Stepped Frequency Modulated Continuous Wave in Guided Wave Radar

Abstract

In industries, level measurement has to be precise and fool-proof at a minimal cost and power consumption. Radar technologies are used in this field along with other float, pressure, and magnetic based instruments. Guided wave radars (GWR) are used to measure levels and multiple material interfaces in a tank containing bulk liquids, solids, or slurries. The existing GWR device sends a short DC pulse down the waveguide made of a stiff metallic rod, flexible wire, or a coaxial construction which hits the media and a small proportion of the energy is reflected back to the device. The level is directly proportional to the time elapsed between the transmission of the pulse from the radar and the reception of the echo from the target.

The use of frequency-modulated continuous waves (FMCW) is expected to improve the accuracy and reliability of the level measurements. Existing non contacting radars (NCR) transmit a frequency modulated signal chirp within a fixed bandwidth through an antenna. A part of this transmitted signal is reflected back by the material whose level is being measured inside the tank. This reflected signal is received back at the radar. The level of the contents of the tank is established by comparing the frequencies of the transmitted and received signals. An initial study on generating a low-frequency FMCW signal, sending it over a waveguide, and receiving it was done internally at Emerson using a Vector network analyzer (VNA) and found that an FMCW based GWR is feasible.

A wider bandwidth in the range of multiple gigahertz (GHz) is required in FMCW to improve the resolution of the targets and a frequency sweep starting from DC or very low frequencies is preferred to get the signed values of the targets and echoes in the echo curve. A traditional FMCW radar requires phase linearity in their transmitted signal. Maintaining phase linearity for the transmitted signal from near DC to multiple GHz is not possible. Therefore, a FMCW technology achieved by stepping in frequency is proposed in this thesis where the phase linearity of the transmitted signal is not necessary. This introduces two main constraints in the Stepped-FMCW GWR, namely the power consumption and measuring time. The objective of this thesis work was to setup and demonstrate a Stepped-FMCW GWR concept operating over DC to 4 GHz range and study its power consumption and measurement time.

The demonstration setup consist of two frequency synthesizers together covering the frequency range from 1 MHz to 4 GHz, radio frequency (RF) gain blocks, mixers, switches, variable attenuator, variable potentiometer, intermediate frequency (IF) filter, IF amplifier and a microcontroller (MCU). The measurement is done by generating a 10 kHz offset sweep for the entire frequency range along with the transmitted v sweep and sampling this offset IF signal for amplitude and phase response. The MCU controls the frequency synthesizers and other variable components via its serial and parallel input/ output (IO) interfaces, and samples the IF signal using its analog to digital converter (ADC) channel. The compatible digital signal processor (DSP) library functions are used for filtering and performing inverse fourier transform to generate the echo curve showing the targets.

The process proved the feasibility of developing a printed circuit based system for the stepped FMCW GWR. The results showed that the demonstrator is capable of measuring level at a good accuracy in a reasonable time. The power measurements were higher for the system. But, as the demonstrator is designed using evaluation kits, the power measured include additional power consumption of those components not used. Design of a dedicated custom board in future is expected to provide more accurate power measurements. Also, adding power control options for the RF gain blocks and mixers could optimize it further. Regarding the measurement time, there is room for further improvements by the choice of frequency range and the frequency synthesizer components.