Acconeer A111 User manual

Hardware and physical integration
guideline PCR Sensor A111
User Guide

Hardware and physical integration guideline PCR Sensor A111
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© 2022 by Acconeer –All rights reserved 2022-03-08
Hardware and physical integration guideline PCR Sensor
A111
User Guide
Author: Acconeer
Version 1.5: 2022-03-07
Acconeer AB

Hardware and physical integration guideline PCR Sensor A111
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Table of Contents
1 Introduction....................................................................................................................................4
1.1 Radar loop equation................................................................................................................4
1.2 Radar loop radiation pattern ...................................................................................................5
2 HW Integration - Schematics .........................................................................................................6
2.1 Power .....................................................................................................................................6
2.2 SPI Interface...........................................................................................................................7
3 PCB layout.....................................................................................................................................8
3.1 Sensor ground plane size........................................................................................................8
3.2 Impact of PCB routing and nearby components .....................................................................8
3.3 Impact of conformal coating.................................................................................................10
3.4 Sensor underfill ....................................................................................................................11
4 Physical Integration......................................................................................................................12
4.1 Radome integration ..............................................................................................................12
4.2 Radome thickness.................................................................................................................14
4.3 Radome distance...................................................................................................................15
4.4 Impact on the radiation pattern.............................................................................................17
4.5 Multi-layer radome integration.............................................................................................19
5 Physical Integration - Lens...........................................................................................................21
5.1 Focal distance.......................................................................................................................22
5.2 Radiation pattern ..................................................................................................................24
5.3 FZP Lens Design..................................................................................................................26
6 Appendix A: Materials .................................................................................................................29
7 References....................................................................................................................................30
8 Revision .......................................................................................................................................31
Disclaimer............................................................................................................................................32

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1 Introduction
This document aims to provide general guidelines for the hardware and physical integration of the
Acconeer A111 radar sensor. The A111 sensor is a fully integrated 60 GHz radar including the
transmitter and receiver antenna. The Tx/Rx antenna is a folded-dipole type and the E-plane and H-
plane of the antennas are indicated in Figure 1. The radiation pattern of the radar transceiver can be
found in the A111 datasheet.
The provided guidelines aim to optimize the radar sensor performance when integrating it into your host
product.
Figure 1. Sensor mounted on a printed circuit board (PCB). E-plane and H-plane are highlighted with blue and red
color, respectively.
1.1 Radar loop equation
Consider a signal transmitted through free space to a radar target located at distance Rfrom the radar.
Assume there are no obstructions between the radar and the radar target, and the signal propagates along
a straight line between the two. The channel model associated with this transmission is called a line-of-
sight (LOS) channel. For the LOS channel, the corresponding received reflected power from a radar
target, i.e. the signal to noise ratio (SNR), can be defined as

󰇛󰇜
Ris the distance of the radar to the target,is the radar loop gain, including both the transmitter and
receiver chain (two-ways signal path), is the Radar Cross Section (RCS) of the scattering object and
determines the reflected power of the object’s material. RCS depends on the roughness, size and shape
of the scattering object. Moreover, SNR depends on the sensor profile setting. A comprehensive
explanation of the sensor profiles can be found in Acconeer’s Radar Sensor Introduction.

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1.2 Radar loop radiation pattern
When characterizing the gain, we refer to the radar loop gain defined in the radar equation section.
Figure 2 shows the radar setup configuration for the radar loop radiation pattern measurement. The
reflector which in this case is a circular trihedral corner (radius of 3.8 cm) is located at the far-field
distance from the sensor (1 m). The far-field distance can be determined by the aperture of the sensor
and the radar target.
 
󰇛󰇜
where A is the largest dimension of either the sensor or the radar target. Envelope service can be used
to collect the reflected power at the fix distance from the radar target at different rotation angle. The
amplitude variation stated in the document shows one direction (TX or RX side). The radar loop gain
(RLG) is in fact twice the amplitude value. This is applicable for all the figures which state the amplitude
variation of the radome and the lens.
Figure 2. Measurement setup for radar loop radiation pattern.

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2 HW Integration - Schematics
2.1 Power
The A111 is powered by 1.8 V and all the control signals and the SPI interface are 1.8 V pins. It must
therefore be ensured that all host MCU pins connected to the A111 are at 1.8 V. If this is not the case,
level-shifters must be used in between the A111 and the host MCU.
As mentioned in the power consumption summary of the A111 datasheet, the A111 consumes
typically 66 µA when the ENABLE pin is set low. If the leakage current is to be even further reduced,
the power to the A111 must be switched off. It can be achieved either by using a low-leakage power
regulator with an enable/disable function (if 1.8 V is not available in the system) or a low-leakage
power switch in between the two 1.8 V domains. In both cases, a control signal, PMU_ENABLE, is
needed. See Figure 3 for details.
Figure 3. Block diagram of how to connect a Power Management Unit for controlling the 1.8V to the A111.
If the power to the A111 is switched off in between sweeps it is important that the control signals and
SPI interface are pulled low during this time, otherwise reverse leakage will occur via the ESD diodes
in the A111. If it is not possible to set the SPI interface in such a state (either via SW or by configuring
any level-shifters that might be used in the design), the problem can be solved by adding a power
switch only to VIO1 and VIO2. This way the leakage will be significantly lower than 66 µA, but the
control signals and SPI interface of A111 will still be supplied by 1.8 V and thus no reverse leakage
will occur. See Figure 4 for details. The Acconeer High Performance Module shows how to integrate a
power switch into the design, refer to the XM112 datasheet.

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Figure 4. An example of how to connect a power switch to reduce the leakage current when A111 is powered off.
2.2 SPI Interface
To optimize signal integrity of the SPI bus, it is recommended to route the SPI bus with as short as
possible trace lengths with an adjacent ground plane.

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3 PCB layout
This chapter describes means of optimizing the sensor performance by properly designing the printed
circuit board (PCB).
3.1 Sensor ground plane size
To maximize the radar loop gain and minimize impact on radiation pattern, it is recommended that the
top PCB layer is a filled copper layer with minimum amount of routing close to the sensor. Figure 5
shows the relative loss in RLG as a function of ground plane size, assuming a solid square ground plane
and the sensor placed at the center. As the ground plane size is increased, the RLG increases because of
increased antenna directivity. However, the RLG doesn't increase monotonically with ground plane size
due to constructive and destructive interference.
Figure 5. Simulated relative radar loop gain as a function of ground plane side length (x). Ground plane is a solid
square ground plane without routing.
In terms of regulatory compliance, any openings in the ground plane inside the A111 BGA footprint
must be significantly smaller than the wavelength of the radiation that is being blocked, to effectively
approximate an unbroken conducting surface.
3.2 Impact of PCB routing and nearby components
The radar sensor can be integrated with other components on the same layer if required in compact
designs. However, the radiation pattern is most sensitive to the ground plane area just outside the sensor.
Therefore, if maximum directivity is important, place other components on the opposite side of the
sensor or keep component count and routing to a minimum close to the sensor footprint. Vias should be
placed as close as possible to the pads to maximize the ground plane area as shown in Figure 6. It is also
recommended to minimize copper clearance for traces, vias and pads. The ground plane area inside the
x
x

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footprint will have a lesser impact on the directivity and therefore some vias and short traces are
preferably placed there while still satisfying regulatory compliance.
If the PCB assembly process allows, connect all ground pads without thermal reliefs as shown in Figure
6 a-b. Figure 7 shows the simulated relative RLG for the different routing examples in Figure 6. Omitting
thermal reliefs as in Figure 6a can increase the boresight RLG by approximately 2 dB, provided there
are no other interfering components or PCB traces close to the sensor. A small drop of approximately
0.5 dB is seen when placing decoupling capacitors (metric 1005) as shown in (Figure 6b).
Figure 6. A111 routing examples with vias placed close to sensor for maximizing ground plane size. (a)
Without GND thermal reliefs, (b) with decoupling capacitors and without GND thermal reliefs, (c) with
GND thermal reliefs. Trace to copper clearance is 0.127 mm (5 mil) and via pad diameter is 0.45 mm.
Figure 7. Relative RLG loss with and without A111 thermal reliefs.
a
b
c

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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 εr< 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.