EP3631492A1

EP3631492A1 - Radio-frequency generating unit

Info

Publication number: EP3631492A1
Application number: EP18717034.5A
Authority: EP
Inventors: Winfried Mayer
Original assignee: Endress and Hauser SE and Co KG
Current assignee: Endress and Hauser SE and Co KG
Priority date: 2017-05-30
Filing date: 2018-04-10
Publication date: 2020-04-08
Also published as: DE102017111820A1; WO2018219534A1

Abstract

The invention relates to a radio-frequency generating unit (11) for an FMCW radar-based fill level measuring device (1) which according to the invention comprises the following components: a clock unit (111) for generating a clock signal (sClock), a digital switch mechanism (114) controlled by the clock signal (sClock) for generating a digital output signal (Sd), and a digital/analogue converter (115) which converts the digital output signal (Sd) of the digital switch mechanism (114) into an analogue radio-frequency signal (SHF). Due to this construction according to the invention, a radio-frequency generating unit (11) for an FMCW-based fill level measurement system can be produced which in particular functions without a voltage-controlled oscillator connected to a sensitive control loop, as is normally required in the prior art. This substantially reduces the circuit complexity, so that the radio-frequency generating unit (11) can be made very compact, if necessary as a single monolithically integrated circuit. With regard to the fill level measurement, the radio-frequency generating unit (11) according to the invention, because of the low-interference signal generation of the radio-frequency generating unit, allows the fill level (L) to be determined more precisely and more reliably.

Description

High-frequency generating unit

The invention relates to a high-frequency generating unit for an FMCW radar-based level gauge.

In automation technology, in particular in process automation technology, field devices are often used which serve for detecting and / or influencing process variables. For the detection of process variables, sensors are used, for example, in level gauges, flowmeters, pressure and temperature measuring devices, pH redox potential measuring devices or

Conductivity meters are used. They record the corresponding process variables, such as level, flow, pressure, temperature, pH value, redox potential or conductivity in the respective process plant. For influencing

Process variables are actuators, such as valves or pumps that allow the flow rate of a fluid in a conduit section or the level in a reservoir to be changed. A large number of these field devices are manufactured and sold by Endress + Hauser.

For level measurement of filling goods in containers have non-contact

Measuring procedures established because they are low in wear and maintenance. Another advantage is the ability to quasi-continuous, so with a high level

To measure resolution. Therefore, in the field of continuous

Level measurement mainly used radar-based distance measuring methods. For this purpose, preferably the so-called FMCW method ("Frequency Modulated Continuous Wave", also known as continuous wave radar) is used, which operates within predefined radar frequency bands (in the context of this invention, the terms "high frequency" and "radar" refer to signals or Electromagnetic waves with frequencies between 0.3 GHz and 300 GHz.) With FMCW-based distance or velocity measurement, there are already technically mature solutions for frequencies up to approximately 79 GHz for generating a high-frequency electrical signal by means of corresponding high-frequency generating units Here, a transmission / reception antenna, whereby the resulting microwave-based transmission signal is generated and radiated, Accordingly, the frequency of the transmission signal is determined by the high-frequency generating unit.

Suitable high-frequency generating units for generating high-frequency signals for FMCW radar are already known. In this context, the example Publication DE 10 2013 105 A1 mentioned. There, a typical high-frequency generation unit according to the prior art is shown:

Core of the local high-frequency generating unit is a voltage-controlled high-frequency oscillator (this is usually a VCO, ie a "Voltage Controlled Oscillator" used), which generates a corresponding high-frequency signal whose frequency is variable by the control voltage typical

Sawtooth-shaped frequency change of the electrical high-frequency signal is impressed by the fact that the oscillator via a feedback circuit, also known as "phase locked loop" or PLL ("Phase Locked Loop"), regulated.

In this case, the feedback is realized by the high frequency signal of the

Oscillator optionally via a frequency divider a control signal is diverted. This is supplied to a phase comparator, which compares the instantaneous phase position with that of a frequency-constant clock signal. The frequency of the clock signal is in the range of preferably 10 MHz to 100 MHz and can be generated by means of a corresponding quartz oscillator. The phase comparator controls the VCO via an analog DC signal, wherein the voltage value of the DC signal depends on the instantaneous phase position at the inputs of the phase comparator. As the name implies, the frequency of the VCO depends on the voltage at its input.

As is apparent from this description, such a high-frequency generation unit according to the prior art requires a comparatively complicated circuit construction. In addition to the complex circuit structure, another disadvantage is that the voltage-controlled oscillator and the frequency divider have a high power consumption and a high noise. The adverse effect of noise on the distance or level measurement, however, is that the

Sensitivity, with which small or poorly reflecting measurement objects can be detected, especially at long distances, drastically decreases. By high

Power consumption makes it difficult to design the level gauge so that it complies with current explosion protection regulations.

The invention is therefore based on the object, a simple and robust

To provide high frequency generating unit for FMCW-based level gauge.

The invention achieves this object by a high-frequency generating unit for an FMCW radar-based fill level measuring device, which comprises at least the following components: A clock unit configured to generate a clock signal having a predetermined clock frequency;

a digital switching device clocked by the clock signal (for example as an arbitrary generator or a circuit operating according to direct digital synthesis) which is designed to generate a digital output signal, ie discrete-time and / or discrete-value signal;

a digital / analogue converter which converts the digital output signal of the digital switching device into an analogue high frequency signal. By this construction according to the invention, a high-frequency generating unit for FMCW-based level measurement can be realized, which in particular manages without a voltage-controlled oscillator and an associated, susceptible control loop. This leads to a significantly reduced circuit complexity, so that the high-frequency generation unit can be realized very compactly, possibly in a single monolithically integrated circuit.

For use in FMCW radar based level measurement is the digital

Rear derailleur so that the digital output signal with a variable frequency, in particular a within a predetermined frequency band linearly changing frequency, is generated.

In addition, the digital switching mechanism is preferably designed so that the frequency of the output signal is at most half as high as the clock frequency of

Clock signal is to satisfy the sampling theorem. For use in FMCW radar-based level measurement, the digital switching mechanism is also preferably designed so that it generates the digital output signal with a frequency (or in a frequency band) at least 1 GHz, in particular 6 GHz. Even applications at 26 GHz or even 79 GHz, which represent further typical areas for radar-based level measurement, would be conceivable.

According to the digital switching mechanism, the digital / analogue converter is to be adapted to the digital output signal of the digital switching mechanism (in particular its level and pulse width or frequency range). On the output side, the digital / analog converter is dependent on the further processing of the high-frequency signal in

Level Gauge: Depending on the downstream components, such as transceivers or transmit / receive antennas, the digital / analog converter shall be designed to produce the analog high frequency signal either as a current-based signal or as a voltage-based signal , With regard to the filtering of any noise or interfering signals which superimpose the electrical high-frequency signal, it is advantageous if the high-frequency generating unit additionally comprises a filter which is designed to handle the

High-frequency signal before coupling into the transmitting / receiving antenna of

Level gauge to filter accordingly. Here, the filter (which is preferably designed as a low-pass or bandpass) at least for the frequency of

Output signal permeable. In particular, if the frequency of the output signal is at most half as high as the clock frequency of the clock signal, the use of a low-pass filter is necessary to smooth the discrete-time output signal of the digital / analog converter.

A very compact design of the high-frequency generating unit according to the invention can be achieved if at least the clock unit, the digital switching mechanism and the digital / analog converter are constructed in the form of an integrated monolithic circuit. Not least in the event that the integrated circuit also comprises the clock unit, there is a particularly favorable embodiment variant in that the clock unit is realized as an acoustic surface resonator.

In addition, it makes sense if the clock unit is executed synchronized by means of a phase locked loop with a reference source. This makes it possible for the

Generation of the high-frequency signal and the sampling of the received measurement signal have the same clock reference and thus are particularly resistant to interference. In addition, such a clock unit can be saved. In this case, it is preferable for the reference source to have a reference frequency which is equal to or corresponds to an integer divisor of the clock frequency.

In an implementation of the high-frequency generating unit according to the invention in an FMCW radar-based level measuring device, which is used to measure a level of a filling material located in a container, has

Level gauge according to the following components:

A high-frequency generation unit according to one of the previously described embodiments,

a transmitting / receiving antenna which emits the analog high frequency signal as an electromagnetic transmission signal in the direction of the contents and / or the reflected signal after reflection on the surface of the contents receives (a separate transmitting and receiving antenna would also be used), a Control / evaluation unit, which determines the level according to the FMCW method by means of the measured frequency difference between the electromagnetic transmission signal and the reflected signal. Based on the following figures, the invention will be explained in more detail. It shows:

1 shows a general representation of an FMCW radar-based level gauge on a container,

Fig. 2: a characteristic of FMCW radar frequency characteristic of the electrical

High frequency signal,

Fig. 3: a high-frequency generation unit for FMCW radar according to the prior art, and

4 shows a high-frequency generation unit according to the invention.

For a basic understanding of the invention, FIG. 1 shows a typical arrangement of a fill level measuring device 1 operating according to the FMCW radar principle on a container 2. In the container 2 is a filling material 3, the level of which L is to be determined by the level gauge 1. In general, that is

Level gauge 1 via an internal control evaluation unit 15 and a

corresponding bus system, such as "PROFIBUS", "HART" or "Wireless HART" is connected to a higher-level unit 4, for example a process control system, which can be used to communicate information about the operating state of the fill level measuring device 1. Information about the fill level L can also be communicated be transmitted to optionally control on the container 2 existing inflows and / or outflows.

To determine the fill level L, the level gauge 1 is mounted in a known installation height h above the filling material 3 on the container 2. Here, the container 2 depending on the application can be up to more than 30 m high. The level gauge 1 is arranged on the container 2 so that it emits an electromagnetic transmission signal SHF in the direction of the surface of the filling material 3. After reflection at the Füllgut- surface receives the level gauge 1, the reflected signal EHF in

Dependence of the distance d = h - L on the product surface. In this case, according to the FMCW principle, the effect is utilized that the difference frequency (ie the difference between the instantaneous frequency IHF of the transmission signal SHF and the frequency of the reflected signal EHF currently received by the level measuring device 1) changes linearly with the distance d to the product surface ,

In order to use this effect, according to the FMCW principle, it is necessary to transmit the transmission signal SHF not at a constant frequency fHF but at a frequency fHF that varies with time within a fixed frequency band Af. Common frequency bands are in the field of level measurement at 6 GHz, 26 GHz or even at 79 GHz. The width of the respective frequency band Af is approximately between 5% and 20% of the absolute frequency.

As shown in Fig. 2, typically a sawtooth

Frequency change, ie a time linearly increasing frequency implemented (a linearly decreasing frequency change would be equally applicable), wherein the frequency change within the frequency band Af cyclically repeats with a preset repetition rate r. In order to transmit the electromagnetic transmission signal SHF having this characteristic via a corresponding transceiver antenna 13 (for example, a horn antenna as shown in FIG

High-frequency generating unit 1 1, 1 1 'of the level gauge 1 a

generate corresponding electrical high frequency signal SHF, which also has to exhibit a sawtooth frequency characteristic. A common high-frequency generating unit 1 1 'according to the prior art is shown in Fig. 3: It is based on a voltage controlled oscillator 1 13 for generating the high-frequency electrical signal SHF. In this case, its oscillation frequency can be adjusted by an applicable DC voltage value Sdc. In the case of the high frequency generating unit 11 'shown in FIG

Setting of the required DC voltage value Sdc by a ramp generator 1 12, which is fed back by default as a phase-controlled loop (also known as "phase locked loop") with the voltage controlled oscillator 1 13. In this case, from the high frequency signal SHF of the oscillator 1 13 (possibly via an additional Frequency divider, not shown in Fig. 3) diverted a control signal.

This is supplied to a phase comparator within the ramp generator 1 12, wherein the phase comparator the instantaneous phase position with a

frequency constant clock signal s _C iock compares. The generation of the clock signal s _C iock is effected by means of a clock unit 1 1 1, which is implemented for example as a quartz oscillator, which clocks the ramp generator 1 13 here in a frequency of 10 MHz to 100 MHz. About the ramp generator 1 12 is also the frequency fHF or the

sawtooth frequency change of the lying in the microwave range transmission signal SHF set by means of a corresponding control signal Sf. This control signal Sf can cause the frequency change, for example, by changing the divider values of the frequency divider in the ramp generator 112.

As known from the prior art, the high-frequency electrical signal SHF via a transmitting / receiving switch 12 of the transmitting / receiving antenna 13 of the

Level meter 1 is supplied to emit for purposes of level measurement, the electrical high frequency signal SHF as electromagnetic transmission signal SHF. In addition, the electromagnetic signal EHF, which is reflected by the surface of the filling material 3, is received via the transmitting / receiving antenna 13. Subsequently, the received electromagnetic signal EHF is supplied via the transmitting / receiving switch 12 to a mixer 14 and through the mixer 14 with the electric

High frequency signal SHF mixed. As indicated in Fig. 3, the electrical

Radio frequency signal SHF for this purpose between the voltage controlled oscillator 1 13 and the transmitting / receiving switch 12, for example by means of a corresponding

Signal divider to be branched off. By mixing with the high frequency electrical signal SHF, an intermediate frequency signal ezF is formed as known in FMCW. Due to the sawtooth-shaped frequency change of the electrical high-frequency signal SHF, the so-called difference frequency of the intermediate frequency signal ezF changes approximately linearly to the distance d between the level measuring device 1 and the surface of the filling material 3.

Accordingly, by measuring this difference frequency of the

Intermediate frequency ezF the level L can be determined. This can be achieved, for example, by means of the corresponding designed control / evaluation unit 15 of the level measuring device 1, in particular by processing the

Intermediate frequency signal ezF by means of a "Fast Fourier Transformation".

The high-frequency generating unit 1 V according to the prior art described with reference to FIG. 3 has several characteristics which have a disadvantageous effect especially in the level measurement: In addition to the complex and thus fault-prone circuit construction, a central disadvantage is that the oscillator 1 13 a comparatively high power consumption and a strong noise and a strong temperature dependence. A high power consumption of the

Level gauge 1 makes it difficult to comply with explosion protection regulations. The disadvantageous effect of noise on the distance or level measurement is that the sensitivity with which poorly reflecting contents 3 can be detected, especially at long distances or low level L, drastically decreases. Accordingly, not only the resolution or the

Accuracy of the level measurement limited, but it also increases the risk that any clutter from the level gauge 1 is erroneously interpreted as a level echo. This endangers not only the safety in the environment of the container 2, but possibly also the operational safety in the entire process plant.

Therefore, a high-frequency generating unit 1 1 according to the invention for an FMCW-based level measuring device 1 is proposed, which has a structure, as shown in Fig. 4: Instead of the feedback oscillator 1 13, the high-frequency generating unit 1 1 according to the invention based on this structure a digital rear derailleur In this case, the digital switching mechanism 14 serves to generate a digital, periodic output signal Sd and accordingly can be implemented, for example, as an arbitrary generator (also known by the term "Arbitrary Waveform Generator") or a circuit operating according to direct digital synthesis In addition, the frequency fHF or the sawtooth-shaped frequency change of the high-frequency signal SHF (and thus also the frequency of the transmission signal SHF, which is in the microwave range), is also produced via the digital switching mechanism 14 The clocking of the digital switching mechanism 1 14 is in turn effected by a separate clock unit 1 1 1. The clock unit

1 1 1 is here preferably designed to generate the clock signal s _c iock with a clock frequency fdock which is at least twice as high as the frequency fHF of the

Output signal is. In addition, the high-frequency generating unit 11 according to the invention comprises a digital / analog converter 15, which purely optimally inputs the digital output signal Sd of the digital switching mechanism 1 14 into the desired analog high-frequency signal SHF

sinusoidal course converts. In this case, the digital / analog converter 1 15 may be designed to output the high-frequency signal SHF either as a voltage-based or as a current-based signal.

Thus, by the high-frequency generating unit 1 1 according to the invention, the high-frequency electrical signal SHF can be realized without a voltage-controlled oscillator and with substantially reduced circuit complexity compared with the prior art. In addition, eliminates any additional circuit to compensate for temperature effects. By eliminating a control loop, the setting of a wide frequency band Af and a high repetition rate r is also possible with the aid of the high-frequency generation unit 1 according to the invention, which in turn results in a higher accuracy in the measurement of the fill level L. A further advantage of the high-frequency generation unit 1 1 according to the invention is that the entire circuit or at least the clock unit 11, the digital switching unit 114 and the digital / analog converter 15 can potentially be realized in a very compact manner in a single integrated circuit , For this purpose, the clock unit 1 1 1 could also be integrated as an acoustic surface resonator within the monolithic circuit.

As shown in Fig. 4, it is optionally possible to filter the high-frequency signal SHF by means of a corresponding filter 16, so as to filter out any interference frequencies from the high-frequency signal SHF. For this, the filter 1 16 but at least for the Frequency f _H F of the output signal SHF be permeable. For this purpose, the filter 1 16 can be designed for example as a low-pass or bandpass.

LIST OF REFERENCE NUMBERS

1 level gauge

2 containers

3 contents

4 Parent unit

1 1, 1 1 'high frequency generating unit

12 transmitting / receiving switch

13 transceiver antenna

14 mixers

15 control / evaluation unit

1 12 clock unit

1 13 ramp generator

1 14 Voltage Controlled Oscillator

1 15 Digital Rear Derailleur

1 16 digital / analog converter

1 17 low pass

EHF Reflected electromagnetic signal ezF Intermediate frequency signal

fHF frequency of the high-frequency signal fciock clock frequency of the clock signal

r repetition rate

Sf control signal for controlling the frequency

SHF electromagnetic transmission signal

Sdc DC voltage value

SHF high frequency signal

Sdock clock signal

Af frequency band

Claims

claims

1 . High-frequency generation unit for an FMCW radar-based level gauge (1), comprising the following components:

- A clock unit (1 1 1) which is designed to generate a clock signal (s _C iock) with a predetermined clock frequency (fciock),

a digital switching device (1 14) clocked by the clock signal (s _C iock) and adapted to generate a digital periodic output signal (Sd); a digital / analogue converter (1 1 5) encoding the digital Output signal (Sd) of the digital switching mechanism (1 14) into an analog high-frequency signal (SHF) converts.

2. High-frequency generating unit according to claim 1, wherein the digital switching mechanism (1 14) is adapted to the digital output signal (Sd) with a variable frequency (fHF), in particular within a predetermined frequency band {AT) linearly changing clock frequency (^ F) to generate.

3. High-frequency generating unit according to claim 1 or 2, wherein the digital

Derailleur (1 14) is designed so that the frequency (^ F) of the output signal (Sd) is at most half as high as the clock frequency () of the clock signal (s _C iock).

4. The high frequency generating unit according to claim 1, 2 or 3, wherein the digital switching circuit (1 14) is adapted to generate the digital output signal (Sd) having a frequency (fHF) of at least 1 GHz.

5. High-frequency generation unit according to one of claims 1 to 4, wherein the digital switching mechanism (1 14) is implemented as an arbitrary generator or a working after the direct digital synthesis circuit.

6. High-frequency generation unit according to one of claims 1 to 5, wherein the digital / analog converter (1 1 5) is designed to generate the analog high-frequency signal (SHF) as a current-based signal or as a voltage-based signal.

The high-frequency generation unit according to any one of claims 1 to 6, comprising:

A filter (16) configured to filter the high frequency signal (SHF), the filter (16) being transmissive at least to the frequency (^ F) of the output signal (SHF).

8. High-frequency generation unit according to claim 7, wherein the filter (1 1 6) is designed as a low-pass or bandpass.

9. High-frequency generation unit according to one of the preceding claims, wherein at least the clock unit (1 1 1), the digital switching mechanism (1 14) and the digital / analog converter (1 15) are constructed as an integrated circuit.

10. High-frequency generation unit according to claim 9, wherein in the event that the integrated circuit comprises the clock unit (1 1 1), the clock unit (1 1 1) is implemented as a surface acoustic resonator.

1 1. High-frequency generating unit according to one of the preceding claims, wherein the clock unit (1 1 1) is synchronized by means of a phase locked loop with a reference source, wherein the reference source has a reference frequency which is the same or an integer divisor of the clock frequency (fcioc _k ) equivalent.

FMCW radar-based level measuring device for measuring a filling level (L) in a container (2) located filling material (3), comprising:

A high-frequency generation unit (1 1) according to one of the preceding claims,

a transmitting / receiving antenna (13) which emits the analog high frequency signal (SHF) as an electromagnetic transmission signal (SHF) in the direction of the filling material (3) and / or the reflected signal (EHF) after reflection at the

Surface of the filling material (3) receives,

a control / evaluation unit (15) which determines the level (L) by means of a frequency difference between the electromagnetic transmission signal (SHF) and the reflected signal (EHF).