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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Hardware and physical integration guideline PCR Sensor
A111
User Guide
Author: Acconeer
Version 1.0: 2019-11-13
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 radiation pattern............................................................................................................ 5
2 HW Integration - Schematics .......................................................................................................... 6
2.1 Power....................................................................................................................................... 6
2.2 SPI Interface............................................................................................................................ 7
3 HW Integration - PCB..................................................................................................................... 8
3.1 Sensor ground plane................................................................................................................ 8
3.2 Sensor underfill ....................................................................................................................... 9
3.3 Integration with other components.......................................................................................... 9
3.4 A111 Decoupling capacitors ................................................................................................. 10
3.5 A111 Crystal.......................................................................................................................... 11
4 Physical Integration....................................................................................................................... 13
4.1 Radome integration ............................................................................................................... 13
4.2 Radome thickness.................................................................................................................. 15
4.3 Radome distance.................................................................................................................... 16
4.4 Impact on the radiation pattern.............................................................................................. 18
4.5 Multi-layer radome integration.............................................................................................. 20
5 Physical Integration - Lens............................................................................................................ 22
5.1 Focal distance........................................................................................................................ 23
5.2 Radiation pattern................................................................................................................... 25
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 [1]
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” [2].

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1.2 Radar 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 radiation pattern measurement. The reflector
which in this case is circular trihedral corner (radius of 5 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.
Figure 2 Measurement setup for sensor radiation pattern.

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2 HW Integration - Schematics
2.1 Power
The A111 is powered by 1.8V and all the control signals and the SPI interface are 1.8V pins. It must
therefore be ensured that all host MCU pins connected to the A111 are at 1.8V. 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 66uA 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 1.8V is not available in the system) or a low-leakage
power switch in between the two 1.8V 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 66uA, but the
control signals and SPI interface of A111 will still be supplied by 1.8V 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 [3].

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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 the performance and speed of the SPI interface, the A111 and the MCU should be placed
on the same PCB. If it is necessary to route the SPI interface via a cable, it is recommended that
differential routing is done for SPI_CLK, SPI_MOSI and SPI_MISO. This is enabled by using a
differential driver circuit as described in the XC122/XR122 EVK HW User Guide [4]. An example is
shown in Figure 5.
Figure 5 An example of how to use differential driver circuit if the SPI interface between A111 and Host MCU is
routed via cable.

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3 HW Integration - PCB
This chapter describes means of optimizing the sensor performance by properly designing the printed
circuit board (PCB).
3.1 Sensor ground plane
To optimize the sensor transceiver gain, the sensor must be shielded by a solid ground plane. The area
of the ground plane relates to the sensor transceiver gain as shown in Figure 6. It is recommended to
place the sensor at the center of the PCB to avoid any tilt on the main radiation beam of the antennas.
Figure 6 Simulated relative transceiver gain loss as a function of extended board ground plane area. The area is
quadratic.
In terms of regulatory compliance, any holes in the ground shield must be significantly smaller than the
wavelength of the radiation that is being blocked (5 mmfor the A111sensor), to effectively approximate
an unbroken conducting surface.
For manufacturing reasons, it is recommended to not flood the ground area in between the A111 ground
balls. It is recommended to connect the ground balls of A111 with individual traces. It is very important
to connect the ground below A111 to the Main ground plane surrounding the A111. An example of how
to properly connect the sensor ground plane is shown in Figure 7.

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Figure 7 An example of a properly connected ground plane below the A111 radar sensor.
3.2 Sensor underfill
Underfills are usually made of materials which are characterized by their dielectric properties. 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.
3.3 Integration with other components
It is optimal to have no electronic components on the same side of the PCB as the sensor is located. The
electronic components located on the same side of the PCB as the sensor will cause the EM waves to be
scattered. The component size, shape and material determine the amount of scattered energy from the
component. If it is necessary to locate a component on the same side as the sensor, the distance between
the sensor and the component must be taken into consideration.
Figure 8 illustrates different strategies for placing a component close to the sensor. This is important to
consider when detection range is set to close-range (distances < 150 mm), as reflections from the
components can cause coherent or non-coherent interference. It is recommended to place the object in
the green areas as highlighted in Figure 8. The highlighted yellow area is more sensitive tothe placement

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of an object. Optimum distance depends on the material, shape, size and orientation of the object.
Moreover, the height of the object should not exceed the sensor’s height.
Figure 8 Guideline for placement ofa component on the side of the sensor.
In case of having multiple components and objects nearby and if the radar performance is impacted by
the objects in proximity, it is recommended to shield the sensor by creating a conductive tapered wall
around the sensor. Figure 9 shows a model of such a shield can. The inner-walls of the can are tapered
and conductive coated, in order to minimize the unwanted reflections.
Figure 9 Tapered shield wall designed to prevent unwanted reflections from components placed close to the radar
sensor.
3.4 A111 Decoupling capacitors
Ideally, the decoupling capacitors should be placed on the opposite side of the PCB from the sensor,
but as close as possible to the via connecting the top and bottom layer. If manufacturing process
allows, the via can be placed directly on the A111 VIO ball. If that is not possible, the impedance
between the A111 VIO ball and the decoupling capacitor should be minimized. In practice, that means
that the via should be placed as close as possible to the VIO balls on A111 and the trace between the
A111 balls and the via, as well as the trace between the via and the decoupling capacitor, should be as
short as possible. In Figure 10 an example of how to place the via and decoupling capacitor is shown.
A four-layer PCB is shown in the example and the components and traces are highlighted in red.