Frequency Modulation for Improved Radar Level Measurement

by | Nov 5, 2019 | Level, Measurement Instrumentation

Jim Cahill

Chief Blogger, Editor

For those that still listen to FM radio over the airwaves given our increasingly streaming digital world, you may know that the FM stands for “frequency modulation”. Information such as the music you hear on an FM music station is encoded in the radio signal through subtle changes in the frequency of the signal.

In a Flow Control magazine article, How frequency modulation can improve effectiveness of radar level measurement, Emerson’s Ingemar Serneby shares how this FM technology is applied to tank level measurement.

Flow Control: How frequency modulation can improve effectiveness of radar level measurementHe opens describing how radar level measurement works by taking:

…advantage of an electromagnetic wave’s ability to bounce off surfaces, including liquids. If it is possible to measure the time required for a signal to travel to the surface of the liquid, reflect and return to the device at the top of the tank, then it is possible to calculate the distance traveled and therefore level.

The two types of radar level measurements are guided wave radar (GWR) and non-contacting radar (NCR). A probe is used with GWR to send the radar pulse down to the liquid level.

Ingemar describes the two methods of measurement:

…pulse or frequency-modulated continuous wave (FMCW). GWR uses pulse, but the more focused signal provided by the wave guide creates different operating characteristics than NCR.

The transmitter for pulse radar:

…measures the time delay between the transmitted and received echo signal, and an on-board microprocessor then calculates the distance to the material’s surface and, consequently, the level measurement.

For FMCW, it:

…transmits a continuous signal from the antenna at the tank top, but with a constantly changing frequency… After the signal is reflected by the surface of the contents, the echo is picked up by the antenna. Since the transmitted signal constantly varies in frequency, the echo has a slightly different frequency compared to the signal being transmitted at that moment.

This use of frequency modulation allows the FMCW method to capture:

…its process variable information in the frequency domain, which supports more accurate signal conversion. Its signal processing can ignore the common interference sources.

Read the article for more why both pulse and FMCW measurement methods exist and applications where FMCW is the best choice. Visit the Level Measurement section on Emerson.com to learn more about GWR, NCR and other level measurement technologies best suited for your applications. You can also connect and interact with other level measurement experts in the Measurement Instrumentation group in the Emerson Exchange 365 community.

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