Hardware and physical integration guideline PCR Sensor A111
Page 10 of 32
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2022-03-08
Example PCB designs can be found on the Acconeer developer page [1].
For designs requiring larger components close to the sensor, a tapered shielding wall can be designed
as shown in Figure 8.
Figure 8. Tapered shield wall designed to prevent unwanted reflections from components placed close to the radar
sensor.
3.3 Impact of conformal coating
Conformal coating may be used to protect the sensor and other electronic components from
environmental factors such as moisture, dust, and chemicals. Conformal coatings are typically made of
polymeric materials with dielectric constants typically in the range ε
r
< 4. Depending on the coating
material properties and layer thickness, one can expect a drop in the sensor’s radar loop gain (RLG).
This gain drop is due to three factors;
1.
Offset in antenna resonance frequency as the antenna becomes electrically longer.
2.
Reflection and refraction loss due to the interface between the sensor and the coating.
3.
Dielectric loss in the coating material. This can usually be neglected for thin coating layers
(e.g. <200 μm).
Figure 9 shows the simulated relative radar loop gain (RLG) after a linear lossless (tan(
δ) = 0
)
isotropic coating has been added to all sides of the radar sensor. As the coating thickness increases
and/or the dielectric constant increases, the RLG decreases. The loss is however non-monotonic, and
this is due to constructive and destructive interference depending on the coating thickness.
The loss due to antenna resonance frequency offset can be seen in Figure 10 where the maximum gain
is shifted in frequency. This means that, as the radar signal is a wideband signal, the resulting loss in
SNR can be expected to be somewhat less than what is estimated in Figure 10.
If it is critical to obtain the maximum gain, it is recommended to use a thin coating layer (< 40 μm)
with low dielectric constant and loss factor. However, as many coating materials are not well
characterized at mm-wave frequencies it may be best analyzed by performing actual tests.
