to the evaluation means  18 . 
   In contrast to the exemplary embodiment of the present invention shown in  FIG. 2 , the circulator or directional coupler  11  has been renounced of in the alternative as per FIG.  4 . By using a monopulse  23  as per  FIG. 9 , the low pass filter  13  and a possible rectifier in the pre-amplifier  12  may be moreover renounced of. The combination of an A/D converter  14  and a terminal memory unit  17  corresponds to the embodiment as per FIG.  3 . As an alternative to the realization as per  FIG. 3 , an apparatus as per  FIG. 5  may also be used in the embodiments of  FIGS. 2 and 4 , which comprise several A/D converters  14   a ,  14   b . By using two or more A/D converters, the sampling rate may be increased. 
   The two A/D converters  14   a  and  14   b  each are connected with a terminal memory unit  17   a  and  17   b . The first value of the echo signal is acquired in the A/D converter  14 , and is stored in the terminal memory unit  17   a . The next value is converted by the A/D converter  14   b  and stored in the associated terminal memory unit  17   b . The next value of the echo signal is then again converted by the first A/D converter. While the digitizing of a value ensues in an A/D converter  14   a ,  14   b , the storage is performed in the other “branch”, so that the A/D converter is again available for the next pulse. 
   A further alternative exemplary embodiment of an inventive method is depicted in FIG.  10 . Thus, a first envelope  24  is sampled by a single A/D converter with a predetermined sampling rate, i.e. the corresponding analog values are converted into digital values without temporal lengthening as otherwise usual in the prior art. These digital values are then directly stored, preferably without using an intermediate memory, in a terminal memory unit not shown here. The further envelope  25  generated from another reflected microwave signal, is sampled with the same sampling rate as the first envelope, the pulses during the sampling of the first envelope  24  and the second envelope  25 , however, are mutually offset, in this case by half a sampling time. These directly digitized values of the second envelope, as well, are written into the terminal memory unit. Therewith, a sufficient number of digital values is now present in the terminal memory unit representing a common envelope by joining the two successively generated envelopes  24 ,  25 , from which common envelope the filling level height will then be determined by the evaluation means. By sampling different envelopes several times, which is then stored in the digital form as a single envelope for the evaluation means, it is possible to perform a direct digitizing at a low frequency, allowing a storage directly in a terminal memory unit, to which the evaluation means may then access. 
   A further exemplary embodiment of the present invention is schematically shown in FIG.  11 . This apparatus differs from the exemplary embodiment of the present invention shown in  FIG. 2  by an analog memory unit  30 . The analog memory unit  30  is connected with a low pass filter  13 , a A/D converter  14 , and a control unit  15 . The analog memory unit  30  works according to the FISO principle. An amplified microwave signal from the low pass filter  13  is stored in the analog memory unit  30  with a very high rate. As soon as the microwave signal is stored, the A/D converter  14  reads out the analog microwave signal stored in the memory unit with a rate less than the rate with which the signal was stored and converts the analog values in digital values. These digital values are then stored in an terminal memory unit  17  (RAM). The further proceeding is as mentioned above, for example, with reference to FIG.  1 . An exemplary embodiment of an analog memory unit  30  useable for the invention is for example described in DE 30 13 256 A1, U.S. Pat. No. 5,200,983, and U.S. Pat. No. 5,144,525 or like the circuit “CompuScope 85G” mentioned above. 
   Of course, it is also possible to combine various features of the single described alternative exemplary embodiments of the present invention with each other. Furthermore, all exemplary embodiments of an apparatus according to the present invention may be integrated in a filling level measurement device working on the transit time principle and emitting microwave signals. Alternatively, an apparatus for directly digitizing microwave signals reflected from a filling product surface of a filling product present in a receptacle according to the present invention may be separated from such a filling level measurement device.