The sensor developed aims at applications where the sensor is mounted very close – but contactless – to the target. In such case, neither FMCW nor pulse radar modes of operations are advantageous, since they would require very large bandwidths. On the other hand, in such a mounting situation only a single reflection is to be expected or – the other way around – the mounting situation has to be chosen in such way that (strong) multiple reflections are avoided. In this case, the distance to the target can be measured by using the continuous wave (CW) mode of operation. In this mode the radar is sending an unmodulated waveform and measures the phase difference between the receive and transmit signals, which depends on the target’s distance. Since the phase is ambiguous the measurement range is limited to half the wavelength. For example, in the case of a frequency of 77 GHz, this measurement range of lamda/2 = 1:95 mm might be located at 1cm distance to the sensor. The sensor’s center frequency is therefor a tradeoff between the measurement range, which decreases with frequency, and the precision, which increases. A very important point for the design is the available space for the sensor. For the whole unit a cutout of only 5mm x 30mm and a depth of 12 mm is sufficient. The LTCC frontend takes 1.4mm x 30mm x 12mm. The LTCC process has been chosen for its miniaturisation capabilities and robustness (against moisture etc..).
MMICs from UMS.