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

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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.

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Figure 9. Simulated impact of lossless dielectric conformal coating on radar loop gain (RLG) at center frequency (60.5
GHz).
Figure 10. Simulated impact of conformal coating with εr=2 and εr=3 for different thicknesses.
3.4 Sensor underfill
Underfills can be used to fill the gap between the sensor package and PCB. The sensor underfill material
should be chosen to have as low dielectric constant and loss tangent as possible. Dielectric constant
determines the reflectivity of the medium and loss tangent indicates the dissipation effect of the material
and both are frequency dependent.

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4 Physical Integration
EM waves can interact with the objects and the media in which they travel. As EM waves propagate,
they can be reflected, refracted, or diffracted. These interactions cause the radar pulse to change the
direction and to reach the areas which would not be possible if the radar pulse travelled in a direct line.
This chapter provides the integration guidelines for simplified sensor cover scenarios. In any case, it is
important to carefully design and characterize the integration to ensure that the desired performance is
obtained. The radiation pattern presented in the A111 datasheet (developer.acconeer.com), shows the
sensor performance when integrated in free space.
4.1 Radome integration
A radome is a structural enclosure that protects a radar assembly. Radomes are generally made of
materials which are mainly characterized by their dielectric properties. Dielectric materials have a
characteristic impedance of


where is the dielectric constant relative to the free space and is the wave impedance in free-space.

= 377
When an EM wave hits a dielectric material at a normal incident angle, the reflection coefficient is
defined as

= 

The transmission coefficient is defined as T = 1 +. It is important to select dielectric materials for the
radome such that the reflection coefficient has a low value. Therefore, a low dielectric constant material
will minimize the reflections and reduce the impact on the radiation pattern and insertion loss. Typically,
radomes are made of plastic and the dielectric constant of common plastic (ABS, PC, Teflon, PP) is less
than 3, therefore, reflection coefficient for plastic materials is usually low. To further optimize the
radome integration, shape of the radome, distance to the radar and the thickness of the radome needs to
be considered in the design.
Figure 11 shows a scenario where a 1mm thick radome made of ABS plastic is placed at different
distances, D, from the radar. The amplitude variation of the reflected waves from the radar target can be
seen in the measured results shown in Figure 12. The variation of the amplitude is explained by the
standing wave phenomena. The transmitted wave (indicated as blue in Figure 13) travels in the air
medium until it reaches the second medium (plastic cover). Depending on the properties of the second
medium, part of the energy will be transmitted through the medium and part of it will be reflected. The
reflected pulse (indicated as yellow in Figure 13) will travel back to the PCB and be reflected again
(indicated as red in Figure 13). Depending on the distance D, the transmitted wave and the reflected
pulse can add up coherently or noncoherently. The maximum and minimum occur at multiples of the
half wavelength of the radar pulse. The impact of the pulse length is visible in the measurement results
(Figure 12). When the distance D gets larger than the pulse length, the standing wave disappears.

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Figure 11. Placement of the radome in relation to the sensor.
Figure 12. Measured reflected power from the target versus the radome to sensor distance for two different service
profiles, Profile 2 (MaxSNR) and Profile 1(MaxDepth). Amplitude is nomalized by the maximum value of Free Space.
Amplitude variation is stated in one direction (Tx or Rx side). For Radar Loop Gain (RLG) the values will be
doubled.
Figure 13. Illustration of the transmitted pulse and the reflected pulses (from plastic cover and the PCB).

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4.2 Radome thickness
When an EM wave travels in a dielectric material, its effective propagation speed and the wavelength
will change depending on the dielectric material:
 
,   
where is the dielectric constant of the material, is the speed of light in free space, i.e. 3×108
andis the free space wavelength (for A111).
Reflections happens at the air-dielectric interfaces and the thickness of the dielectric medium determines
the phase shift of the reflected wave from dielectric-air interface. Figure 14 shows a simplified model
of the transmission and reflections of the waves between air-dielectric. Reflections from the second
bounce and above are neglected in this model. When the wave impinges the air-dielectric interface, part
of it reflects () and part of it transmits (). The thickness of the dielectric material will cause the
wave to shift phase by:
 

where is the thickness of the dielectric. When the wave reaches the dielectric-air interface (),
another transmission () and reflections () happens. Notice that the wave will have
a sign change when the reflection happens,  . At the lower dielectric-air interface, the
amplitude of the wave is , where the wave has traveled a distance of .
If we assume a half-wavelength-thick radome, the round trip of the wave inside the radome will
introduce a 360-degree phase shift. Hence, the reflected wave into the air has a  amplitude.
In this case, the reflected waves  will cancel out. Therefore, to minimize the
reflections, the thickness of a single layer radome should be half-a-wavelength or a multiple of it.
 

Figure 14. Incident wave impinging the radome at the normal incident angle. Transmission and reflection of the wave
at the interfaces of two mediums.
As an example, a plastic sheet made of ABS with different thicknesses was selected for the test
measurement. If we assume dielectric constant of ABS to be 2.6 (the dielectric constant can vary
depending on the vendor), the thickness of the half-a-wavelength-thick radome is calculated to be

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

Figure 15 shows the amplitude variation of the reflected wave from the radar target when the distance
between the sensor and the radome is varied for different radome thicknesses. A thickness of 1.6 mm,
which is very close to the thickness of half-a-wavelength, has the minimum impact on the amplitude
variation. It is worth to mention that when the thickness is set to quarter-of-a-wavelength, the amplitude
variation becomes maximum. When the radome thickness is set to quarter-of-a-wavelength, the round
trip of the wave inside the radome leads to a 180° phaseshift.
Figure 15. Impact of the thickness of the radome (made of ABS) on the reflected amplitude variation. The amplitude
is normalized to maximum value of the Free Space (FS). Profile set to Profile 2. Amplitude variation is stated in one
direction (Tx or Rx side). For Radar Loop Gain (RLG) the values will be doubled.
If absolute measurements are required for a certain use case, it is advised that an additional offset is
added to the distance measurement. The reason for this offset is that the propagation delay caused by
the radome must be compensated. This additional offset is obtained by making reference measurements,
and it will also allow you to place the reference plane at the desired location for your product.
4.3 Radome distance
The reflected waves from the radome wall should be in-phase with the transmit waves, which leads to
the optimum distance of
 


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The optimum distance is valid if the thickness of the radome is optimum as well. Otherwise, to have a
minimum insertion loss on the received signal, the distance to the sensor should follow the marginal
criteria below:

 
In order to have zero insertion loss is determined to be 0.5 mm.

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4.4 Impact on the radiation pattern
The radar loop radiation pattern of the integrated antennas will be affected by the dome that is put on
top of the sensor. Figure 16 shows the measured radar loop pattern for three different materials, ABS
plastic, gorilla glass and free space.
Figure 16. Impact of the different materials on the radiation pattern, H-plane. Amplitude is stated in one direction
(Tx or Rx side). For Radar Loop Gain (RLG) the values will be doubled.
Figure 17 shows the radar radiation pattern when a radome made of ABS with 2mm thickness is placed
at distances corresponding to the harmonics of the quarter-of-a-wavelength of the radar pulse. The
reference case is FS (Free Space i.e. no cover). Since the radome thickness is not optimal, distances
which correspond to odd harmonics of the quarter-of-a-wavelength have the minimum impact on the
radiation pattern.

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Figure 17. Impact of the radome-to-sensor distance on the radiation pattern (H-plane). Amplitude is stated in one
direction (Tx or Rx side). For Radar Loop Gain (RLG) the values will be doubled.
It is not recommended to place the cover directly on the sensor. Figure 18 shows the radiation pattern
on the H-plane when the cover (ABS plastic sheet) is located on top of the sensor. In comparison with
Free Space, there is around 3 dB loss on both max. power and total radiated power for this case.
Figure 18. Radiation pattern on H-plane, cover placed directly on the sensor vs Free Space. Amplitude is stated in one
direction (Tx or Rx side). For Radar Loop Gain (RLG) the values will be doubled.

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4.5 Multi-layer radome integration
Radomes can also be constructed from a multi-layer dielectric. Particularly, where the thickness of a
single layer dielectric is fixed, thus additional layer can be added to the radome which can act as an
anti-reflection layer. Figure 19 shows the stack-up of a radome made of two dielectric layers and
illustrates the scattering of waves in this scenario. In this scenario, the reflections formed at the
interfaces of the different mediums can be cancelled if the condition below is satisfied:
󰇛󰇜 
The layer indicated with dielectric constant is the main layer and the layer with the dielectric
constant is the matching layer. Depending on the degree of the freedom, one can find the optimum
thickness or the optimum dielectric constant for the anti-reflection layer.
Figure 19. Illustration of the scattering waves for a two-layer radome.
Another approach is to use an EM simulation tool to find the characteristic of the anti-reflection layer.
For example, Figure 20 shows a sensor integration scenario where the main layer of the radome is made
of a special glass which has a high dielectric constant (~6-7). Therefore, the wave impedance mismatch
between the glass and the air becomes large. To reduce the mismatch, a matching layer with a lower
dielectric is applied between sensor and the screen. To find the optimumdielectric value of the matching
layer, the dielectric constant is varied in the simulation. It can be seen from the results shown in Figure
21 that the gain drops at least 2 dB in case 1, compared to case 2 where the matching layer has a dielectric
constant of 3.

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Figure 20. Simulation scenario: Sensor, air-gap, the matching layer and the screen.
Figure 21. Normalized antenna gain (Tx or Rx antenna gain) where the matching layer dielectric constant is set to 1.1
and 3.