Field of the invention

The present invention relates to a pulsed radar level gauge including a frequency generator for providing Tx and Rx frequency signals. Specifically, the invention relates to improved synchronization of the Tx/Rx signals.

Background of the invention

Radar level gauge (RLG) systems are in wide use for determining the filling level of a product contained in a tank. Radar level gauging is generally performed either by means of non-contact measurement, whereby

electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a probe acting as a waveguide. The probe is generally arranged to extend vertically from the top towards the bottom of the tank. The probe may also be arranged in a measurement tube, a so-called chamber, which is connected to the outer wall of the tank and is in fluid connection with the inside of the tank.

The transmitted electromagnetic signals are reflected at the surface of the product, and the reflected signals are received by a receiver or

transceiver comprised in the radar level gauge system. Based on the transmitted and reflected signals, the distance to the surface of the product can be determined.

One specific type of radar level gauges employs pulsed radar. More specifically, the distance to the surface of the product contained in the tank is determined based on the difference in time (time-of-flight) between

transmission of a pulse and reception of its reflection at the surface of the product. Most pulsed radar level gauge systems employ Time Domain

Reflectometry (TDR), which provides a time expansion of the (extremely short) time-of-flight. Such TDR radar level gauge systems generate a transmit pulse train having a first pulse repetition frequency Tx, and a reference pulse train having a second pulse repetition frequency Rx that differs from the transmitted pulse repetition frequency by a known frequency difference D1 This frequency difference Dί is typically in the range of Hz or tens of Hz.

The transmit pulse train is emitted by a propagating device towards the surface of a product contained in a tank, and the reflected signal is received and sampled with the reference pulse train. In the present disclosure, the propagation device is a transmission line probe, and the gauge is referred to as a“guided wave radar” (GWR) level gauge.

At the beginning of a measurement sweep, the transmission signal and the reference signal are synchronized to have the same phase. Due to the frequency difference, the phase difference between the transmission signal and the reference signal will gradually increase during the measurement sweep. This gradually shifting time sampling of the reflected signal will provide a time expanded version of the time-of-flight of the reflected pulses, from which the distance to the surface of the product contained in the tank can be determined.

As mentioned, a synchronization of the Rx and Tx pulse trains is required before a sweep is initialized. Such synchronization is typically accomplished by a phase detector, which detects a zero phase difference between the signals. This means that the time required to ensure

synchronization will depend on how often the signals are in phase, or in other words be inversely proportional to the difference frequency Dί. So, as an example, if the difference frequency is in the order of 10 Hz, the waiting time until synchronization may be as long as 100 ms.

It would be desirable to decrease this waiting time. A shorter wait time may decrease time between measurement sweeps, or, if a faster repetition frequency is not desired, it may save energy as less time in each sweep is spent on synchronization.

General disclosure of the invention

It is an object of the present invention to decrease the waiting time required to synchronize two pulse repetition frequencies. According to a first aspect of the present invention, this and other objects are achieved by a pulsed radar level gauge for determining the filling level of a product contained in a tank, comprising a frequency generator for generating a high Tx frequency signal and a high Rx frequency signal, a first frequency divider connected to receive the high Tx frequency signal and deliver a low Tx frequency signal, wherein the low Tx frequency is equal to the high Tx frequency divided by a first integer factor P, a second frequency divider connected to receive the high Rx frequency signal and deliver a low Rx frequency signal, wherein the low Rx frequency is equal to the high Rx frequency divided by a second integer factor Q, a phase detector for detecting a phase difference between the high Tx frequency signal and the high Rx frequency signal, wherein the phase detector is configured to activate the frequency dividers in a common predefined state when a zero phase difference between the high Tx frequency signal and the high Rx frequency signal is detected, a transceiver for providing a transmit signal in the form of a pulse train having a pulse repetition frequency equal to the low Tx frequency, and receiving a reflected signal resulting from a reflection of the transmit signal at a surface of the product, a propagating device connected to the transceiver for propagating the transmit signal towards the surface, and to return the reflected signal to the transceiver, sampling circuitry connected to the transceiver and to the frequency generator, and configured to sample the reflected signal with a sampling frequency equal to the low Rx frequency in order to provide a time expanded tank signal, and processing circuitry for determining the distance based on the time expanded tank signal.

According to a second aspect of the present invention, this and other objects are achieved by a method for synchronizing a low Tx frequency and a low Rx frequency of a radar level gauge, comprising generating a high Tx frequency signal and a high Rx frequency signal, detecting a phase difference between the high Tx frequency signal and the high Rx frequency signal, in response to detecting a zero phase difference between the high Tx frequency signal and the high Rx frequency signal, generating the low Tx frequency by dividing the high Tx frequency by a first integer factor P, and generating the low Rx frequency signal by dividing the high Rx frequency divided by a second integer factor Q.

The phase detection according to the present invention is thus performed on the high Tx and high Rx signals, which have a factor P/Q higher frequency than the low Tx and low Rx frequencies actually used by the radar level gauge. As a consequence, a difference frequency Af between the high TX and high Rx will also be higher, and the time required to detect

synchronization will consequently be shorter.

Exactly how much shorter the synchronization time will be depends on how the phase detector operates. In some embodiments, the phase detector works at half the frequency of the signals it is phase detecting, i.e. here half the Rx and Tx frequencies. So, when the detection is made on frequencies which are a factor P higher, the detection will be P/2 times faster. With a different type of phase detector, the improvement might be even more significant.

The frequency dividers used to perform the frequency division should be activated in some defined state, to ensure that the low Tx and low Rx frequencies are synchronized. A preferred way to accomplish this is to activate the dividers in their“reset” state.

In a pulsed radar level gauge, the Tx frequency (also referred to as pulse repetition frequency, prf) is typically around 10000 times greater than their difference frequency. A smaller difference may be possible, but at the cost of inferior performance. As a practical example, the low Tx and low Rx frequencies may be in the range 0.5 - 5 MHz, with a difference frequency smaller than e.g. 20 Hz or even smaller than 10 Hz.

As an example, the factors P and Q may be in the range 5-25, and preferably in the range 10-20. The high Tx and high Rx frequencies may then be in the range 5 - 100 MHz.

The first and second integer factors P and Q are preferably equal, although this is not a requirement. Indeed, with different P and Q the difference frequency between the high Tx and high Rx would be even greater, and the time to synchronization shorter. In one embodiment, the frequency generator includes a first oscillator for generating the high Tx frequency signal, and a second oscillator for generating the high Rx frequency signal. Such dual oscillator design is quite common in the art.

In another embodiment, the frequency generator includes a single oscillator for providing one single oscillator frequency and frequency modifying circuitry capable of generating the high Tx frequency signal and the high Rx frequency signal from the oscillator frequency . Such single oscillator design is disclosed in US patent 10,132,671 , hereby incorporated by reference.

The RLG may include an energy store for temporary storage of electrical power received from the power supply, and power management circuitry configured to power the frequency modifying circuitry from the energy store during a measurement sweep, and to charging the energy store during an idle period between two measurement sweep. Such power management may be useful in situations where the available power is limited, for example when the power supply is a battery or a two wire control loop.

Brief description of the drawings

The present invention will be described in more detail with reference to the appended drawings, showing currently preferred embodiments of the invention.

Figure 1 shows schematically a radar level gauge.

Figure 2 a shows a schematic block diagram of the circuitry in the RLG in figure 1.

Figure 3 shows a block diagram of a frequency generator according to a first embodiment of the invention.

Figure 4 shows a block diagram of a frequency generator according to a second embodiment of the invention.

Detailed description of preferred embodiments

Figure 1 shows schematically a guided wave pulsed radar level gauge (RLG) 1 arranged to measure a distance to an interface 2 between two (or more) materials 3, 4 in the tank 5. Typically, the first material 3 is a product stored in the tank, e.g. a liquid such as gasoline, while the second material 4 is air or some other atmosphere. In that case, the RLG will enable detection of the distance to the surface 2 of the content 3 in the tank, and from this determine the filling level L.

The tank 5 is provided with a fastening structure 6 securing a housing 9 of the RLG 1 in a measuring position fixed relative the bottom of the tank 5. The housing 9 houses RLG circuitry (described in more detail below with reference to figure 2), and includes a feed through structure 7, allowing transmission of signals into and out of the tank. The feed through structure 7 may be arranged to provide process seal, capable of withstanding

temperature, pressure, and any chemicals contained in the tank.

The RLG 1 further comprises a propagating device 20 connected to the RLG circuitry via the feed through structure 7. The feed through structure 7 thus acts as an interface between a transceiver 1 1 (see figure 2) and the propagating device 20. The propagating device 20 is arranged to allow propagation of the transmit signal ST towards the surface 2, and to return a reflected signal SR resulting from a reflection of the transmit signal at a surface 2 of the product 3.

In the illustrated example, the RLG 1 is a guided waver radar (GWR), and the signal propagating device 20 is a probe extending from the RLG 1 to the bottom of the tank 5. The probe can be e.g. a coaxial wire probe, a twin wire probe, or a single wire probe (also referred to as a surface wave guide). Electromagnetic waves transmitted along the probe 20 will be reflected by any interface 2 between materials in the tank, and the reflection will be transmitted back to the transceiver 11 (see figure 2) via the feed through structure 7.

Alternatively, RLG is a non-contact RLG, and the propagating device is a directional antenna, such as a horn antenna, arranged to emit the transmitted waves to freely propagate into the tank, and to receive waves that are reflected by any interface 2 between materials in the tank. With reference now to figure 2, the RLG circuitry in the housing 9 includes a frequency generator 10 for generating a Tx frequency signal connected to a transceiver 1 1 and a Rx frequency signal connected to sampling circuitry 12. As an example, suitable Tx and Rx frequencies are in the range 0.5 - 10 MHz, typically 1 -2 MHz. The Tx frequency is preferably greater than Rx, although the opposite relationship is also possible. A critical aspect is the difference between the Tx and Rx frequencies, which needs to be several orders of magnitude smaller than the Tx and Rx frequencies. As an example, the difference frequency is in the order of Hz, smaller than 15 Hz, although a slightly larger difference frequency may also be compatible with the technology.

The transceiver 1 1 is arranged to generate a transmit signal in the form of a pulse train having a pulse repetition frequency equal to the Tx frequency. The pulses may be DC pulses or be modulated by a carrier frequency. The carrier frequency may be in the order of GHz, e.g. 16 GHz or 25 GHz. The duration of the pulses may be in the order of ns, e.g. around 2 ns or less, in order to enable measurement of the relatively short distance between the gauge 1 and the surface 2. The pulses may have average power levels in the order of mW or pW.

The transceiver includes a coupling device (not shown) allowing the transceiver to transmit the transmit signal to the propagating device while simultaneously receiving the reflected signal from the propagating device 20. The coupling device may be some sort of directional coupler, a circulator, or a solid state switch with sufficient switching frequency.

The sampling circuitry 12, which is connected to the transceiver 1 1 and to the frequency generator 10, is configured to sample the reflected signal with a sampling frequency equal to the Rx frequency in order to provide a time expanded tank signal. The time expanded tank signal, also referred to as a time domain reflectometry (TDR) signal, is A/D converted by an A/D converter 18 into a digital TDR signal.

The RLG further comprises processing circuitry 13 for determining the distance based on the digitized TDR signal. The circuitry 13 is provided with software for analyzing the TDR signal in order to determine a process variable in the tank, typically the level L of the surface 2. The processing circuitry may include a microprocessor (MCU), a FLASH memory for storing program code, a ROM (e.g. an EEPROM) for storing pre-programmed parameters, and a RAM for storing variable parameters.

A communication interface 14 provides external access to the level gauge, and is configured to receive operating power and allow

communicating measurement data externally of the RLG 1. In the illustrated example, the interface 14 provides a two-wire interface, and may be connected e.g. to a 4-20 mA control loop 15. The current in the loop may correspond to an analogue measurement value (e.g. indicating the filling level L). Alternatively, digital data may be sent across the two-wire loop, using an appropriate protocol such as HART.

The interface 14 is connected to a power management circuitry 16 which is configured to receive and distribute power to the RLG circuitry in the housing 9. The power management circuitry 16 may be connected to an energy store 17, e.g. a capacitance, configured to store energy such that power exceeding the power available from the interface 14 may intermittently be made available. This is particularly useful when using a two-wire control loop with limited current. The power management circuitry 16 may then “scavenge” available power that is not required for immediate operation, and store it in the energy store 17. This stored energy may then be used during a measurement sweep.

Figure 3 shows a frequency generator 110 according to a first embodiment of the present invention. The frequency generator 110 here has two separate oscillators 1 1 1 , 112, configured to generate a high Tx frequency signal and a high Rx frequency signal, respectively. The high Tx frequency is at least 10000 times greater than a difference frequency defined as a difference between the high Tx and high Rx frequencies. Further, the high Tx frequency is an integer factor P greater than an output Tx frequency to be applied to the transceiver 11 , and the high Rx frequency is an integer factor Q greater than an output Rx frequency to be applied to the sampler 12. The integers P and Q may be equal.

The high Tx and high Rx signals are connected to a first and a second frequency divider 1 14, 1 15, respectively. The first frequency divider 1 14 is configured to divide the high Tx frequency signal by the first integer P and deliver an output Tx frequency signal. Similarly, the second frequency divider 1 15 is configured to divide the high Rx frequency signal by the second integer Q (which may be equal to P) and deliver an output Rx frequency signal.

The high Tx and high Rx signals are also connected to a phase detector 113 configured to detect a phase difference between the high Tx frequency signal and the high Rx frequency signal. The phase detector 1 13 is further configured to activate the first and second frequency dividers 1 14, 1 15 when a zero phase difference between the high Tx frequency signal and the high Rx frequency signal is detected (i.e. when they are in synch). The activation must be done in a defined state, so that the output frequencies (the low Tx and low Rx) remain in synch. In the illustrated example, the phase detector is configured to reset the frequency dividers when a zero phase difference is detected. By resetting the frequency dividers, it is ensured that also the low Tx and low Rx will be synchronized.

In operation, the phase detection will take place at a frequency which is higher than the actual output TX and Rx frequencies. As a consequence, the time to synchronization will be correspondingly reduced. Consider, for example, a case where the output Tx and Rx are 1.56 MHz with a difference frequency of 10 Hz. The two frequencies are in synch only once every tenth of a second. By setting the oscillators to generate a high Tx frequency signal and a high Rx frequency signal, which are both 16 times higher (P=Q=16), the high Tx and high Rx frequencies will both be 25 MHz with a difference frequency of 160 Hz, potentially reducing the time to synchronization also by a factor 16. In some applications, the phase detector operates at only half the input frequency (high Tx/Rx), so that the reduction of the synchronization time will only be half of the frequency increase, i.e. a factor eight in the present example. Figure 4 shows a frequency generator 210 according to a further embodiment of the present invention. Here, both the high Tx and high Rx frequencies are generated by one single oscillating crystal 21 1 and a frequency modifying circuitry 212. The remaining elements of the frequency generator 210 can be identical to the elements of the frequency generator 1 10 in figure 3.

The oscillating crystal 21 1 provides an oscillator frequency , preferably in the range 10-100 MHz, and typically 50 MHz or lower, e.g. 25 MHz. This oscillator frequency is provided to two paths of the frequency modifying circuitry 212 for generating the high Tx and high Rx frequencies mentioned above. For example, the first path may include a frequency divider configured to receive the oscillator frequency as input frequency and deliver a first output frequency equal to the oscillator frequency divided by an integer value. The second path may include a PLL configured to receive multiply the oscillator frequency by a factor M/N, where M and N are integers with M>N, and a frequency divider connected to receive the regulated output frequency and deliver an output frequency equal to the regulated output frequency divided by an integer value. Further details of such a single oscillator chip capable of delivering two frequencies in the order or MHz having a difference frequency in the order of Hz is disclosed in

US patent 10,132,671 , hereby incorporated by reference.

The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, the choice of frequencies may be different that those discussed above. Also, the radar level gauge disclosed above is merely an example, and the principles of the frequency generator according to the invention may be applicable to any radar level gauge where two frequencies having a smaller difference frequency need to be synchronized.