Frequency Modulated Continuous Wave (FMCW)

A radar signal is given via an antenna, reflected on the measuring surface and received after a delay time t.

FMCW: Frequency Modulated Continuous Wave

The FMCW-radar uses a high frequency signal (~10 GHz) which transmit frequency increasing linearly to 1 GHz during the measurement (frequency sweep) (1). The signal is emitted, reflected on the measuring surface and received time-delayed (2). For further signal processing the difference D f
is calculated from the actual transmit frequency and the receive frequency (3). The difference is directly proportional to the distance i.e. a large frequency difference correspond to a large distance and vice versa.

The frequency difference is transformed via a Fourier transformation (FFT) into a frequency spectrum and then the distance is calculated from the spectrum. The level results from the difference between tank height and distance.
  


Linearity of frequency sweeps

The measuring accuracy of a FMCW radar is determined from the linearity of the frequency sweeps and their reproducibility. The linearity correction is deduced via reference measurement of the oscillator.

The non-linearity is corrected up to 98% (BM 700/BM 70 A).

An immediate frequency regulation is necessary with the BM 70 P device because of the higher demand on the measuring accuracy. With the PLL technology (Phase Locked Loop) the signal frequency is directly recorded as a digital data and the converter oscillator locks automatically on the right frequency.

As the transmit frequency is changed during a short time interval by FMCW radar the transmitted frequency has to follow the nominal frequency within micro seconds. This ensues via modern, fast electronic and processors.
  non-linear sweep   


                  linear sweep


Advantages of FMCW

Compared to the simple pulse radar procedure the use of FMCW radar offers the following advantages:

	Higher band width of the microwave signal - better reflection separation - reliable reduction of noise
	Higher transmitting frequency - small angle - fewer disturbing reflections
	Higher transmitting frequency - smaller antenna diameter for same measuring range