AMS AS3992 Installation and operating instructions

Application Note
DN[Document ID]
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AS3992
Arnie –Hardware Description

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Content Guide
1Introduction .......................................................................................................................... 3
2Block Diagram...................................................................................................................... 4
2.1 Building Blocks..................................................................................................................... 4
3Supply Concept.................................................................................................................. 16
4Connectors / Interfaces......................................................................................................16
5PCB....................................................................................................................................17
5.1 PCP Specification .............................................................................................................. 17
5.2 Layer Stack........................................................................................................................17
5.3 Impedance Controlled Traces............................................................................................18
6Test Points .........................................................................................................................20
7Contact Information............................................................................................................21
8Copyrights & Disclaimer..................................................................................................... 22
9Revision Information ..........................................................................................................23

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1 Introduction
The Arnie demo reader is an application example for a long range Gen2 compliant UHF RFID
reader with dense reader mode capability. The reader can output up to 30 dBm of RF power at
two selectable antenna Ports intended to switch between polarization planes. The Arnie demo
reader features an antenna tuning circuit to reduce reflected RF power from antennas. This is
accomplished by tuning the output impedance of the reader system to the impedance of the
antenna. The antenna tuning can be done on different frequencies required for FHSS using a look-
up table. The Arnie demo reader can also adjust the antenna tuning during operation in case the
antenna environment changes. A power detector is used to control and limit the output power to
regulatory requirements. The default operating voltage of the Arnie demo reader is 6 Volt. Since
most of the Gen2 related processing is done on the AS3992 device the computational load is kept
at a minimum. The used MCU is a widely used 8051 controller using USB for host communication.
UART communication is possible by changing FW via the graphical user interface.
In this document the hardware of the UHF RFID Demo Reader: Arnie 2V4 is described.

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2 Block Diagram
MCU
C8051F340 Balun
0900BL18B100E
TCXO
DSA321SC
Loop
Filter
discrete
LDO
AS1364
LPF
LFCN-1000D+
Power
Amp.
SKY65111
Op Amp.
AS1710
Power
Detector Directional
Coupler
RCP890Q10
UHF RFID IC
AS3992
Tuner
PE64904
RF Switch
PE4259
LDO
AS1364
Balun
0900BL18B100E
Debug/
Progr.
Interface
General
Purpose
IOs
OAD
Pins
UART
Interface
LED
Diff. Tx
Ant. 1
Differential Rx
DAC Output
ADC Input
Inp.Cpl.
Iso. Out.
SPI
4
± D
USB
Interface
C2D/C2CK
IRQ
EN
Supply
VBUS 5V
Tx/Rx
Power
Supply
+6V
10
+ 6V
+ 6V
+ 5.3V
+ 4.7V
PI-Pad
Ant. 2
Arnie 2.4
AS3992
UHF RFID Gen2 Reader
Bias
2
Figure 1: Block Diagram
2.1 Building Blocks
This section briefly describes the various building blocks present on the PCB. For more details
please refer to the dedicated data sheets of the components.
UHF RFID IC –AS3992
The Gen2 compliant UHF RFID reader IC is the central
device on the Arnie demo reader as it handles most of
the Gen2 protocol related tasks and thereby
significantly freeing-up processing resources of the
MCU. At the same time AS3992 acts as RF front end
for transmission and reception covering modulation,
signal shaping, I/Q demodulation and decoding. The
location of the AS3992 device is shown in Error!
Reference source not found.
The supply voltage of the AS3992 device is 5.3 V which is down-regulated from the external 6 volt
power supply via the LDO device AS1364.
Figure 2: AS3992 Location

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On the Arnie demo reader the AS3992 device is configured to communicate with the MCU via the
SPI interface which complies with SPI mode 1. Beside the available parallel interface the SPI
interface is selected by setting pin IO0 to “low” and pin IO1 to “high” before AS3992 is enabled
(rising edge on pin EN).
For transmission of the RF signals the 0-dBm differential ports are used. The internal power
amplifier is not utilized in the Arnie demo reader design.
For reception the differential mixer inputs are used while the single-ended mixer input is left
floating.
The clock source of the AS3992 device is a 20 MHz TCXO in order to provide the frequency
stability and accuracy needed to comply with Gen2 operation.
MCU –C8051F240
For the Arnie demo reader a common 8 bit
microcontroller is used with 64 kB of flash memory and
4 kB of RAM in a TQFP package. The location of the
MCU device is shown in Error! Reference source not
found.
The MCU interacts with the AS3992 devices via the
SPI interface and with the host computer via the USB
interface.
The MCU is supplied with 5 V via the VSUB line of the USB interface. This way the MCUs power
supply is independent of the AS3992 device and programming of the boot-loader or a firmware
update can be done without connection the external power supply to the Arnie demo reader.
In order to provide a common interface voltage to the AS3992 device the MCU provides 3.3 V to
the pin VDD_IO of AS3992. 3.3 V is generated with the internal voltage regulator of the MCU.
Since the MCU and AS3992 have no common power supply a power up mechanism for the Arnie
demo reader was implemented to prevent unintended power-up of AS3992 via pin VDD_IO when
only USB is connected. The controller senses if the external power supply is connected and only
then the MCU puts 3.3 V to pin VDD_IO. As the current consumption of pin VDD_IO is low a direct
connection with a port pin is possible.
Figure 3: MCU

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Boot Loader Programming
To program the boot loader to the demo board the debug adapter from Silabs must be used (see
Error! Reference source not found.). Boot-loader programming is done via the C2 interface of
the MCU for which pads are provided on the top side of the PCB
Figure 4: Silabs USB Debug Adapter
Additional an interface cable is required to connect the Silabs debug adapter with the demo board.
The connection plan of the interface cable is shown in Error! Reference source not found. below:
Figure 5: USB Debug Adapter - Interface Cable
The interface cable should be connected to the demo board as shown in Error! Reference source
not found. below:

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Figure 6: Controller C2 Interface for Boot Loader Programming
C2CK → J12
C2D → J20
GND → J21
Follow the steps below to program the boot loader with the flash programming utility:
1 Connect the USB cable to the Arnie demo reader to supply the controller.
2 The Silabs debug adapter should be connected via USB interface to the host computer.
3 Start the IDE software .
4 In the Options menu (see Error! Reference source not found.) of the IDE software select
Connection Options.
5 Serial Adapter should be set to USB Debug Adapter and the Debug Interface should be
set to C2.
6 Press OK.

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Figure 7: Silabs IDE - Connection Settings
7 Establish a connection between the Silabs debug adapter and the Arnie demo reader
using the interface cable (see above for details).
8 Click Connect in the Debug menu.
9 Press Download Object File (see Error! Reference source not found.).
10 Select the boot loader file programmer.hex.
11 Click on Download.
Figure 8: Downloading the boot loader
12 When the download process is finished click on auf Disconnect in the Debug menu.

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Balun
To transform the 100 ohm differential transmit signal
of the AS3992 device to a single-ended signal with 50
ohm a 2:1 LTCC balun is used. On the receiving side
a second balun is used to convert from 50 ohm to 100
ohm impedance. The locations of the Baluns are
shown in Error! Reference source not found..
The insertion loss of this balun type is 1 dB (max).
Power Amplifier –Sky65111
To amplify the transmission signal output from
AS3992 up to 30 dBm an external 3-stage power
amplifier is used. The location of the power amplifier
is shown in Error! Reference source not found..The
PA is supply with 4.7 V via a voltage regulator. The
power amplifier has two bias voltage inputs. The bias
for the first two amplifier stages (VAPC1) is held at a
fixed voltage of 2.6 V via a voltage divider. The second
bias voltage input (VAPC2) is controlled from the DAC
output of AS3992. The DAC output is buffered with a
OP-Amp (AS1710).
Low Pass Filter
To attenuate the higher-order harmonics of the
amplified transmit signal a LTCC low pass filter is
used. The location of the low pass filter is shown in
Error! Reference source not found..The part has a
typical insertion loss of 0.7 dB at 900 MHz and should
provide typically 30 dB of attenuation.
Directional Coupler
Since the transmit signal is on even during reception
good isolation between Tx and Rx is required. If
isolation is poor the sensitivity of the reader system is
severely degraded. The used coupler used with the
Arnie demo reader has a coupling of 10 dB and
provides excellent isolation and hence directivity. The
location of the directional coupler is shown in Error!
Figure 9: Balun Locations
Figure 10: Power Amplifier Location
Figure 11: Low Pass Filter Location
Figure 12: Directional Coupler

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Reference source not found.. The insertion loss is stated with 0.3 dB (min).
Attenuator - PI-Pad
A PI-pad attenuator structures is present on the Arnie
demo reader in order to limit the input power for the
power detector. The location of the PI-Pad is shown in
Error! Reference source not found.. The maximum
input power for the power detector is 20 dBm. With 10
dB of coupling loss it still might be that the input power
is too high if the PA outputs more than 30 dBm.
Therefore the attenuation of the PI-pad is 3 dB to stay
below the limit.
Calculation:
  
  
  
    
   
        
 󰇡
󰇢󰇡 󰇢 󰇡 󰇢󰇡  󰇢   17.8 Ohm
Figure 13: Pl-Pad Attenuator Location
RL
RsRs

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Power Detector
To control and limit the output power that the Arnie
demo reader outputs at the antenna ports a power
detector is used. The power detector is located at the
coupled port of the directional coupler as can be seen
in Error! Reference source not found.. At the input
of the power detector a PI-Pad attenuator is placed in
order to limit the input power seen by the power
detector. Depending on the RF power at the detector
input a DC voltage is generated. This DC voltage is
routed to the ADC input of the AS3992 device. Based on the DC voltage from the power detector
the bias voltage for the third amplifier stage of the PA is controlled via the DAC output of AS3992.
A lookup table is used to choose the correct bias voltage. Note: Loading a custom lookup table in
a CSV format is possible using the GUI.
Tuner
The tuner is one of the most important building blocks
in the Arnie demo reader design. This tuning circuit is
used to compensate reflections of transmit power at
the reader antenna. The location of the tuner is shown
in Error! Reference source not found.. In an ideal
case none of the transmit power which is fed to the
antenna would be reflected. In reality every antenna
reflects some amount of power back into the system.
Reason for this effect may be changes in the vicinity of
the antenna causing a mismatch in the antenna impedance. For instance, by placing objects near
the antenna the impedance of the antenna is changed. The effect of impedance change depends
on the material properties of the object. In a 50 ohm system, an impedance which is different from
50 ohm increases the reflection coefficient (S11). Often antenna design constraints (size, material,
etc…) also result in a considerable high reflection coefficient (S11). In RFID this reflected power is
called self-jammer and couples into the receiving part of the reader, limiting its sensitivity. The
tuning circuit can compensate a mismatch at the antenna utilizing AS3992s unique features .
Figure 14: Power Detector Location
Figure 15: Turner Location

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Tuning Circuit
The core of the tuning circuit consists of three digital tuneable capacitors (DTC) C94, C95 and
C96 (PE64904
1
) which capacitance value can range from 1.05 pF up to 5.1 pF with a step value
of 131 fF. The DTCs can be controlled via a SPI interface. Since the AS3992 also has a SPI
interface the whole reader can operate with one microcontroller.
Additional lumped elements (capacitors and inductors) are used to set the center impedance of
the tuning circuit around 50 ohms and to set the tuning range.
Figure 16 - Tuner Circuit Principle shows a simplified representation of the tuner circuit with its
underlying π-network structure.
Figure 16 - Tuner Circuit Principle
Tuning Range
Error! Reference source not found. shows the impedance range of the tuning circuit. Every
curve represents a frequency sweep from 800 MHz to 1 GHz.
The default impedance (green curve) of the tuning circuit was set with the lumped elements to run
through the 50 ohm point while the DTCs are at their center value (15). The other curves
demonstrate permutations of having the three DTCs set to their minimum, mid and maximum
value. This way the span of the tuning range can be shown without measuring brute force all of
the 32768
2
curves. Note that the overall impedance of the tuning circuit is sensitive to component
variations of the fixed components but this poses no real problem as the DTCs can compensate
these variations.
1
Manufacturer: Peregrine
2
Three DTC with each 5 bits = (25)3 = 32768 steps
C90

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Figure 17: Turning Range
RF (Antenna) –Switch
The Arnie demo reader offers the possibility to
connect two antennas. The RF switch is a SPDT type
of switch with two control lines which are driven by the
MCU complementary. The location of the RF switch is
shown in Error! Reference source not found.. The
switch has a typical insertion loss of 0.35 dB.
LDO –AS3992 Supply
Figure 18: Antenna Switch Location

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The voltage regulator AS1364 (U1) is used to power
the AS3992 device and the OP- Amp. AS1710. The
output voltage of the voltage regulator is adjustable
and is set to 5.3 V by an external voltage divider. The
location of the LDO is shown in Error! Reference
source not found..
Voltage Divider Calculation:
  
 
 
         
    
       
LDO –Power Amplifier Supply
The voltage regulator AS1364 (U2) is used to supply
the power amplifier and provide the bias reference
voltage for the PA. The output voltage of the voltage
regulator is adjustable and is set to 4.7 V by an
external voltage divider. The location of the LDO is
shown in Error! Reference source not found..
Figure 19: AS3992 Supply LDO Location
Figure 20: Power Amplifier Supply LDO
Location

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Voltage Divider Calculation:
  


         
    
      
OP-Amp (Power Supply Bias Buffer)
The OP-Amp AS1710 (U10) is used to buffer the DAC
voltage output. The output of the OP-Amplifier is
routed to the bias voltage input of the third amplifier
stage of the PA. The location of the OP-Amplifier is
shown in Error! Reference source not found..
TCXO
The TCXO (U4) provides the clock for the AS3992
device. The location of the TCXO is shown in Error!
Reference source not found.. The TCXO outputs a
clipped sine wave signal with 20 MHz. The 3.3V supply
for the TCXO is generated by the internal voltage
regulator of the MCU.
Figure 21: OP Amplifier Location
Figure 22: TCXO Location

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3 Supply Concept
MCU
C8051F340 TCXO
DSA321SC
LDO
AS1364
Power Amp.
SKY65111
Op Amp.
AS1710
Power
Detector
UHF RFID IC
AS3992
Tuner
PE64904
LDO
AS1364
LED
USB
Interface
Power
Supply
+6V
+ 6V
Arnie 2.4
AS3992
UHF RFID Gen2 Reader
4.7 V5.3 V
3.3 V
3.3V
3.3V
Figure 23: Power Supply Concept
4 Connectors / Interfaces
The Arnie demo reader features a USB interface and a UART for communication with the host
system. The USB interface uses a USB Mini A/B connector. Connection to the UART interface is
possible via pin headers and requires a firmware update.
For the 6V Power supply of the Arnie demo reader a low voltage DC socket is used. Outer diameter
of the plug should be 6 mm and the inner contact diameter is 1.95 mm.
The antenna connectors are 50 ohm SMA receptacles. Switching between two antennas is
intended to switch between horizontal and vertical polarization planes.

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5 PCB
5.1 PCP Specification
PCB Dimension: 61.8 mm x 41.3 mm
PCB Thickness: 1.88 mm
PCB Material: FR4
PCB Material Specification:
http://www.piu-printex.at/printex/multilayer%20de-104ml-1105d.pdf
5.2 Layer Stack
The PCB of the Arnie demo reader has 6 layers. In Error! Reference source not found. the exact
PCB stack up can be seen. It is important to keep the thickness between TOP and INTERNAL
PLANE 1 as this thickness influences the impedance of the coplanar waveguides. All other
thicknesses can be adjusted to requirements of the PCB manufacturer.
Layer 1: TOP LAYER
Layer 2: INTERNAL PLANE 1
Layer 3: MID-LAYER 1
Layer 4: INTERNAL PLANE 2
Layer 5: MID-LAYER 2
Layer 6: BOTTOM LAYER
Figure 24: Layer Stack

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5.3 Impedance Controlled Traces
To minimize reflections and hence losses, the impedance of the single- ended coplanar
waveguides are matched to 50 ohms. Since transmitting and receiving at the AS3992 device is
done differential a differential coplanar waveguide is required which is matched to 100 ohms. In
section 0 and Error! Reference source not found. the dimensions and the location of the
coplanar waveguides types are defined.
Singled-Ended Coplanar Waveguides
Figure 25: Sinled - Ended Coplanar Waveguide Dimensions
Figure 26: Single - Ended coplanar Waveguides
Figure 27: Differential Coplanar Waveguide

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Figure 28: Differential Coplanar Waveguide Dimensions

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6 Test Points
A number of test pads are available on the Arnie demo reader to gain access to the various
voltages and important lines for measurement.
Voltages:
5V3: J17
4V7: J19
3V3_MCU: J22
GND: J10
GND_DIG: J21
Vapc2: J8
ADC_In: J7
Signals Tx/Rx:
OAD: J9
OAD2: J11
SPI Signals:
MISO: J24
SDAT: J23
SCLK: J16
SEN_TUNE1: J26
SEN_TUNE2: J18
SEN_TUNE3: J27
AS3992 Signals:
IRQ: J25
EN_AS3992: J28
MCU Programming Interface:
C2CK: J12
C2D: J20