Atmel AT08973: SAMR21 Basic Connections
Contents
1. e Jamweeaee ane eee ene eer ee ee ee ee ere eee 11 Bee WOW CN CAG EE 11 2 2 6 Current Measurement Ghunt cess eeeaeeeeseecessecesseeesseeeeseecessesetsaeessneeeseeesanes 11 2 2 7 AMb Serial Flash Memonm 11 3 Printed Circuit Eat CC Ce Le E 12 3 1 Stack up and Material cece 12 L OUNO ca fo VE 12 a H 14 329 2 6 0 E EE 15 E SE UMN E 16 22S al gt e BAe enn meee ee ee eee ne ee ee ee ee eee ene ee 16 320 CICA ASS OMY EE 16 Ae E CHE 17 4 1 Sample Group EE 17 Ae RS le Di Un ME 17 4 3 Receiver Testing EE 19 AA intentional ne D E ER 4 5 Unintentional EMISSIONS cccccccceccc cece eeee ee ee eee cee ee eee se eee eens ae ees a eee ae eeG A ees a ees Geese eesa SAGA a eesa Seta Sessa eeaa Seta Seta eeaa EEE 23 Medel Ee EA ne EE 23 Aoa ee CN 24 4 5 3 High Frequency EMISSIONS EE 24 3 O50 0 ei EL 0 EE 27 RT CO E 27 LE PROVISION TRISCONY E 28 SAMR21 Basic Connections and Wireless Design APPLICATION NOTE Atmel Atmel 42391A SAMR21 Basic Connections and Wireless Design ApplicationNote_012015 1 1 1 2 Overview Description This application note illustrates the basic circuitry needed to use the Atmel ATSAMR21G 48 Pin QFN SAMR21 The SAMR21 is a System in Package SIP device that includes a 2 4GHz IEEE 802 15 4 Transceiver and an ARM Cortex M0 microcontroller To archive satisfactory performance designers need to adhere to good RF design practices This reference design sho2. can be used to guide the placement of the automated via stitching tool in this CAD tool 3 2 5 QA Testing To verify the microstrip a test coupon was generated The test coupon is built along with the PCB so it has identical properties The fabrication house can verify the impedance of the test coupon using a Time Domain Reflectometer TDR Because the Test Coupon and Microstrip transmission line are identical they will have the same impedance This final QA step insures the design plan has been successful we will get maximum efficiency from the RF transmission line and consistent lot to lot yield 3 2 6 Clean Assembly Cleanliness counts The ATSAMR21 is an ultra low power wireless device lonic contamination and debris on PCBAs can rob precious battery power and de tune the RF signal path No Clean assembly processes will give the best results If water soluble flux is used double washing in de ionized water and twenty four hour bake under heat lamps is recommended 16 SAMR21 Basic Connections and Wireless Design APPLICATION NOTE Atmel Atmel 42391A SAMR21 Basic Connections and Wireless Design ApplicationNote_012015 4 1 4 2 Results Sample Group The sample group is shown in Figure 4 1 The project name is Ramen a popular food and an anagram of ATSAMR2 1 Individual PCBAs were identified with sea creature stickers Testing includes Rx Sensitivity and TX emissions testing Figure 4 1 Sample Lot of Ramen Assemblies Read
3. 2 This gives a more consistent dielectric thickness than prepreg Prepreg is used between Layer 2 and Layer 3 as filler for compliance The stack up is shown in Figure 3 1 Figure 3 1 Ramen PCB Stack up Layer Stack Manager 8 T Layer Name Bi Material Thickness mil Dtk Kees Top Solder Solder Mask Co Surface Material Fullback mil Top Layer Signal Copper Dielectricl Dielectric Core Internal Plane Copper Dielectrics Dielectric Prepreg DGOND SV3 Signal Copper Dielectric Dielectric 1 Bottom Layer Signal Copper 14 a o y O E 6Boattem Solder Solder Mask Co Surface Material 4 Solder Resist 4 pees ee Total Thickness 61 4mil Add Layer e Delete Layer ove Up ove Down Drill Pairs Impedance Calculation 3 2 Grounding Plan The layer stack up was assigned to provide a continuous RF ground plane and keep the multiple ground domains organized in a straight forward manner Close review of the ATSAMR21 datasheet Section 5 2 3 2 4 ISM Band Transceiver Connections reveals several different ground domains and how they are internally connected The challenge is to manage the ground domains on the PCB rationally 12 SAMR21 Basic Connections and Wireless Design APPLICATION NOTE Atmel Atmel 42391A SAMR21 Basic Connections and Wireless Design Appli
4. Elements Area is the minimum recommended layout size Reducing the ground plane on RF designs has many risks reduced receiver efficiency from loss of area de tuning of antenna match and increased influence of random parasitic reactance These problems can reduce radio link range and reduce yield in mass production Chip antennas are often suggested as a compact alternative to PCB antennas but critical evaluation of datasheets reveals the ground planes used to characterize these components are often quite large To reduce size designers can add components to the back side of the required elements area as long the gap between the antenna and the ground plane is kept clear For those that want to expand the ground plane the lateral dimension X of the RF ground plane should be preserved additional ground planes to the sides should be isolated with slots This isolates the resonant structure and allows designers to expand their base band circuitry with minimal interferences Figure 2 3 Expanded Ground plane with Slot Partition RF Test Point RFTP1 is an RF test point This is used to test the RF signal using a coaxial probe This is very useful for engineering characterization regulatory testing and mass production testing with ATE The test point is a Switch Connector type when the probe is inserted the RF signal is routed to the probe When the probe is removed the signal passes through the component to the PCB antenna The test point ha
5. and will not be used in connection with any applications where the failure of such products would reasonably be expected to result in significant personal injury or death Safety Critical Applications without an Atmel officer s specific written consent Safety Critical Applications include without limitation life support devices and systems equipment or systems for the operation of nuclear facilities and weapons systems Atmel products are not designed nor intended for use in military or aerospace applications or environments unless specifically designated by Atmel as military grade Atmel products are not designed nor intended for use in automotive applications unless specifically designated by Atmel as automotive grade
6. bottom layers Copper pour is a popular design technique used to reduce EMI and it also gives a good solid ground connection to the RF components on the top layer We want to do all this without confusing the layout tool and suffering a barrage of DRC Errors Table 3 1 Stack up Ground Plan Layer Name Ground domains allowed The layout plan was developed to keep the signals rationally organized In the PCB design three ground domains were assigned RFGND AGND and DGND RFGND was used for the primary return path for RF signals RFGND was implemented with a full Ground Plane on Layer 2 AGND is used for the local oscillators and analog systems on the Transceiver and CPU AGND was implemented on Layer 1 with traces DGND is the digital return path for baseband communication busses and digital logic DGND was placed on Layer 3 as a mixed plane with power traces In this way each ground domain is compartmentalized to a single layer NET TIES To join the ground domains at the IC all three domains flow to the Paddle under the SAMR21 then NET TIES are used on the top layer to join the various ground pins of the SAMR21 to the Paddle NET TIEs are metal shoring links that directly connect the pad to the paddle More information on NET TIES is available in AT11309 Advanced Layout Techniques for Wireless PCBs SAMR21 Basic Connections and Wireless Design APPLICATION NOTE Atmel Atmel 42391A SAMR21 Basic Connections and Wireless Design Appl
7. e N wc Oo S lt XPRO CHIP STRONGER SIGNAL gt Figure 4 6 Testing Rx Sensitivity Azimuth 0 DUT Vertical Atmel SAMR21 Basic Connections and Wireless Design APPLICATION NOTE 21 Atmel 42391A SAMR21 Basic Connections and Wireless Design ApplicationNote_012015 Table 4 2 Rx Sensitivity DUT Vertical AZIMUTH SEA TURTLE V ANGEL FISH V XPRO WHIP H XPRO CHIP V il Figure 4 7 Graph of Rx Sensitivity DUT Vertical RSSI PCBA Vertical 135 180 225 270 315 360 SEA TURTLE E ANGEL FISH XPRO WHIP gt XPRO CHIP A lt v D Lu O Z O D ke OI 4 4 Intentional Emissions Lab testing included conducted power measurements of in band emissions The FCC prefers conducted measurements and once again the RFTP came in handy 22 SAMR21 Basic Connections and Wireless Design APPLICATION NOTE Atmel Atmel 42391A SAMR21 Basic Connections and Wireless Design ApplicationNote_012015 Figure 4 8 Conducted Testing of In Band Emissions Table 4 3 Conducted Tx Power in dBm 2405MHz 2440M Hz 2480M Hz These measurements were taken at maximum transmit power settings They correct for the insertion loss of the RF Probe and cable The ramen boards are showing healthy Tx power The SAMR21 XPRO includes an additional AS222 92 RF switch for diversity The insertion loss of the switch is 0 5dB Even with this factored in Ramen is looking better than the XPRO showing the benefits of microstrip
8. ground domains Star Ground techniques are desirable to reduce common impedance coupling of electro Magnetic Interference EMI and baseband crosstalk These objectives conflict with normal CAD practice Another exception is management of copper pouring on top and bottom layers There is some debate of the usefulness of copper pour with microstrip however if this technique is desired some basic guidelines need to be followed Effective techniques to manage these exceptions are outlined in this paper Atmel SAMR21 Basic Connections and Wireless Design APPLICATION NOTE 3 Atmel 42391A SAMR21 Basic Connections and Wireless Design ApplicationNote_012015 1 3 Design Partitions This design has two sections the required elements and the recommended accessories These have been partitioned into two schematic sheets and separate areas in the PCB layout Refer to the schematics in the RAMEN_DOCS zip design folder The RF section includes a 500 unbalanced microstrip transmission line and a PCB antenna Figure 1 2 Ramen Layout IC Je ake ite aCe LO Doo 60 ols The PCB uses a four layer stack up to implement controlled impedance and a low impedance ground return for the antenna system The ground plane of the required section is the correct size and orientation to counterpoise the PCB antenna The required section also includes programming and UART headers These two interfaces are necessary for debug and mass production with ATE The acces
9. 4 13 15 1 Horizont e ei Ai ag DL S Aly Gel 400 210 p Meas Dist 3m 100 Spec Dist 3m Uu k 100010 130000 Filename c program files x20 emisoft vasonawesults AIMEL 1 12GHz GH t em Table 4 7 Spurious Emissions Tx Full Power on CH18 2440MHz Frequency imi i Result Atmel SAMR21 Basic Connections and Wireless Design APPLICATION NOTE 25 Atmel 42391A SAMR21 Basic Connections and Wireless Design ApplicationNote_012015 26 Figure 4 11 Spurious Emissions Tx Full Power on CH18 2440MHz Ch TUV Rheinland of North America Saal a10 Jerta Av Lmt rag 510 1g H 410 10 j Meas Dist 3m Vo Spec Dist 3m Frequency MHz UU 10000 180000 ATMEL Zigbee radio Module Ramen Blue Fish TX On 2440 Max Power Setting Filename c program files x88 emisoft vasonalresults ATMEL 1 18GHz CHT8 emi Table 4 8 Spurious Emissions Tx Full Power on CH25 2475MHz DUT Frequency Level Limit Margin Result Figure 4 12 Spurious Emissions Tx Full Power on CH25 2475MHz il TUV Rheinland of North America 28 Oct 14 13 42 i m0 srties DN Lmt i A Formal 1g aa Gi 40 R 0 em Meas Dist 3m 10 0 Spec Dist 3m Frequency MHz UO 1000 0 100000 150010 ATMEL EN E Blue Fish TX On 2475Max Power CZE Filename CG files c86 emisoft vasonalesults ATMEL 1 18GHz CH25 The results above are a great looking FCC pre scan This radio should easily pass FCC certification It is r
10. ATSAMR21 User s manual 2 AT0O2865 RF Layout with Microstrip 3 AVR2067 Crystal Characterization for AVR RF 4 AT10216 Transmitter Test with Remote Start 5 Simulations done with Agilent AppCAD 6 AT11309 Advanced Layout Techniques for Wireless PCBs Atmel SAMR21 Basic Connections and Wireless Design APPLICATION NOTE 27 Atmel 42391A SAMR21 Basic Connections and Wireless Design ApplicationNote_012015 T Revision History Doc Rev Date Comments 42391A 01 2015 Initial document release 28 SAMR21 Basic Connections and Wireless Design APPLICATION NOTE Atmel Atmel 42391A SAMR21 Basic Connections and Wireless Design ApplicationNote_012015 CONNECTED Altmel Enabling Unlimited Possibilities fiy fin w Atmel Corporation 1600 Technology Drive San Jose CA 95110 USA T 1 408 441 0311 F 1 408 436 4200 www atmel com 2015 Atmel Corporation Rev Atmel 42391A SAMR21 Basic Connections and Wireless Design ApplicationNote_012015 Atmel Atmel logo and combinations thereof Enabling Unlimited Possibilities and others are registered trademarks or trademarks of Atmel Corporation in U S and other countries ARM ARM Connected logo Cortex and others are the registered trademarks or trademarks of ARM Ltd Other terms and product names may be trademarks of others DISCLAIMER The information in this document is provided in connection with Atmel products No license express or implied by estoppel or o
11. Atmel APPLICATION NOTE AT08973 SAMR21 Basic Connections and Wireless Design SAMR21 Introduction This reference design showcases ATSAMR21G Wireless System in Package and several design techniques for RF PCB construction controlled impedance PCB antenna and interface microstrip layout ground plane segregation star grounding conducted RF test point and several other features that are good practice for RF layout Features e Basic Connections for SAMR21 e RF Layout for SAMR21 e Microstrip e PCB Antenna e Ground plane System e Reference design and analysis e Schematics and BOM e Altium design database e FCC Pre scan data Atmel 42391A SAMR21 Basic Connections and Wireless Design ApplicationNote_012015 Atmel SMART Table of Contents T e e 3 E We ed e E 3 ka e ele e 3 e NOS SIOMT Fr AION E 4 2 dl TENG de EE 5 ZN WRC SG Elements EE 5 2 1 2 Antenna System 8 ce 5 2 193 Adj sting the Een Le e TEE 6 2 1 4 RF Test POU E 6 AlS PI TUNNI e T 2 1 6 Transmseonl me 8 2k AMUN EE 8 219 EE Gene ee eee ene ee ee eee eee EE 8 2 1 9 Power Rails and Bypass Capacitors 9 2 1 10 External Power Connections ccccccccccceececececeececeeeeeceeceeeaceseaeeeseeeeseueessaeessaeeeseusessaeessneessuecesneessaes 9 2 1 11 Programming and UART Headers ertt atttarttartrnrrrarEEerEn etrn a reeet rea 9 2 2 Recommended Accessories Area 10 222 User Demed BURON EE 11 223 UserDeined RE 11 2 2 4 User Defined
12. TION NOTE Atmel Atmel 42391A SAMR21 Basic Connections and Wireless Design ApplicationNote_012015 2 2 2 User Defined Button This button is provided for general use or binding stimulus The button is active low and connected to PAO6 2 2 3 User Defined LEDs These three green lamps are provided for general use The connections are shown in Table 2 3 Table 2 3 User Defined Lamps SIGNAL 2 2 4 User Defined GPIO The four GPIO headers are detailed in Table 2 4 By convention pin 1 is ground Table 2 4 GPIO Header Pin Assignments PIN SIGNAL PIN SIGNAL PIN SIGNAL PIN SIGNAL PA18 SPI_MOSI PA08 I2C_ SCA PA27 PB22 PA19 SPI_SCLK PA09 I2C_SCL PA28 PB23 2 2 5 Power Header Header J3 is provided as an auxiliary power input If users remove the batteries they may power the board using this jack Alternatively they can draw battery power from this header to power off board circuitry 2 2 6 Current Measurement Shunt Header J4 is provided to probe the current shunt The shunt allows developers to measure fast transient currents with an oscilloscope This is a low side shunt that can be connected directly to earth grounded lab equipment For example line current and conducted RF signals can be measured simultaneously while using grounded ICE programmer Pin 1 should be grounded Pin 2 is used for measurement A positive current through the SAMR21 will produce a negative voltage across this shunt For critical measurements of low powe
13. a pattern and radiated emissions to get a preliminary indication of FCC compliance This needs to be done in an anechoic chamber using the Remote Start Transmitter Test TT The sensitivity measurements compare the Ramen boards vs the ATSAMR21 XPRO board The same source was used for all measurements The source was an SAMR21 XPRO board using the chip antenna horizontally polarized and the transmitter power level set to 12dB These measurements are Relative Signal Strength RSSI observed by the DUT Numbers closer to 0 are better The Ramen boards are on par with the control Angel Fish shows best in class reception at 62dBm Note The XPRO Whip antenna and Chip antennas are orthogonal This contributes to the relatively low performance of the Whip with the DUT in the horizontal mode Atmel SAMR21 Basic Connections and Wireless Design APPLICATION NOTE 19 Atmel 42391A SAMR21 Basic Connections and Wireless Design ApplicationNote_012015 Figure 4 4 Testing Rx Sensitivity Azimuth 0 DUT Horizontal Table 4 1 Rx Sensitivity DUT Horizontal AZIMUTH SEA TURTLE H ANGEL FISH H XPRO WHIP V XPRO CHIP H 20 SAMR21 Basic Connections and Wireless Design APPLICATION NOTE Atmel Atmel 42391A SAMR21 Basic Connections and Wireless Design ApplicationNote_012015 Figure 4 5 Graph of Rx Sensitivity DUT Horizontal RSSI PCBA Horizontal 45 90 135 180 225 270 315 360 SEA TURTLE AN a E ANGEL FISH WE as gt XPRO WHIP 0 SS
14. and efficient RF ground return layout 4 5 Unintentional Emissions 4 5 1 Sub GHz Emissions These tests are part of the standard FCC part B and C protocols The Ramen boards were tested with the transmitter at full power on CH18 2440MHz They passed with wide margin This is the benefit of the ground plane structure and via stitching SAMR21 Basic Connections and Wireless Design APPLICATION NOTE 23 Atmel 42391A SAMR21 Basic Connections and Wireless Design ApplicationNote_012015 4 5 2 4 5 3 24 Table 4 4 Sub GHz Spurious Emissions DUT Frequency MHz Level dBuV Limit dBuv Margin dBpV Figure 4 9 Sub GHz Spectrum with Very Low Emissions Si TUV Rheinland of North America 28 Oct 14 08 32 fi Horizont R Re DIV Trace MACH 0 055 i Ref ag im Att 10d VEw 200kKHz RBw 120kKHz e Che Keng Spec Dist Am Frequency MHz UU ATMEL igbee radio Fish TX ON at 2440MHz EUT Vertical a Filename c program files x26 emisoft vasonaresultsiatmel 2440 TX oh em Band Edge In the U S the 2 4GHz ISM band is surrounded by two restricted bands 2310 to 2390MHz and 2483 5 to 2500MHz Typically IEEE 802 15 4 radios have trouble meeting the band edge requirements for Channel 26 2480MHz This is due to the skirts of O QPSK modulation spilling into the adjacent restricted band The SAMR21 is no exception Typically IEEE 802 15 4 systems designers do not use Channel 26 We included band edge testing for bo
15. cationNote_012015 Figure 3 2 SAMR21 Internal Connections PAOD A EIGEXTINT 0 i i D EE SERCOMJPADE SERCOM4PAD 1 PA08 09 12 15 F RFCTRUFECTRLO 5 Paddle connected to digital gound D VSS GND SAMR 1 GND VODOORE Woo 5 LLL There are several opposing design objectives one design objective is prevention of common impedance coupling For example the local oscillator grounds or digital communication bus returns from mixing with the RF ground currents Atmel 42391A SAMR21 Basic Connections and Wireless Design ApplicationNote_012015 Atmel SAMR21 Basic Connections and Wireless Design APPLICATION NOTE 13 3 2 2 14 Figure 3 3 Undesirable Common Impedance Coupling Star grounding is the usual technique employed to reduce common impedance coupling Classical Analog layout implements star grounding by connecting ground domains at the IC Another objective is to provide a large and stable RF ground plane as a counterpoise to the PCB antenna and low impedance return path for RF signals For superior signal strength these return currents need to freely flow to the Paddle of the SAMR21 and to the ground pins of the Balun The paddle of the SAMR21 is for RF Ground return not thermal conduction ATSAMR21 is an ultra low power device that does not need extra cooling Because we are using microstrip Layer 2 is the primary RF ground however this design also uses Copper pour on the top and
16. d signals have a common mode voltage of 900mV shorting this to grounded test equipment will kill the RF signal The Low Pass filter suppresses harmonic emissions EMI and improves ESD resilience This example uses a balun which has been customized to match the Atmel ATSAMR21 Matching generic or discrete baluns to Atmel ATSAMR21 is beyond the scope of this document and is not recommended Local Oscillator This design uses a 16MHz crystal oscillator for the RF synthesizer The ATSAMR21 uses special circuitry to drive the PLL in coordination with sleep management For low power designs a dedicated crystal is SAMR21 Basic Connections and Wireless Design APPLICATION NOTE Atmel Atmel 42391A SAMR21 Basic Connections and Wireless Design ApplicationNote_012015 2 1 10 2 1 11 recommended Atmel application note AVR2067 Crystal Characterization for AVR RF BI covers the use of crystals in detail In general the crystal signal quality directly influences the RF signal quality Jitter phase noise and drift are all mixed with the carrier and impact modulation quality radio link range and spurious emissions Good quality crystals with low drift are recommended for high performance designs Designers may want to add a resistor in series with the crystal to test safety margin but this was omitted in this minimal implementation Power Rails and Bypass Capacitors The ATSAMR21 has several internal power domains that need to be factored into the design In
17. ecommended to have an FCC pre scan done as early as possible in the product development cycle in case corrections are needed Scan of record will have to be performed with the radio in the final enclosure as the product is sold to the public Enclosures will de tune the PCB antenna and should be included in the antenna tuning process These results were generated using firmware included in Atmel application note AT10216 TX Test with Remote Start The Remote Start feature is design to test enclosed products that do not have control interfaces SAMR21 Basic Connections and Wireless Design APPLICATION NOTE Atmel Atmel 42391A SAMR21 Basic Connections and Wireless Design ApplicationNote_012015 5 Conclusion A considerable amount of craft is required to achieve acceptable radio performance with ATSAMR21 This application note has detailed several successful techniques including microstrip 4 layer PCB construction and a very effective RF ground plane system The Required Elements section of this design can be used as is witha low degree of risk Additionally useful accessories and common connections have been shown and can be applied as needed The test results are conclusive this reference design is excellent in terms of RF performance and exhibits satisfactory regulatory compliance Wireless Product developers seeking a low risk path and high yield in mass production should be very successful with derivatives of this design 6 Reference 1
18. he impact of firmware updates and product recalls Programming and UART headers have been included in the Required Elements Area The Debugging header is the industry standard pin assignment for Cortex This board can be programmed using the Atmel ICE Basic programmer dongle The UART is simple 3x1 x 100mil header with a CMOS interface This can be connected to generic USB UART converter cables The pin assignments for the Programming and UART headers are shown in Table 2 1 and Table 2 2 Table 2 1 Programming Header Pin Assignment PIN SIGNAL Atmel SAMR21 Basic Connections and Wireless Design APPLICATION NOTE 9 Atmel 42391A SAMR21 Basic Connections and Wireless Design ApplicationNote_012015 2 2 10 Table 2 2 UART Header Pin Assignment SIGNAL PIN Recommended Accessories Area Several features were added to the recommended accessories area The recommended accessories selected have proven useful in many development projects but they are optional and can be taken as needed in derivative designs The accessories include e On Off Switch e Battery Pack e Manual Reset e User defined button e User defined LEDs e GPIO Headers e Power Header e Current measurement shunt e 4Mb Serial Flash The utility of most of these accessories is obvious but a few need expanded comments Figure 2 7 Recommended Accessories Area Ip D 0 sl Je 0 OG o Je o DO 0 Go SAMR21 Basic Connections and Wireless Design APPLICA
19. icationNote_012015 Figure 3 4 NET TIES NT29 and NT35 Connect RFGND to DGND At the IC 3 2 3 Copper pour In this design Copper pour is used on the top and bottom layers The Copper pour was assigned to the RFGND Domain and is connected to Layer 2 with via stitching Care should be taken with Copper pour to avoid floating islands and narrow peninsulas Floating Islands act as shields and peninsulas act as radiators In this design the Copper Pour tool of the CAD system was configured to connect all pour areas to RFGND remove small areas of dead copper and keep the pour away from the microstrip transmission line Microstrip works because of the controlled dielectric thickness and the idyllic assumption that the primary ground on Layer 2 is an infinite plane Pulling back the Copper pour on Layer 1 by four times the dielectric thickness reduces secondary ground effects to less than 10 of the 50Q impedance Figure 3 5 Copper pour Near U1 and Transmission Lines Atmel SAMR21 Basic Connections and Wireless Design APPLICATION NOTE 15 Atmel 42391A SAMR21 Basic Connections and Wireless Design ApplicationNote_012015 3 2 4 Via stitching And finally via stitching joins the RFGND Layers together near the edges of the PCB to reduce unwanted spurious emissions and edge effects The via stitching connects the RF Ground plane on Layer 2 with Copper pour on Layer 1 and Layer 4 The via stitching does not connect to the DGND plane The DGND plane
20. lane The nominal size of the ground plane is one quarter by one half wavelengths The ground plane collects RF energy in receive mode the amount of energy collected is directly proportional to the area or effective aperture In transmission mode an African kalimba is a good allegory for the IFA System The tines of the kalimba are quarter wave resonators and the body is similar to the ground plane The body has sustained standing waves resonating in multiple modes size and shape influences the efficiency and tone of the system The same is true for the ground plane of our IFA system size and shape influence efficiency and tone Figure 2 2 Kalimba and Ramen Atmel SAMR21 Basic Connections and Wireless Design APPLICATION NOTE 5 Atmel 42391A SAMR21 Basic Connections and Wireless Design ApplicationNote_012015 This transmitter operates in the 2 4GHz ISM band In free space the wave length of 2 4GHz is approximately 4 8 inches 123mm Hence a quarter wave is 1 2 inches When the IFA is placed on a PCB the relative permittivity of the material reduces the wavelength by slowing the propagation of the TEM wave In this design we are assuming the PCB material is FR 4 and has a velocity factor of about 0 7 This results in a quarter wavelength of 880mils 22mm The nominal dimensions of the resonator and the ground plane are defined using this length Adjusting the Ground plane Many designers want smaller dimensions everywhere The Required
21. n this design we use a 20mil 0 5mm transmission line and a 10mil 0 25mm dielectric to achieve the 500 impedance 20mil transmission lines neatly abut to 0402 components Layer 1 is pushed back by 40mils from the microstrip to reduce parasitic coupling In Figure 2 6 the RF Ground plane is not visible because the CAD tool uses a negative image Figure 2 6 Transmission Lines inside Green Boarders To connect the balun to the ATSAMR21 balanced 1000 transmission lines are used This technique is also detailed in RF Layout with Microstrip 2I The balanced transmission lines are designed with a 20mil center to center spacing that mates directly to the pins of the SAMR21 Balun The balun is an essential component It converts the balanced output of the ATSAMR21 to the unbalanced 500 transmission line 500 transmission lines are easy to use and are the interface of choice for RF test equipment and RF components such as switches filters and PA LNA range extenders Internally the ATSAMR21 uses a balanced 1000 signals Balanced signals have several advantages for IC designers they increase the receiver signal to noise ratio suppress even harmonics of the transmitter and can be controlled with high speed current steering circuitry Unfortunately balanced signals are difficult for PCB designers to route and use The balun used in this example provides two additional features DC blocking and Low Pass Filtering The DC blocking is needed because the balance
22. next procedure is to match the complex impedance The Smith chart for the Ramen demo board is shown in Figure 4 3 18 SAMR21 Basic Connections and Wireless Design APPLICATION NOTE Atmel Atmel 42391A SAMR21 Basic Connections and Wireless Design ApplicationNote_012015 Figure 4 3 Complex Impedance of Ramen Antenna HP7470A Plotter Emulator Version 2 00 C application_notes at089 3_basic_atsamr21_connections trunk documents rame fo File View Display Caption Acquire GPIB Help CH1 Sun 1 U F Li ozro 11i 133 E 726 16 6H 4460 550 SR MHZ START d 398 888 Baa MHz STOP d 496 686 HBH MHz Without an enclosure the Mid band impedance is 52 8 j11 1 This is slightly capacitive Using a slightly smaller value capacitor in series or a shunt inductor could bring the trace closer to the center but these results are acceptable as is An enclosed assembly may require more correction After trimming and matching the PCBA was assembled and functionally tested 4 3 Receiver Testing The primary tool for characterization is the Atmel Performance Analyzer included in Atmel Studio With this tool we can run peer to peer Packet Error Rate tests PER to verify the range and sensitivity of the design With a spectrum analyzer we can also run conducted testing to verify RF output power channel equalization and carrier center frequency As a control we will compare to the ATSAMR21 XPRO demo board Additionally we want verify the antenn
23. r nonessential devices will have to be de populated from the PCBA 2 2 7 4Mb Serial Flash Memory A socket is provided for Serial Memory This can be used to Over the Air OTA upgrades storing credentials or general purpose memory The SPI bus connected to this memory is also routed to the J5 header Atmel SAMR21 Basic Connections and Wireless Design APPLICATION NOTE 11 Atmel 42391A SAMR21 Basic Connections and Wireless Design ApplicationNote_012015 3 Printed Circuit Board Design 3 1 Stack up and Material The board was designed as an example for RF layout for the ATSAMR21G This board is a four layer PCB that uses microstrip and a PCB antenna Four layers are recommended for low risk development More than four layers can be used if the additional layers are inserted beneath the RF Ground plane on Layer 2 Two Layer PCBs are possible using Coplanar Waveguide techniques but they have inferior grounding which typically results in abundant spurious emissions The layers used in this design are shown below e Layer 1 RF signals and copper pour e Layer 2 RF ground plane exclusively e Layer 3 Digital ground plane and power traces e Layer 4 General purpose signals and copper pour To implement microstrip transmission lines on Layer 1 the dielectric thickness and trace width are chosen using a 2 1 rule of thumb The traces are 20mils wide and the dielectric is 10mils thick A 10mil sheet of good quality core was used between Layer 1 and Layer
24. s been oriented to connect the ATSAMR271 to the test instrument In this way spectrum analyzers and RF test instruments can be used to make conducted measurements of RF output SAMR21 Basic Connections and Wireless Design APPLICATION NOTE Atmel Atmel 42391A SAMR21 Basic Connections and Wireless Design ApplicationNote_012015 Figure 2 4 RF Automated Test Equipment Using RFTP 2 1 5 Pl Tuning Network At the base of the antenna we have placed a PI network for tuning the match The final values will be selected after the PCBA has been assembled and the radio can be tuned inside an enclosure For initial engineering prototyping the series position C6 is loaded with a DC blocking capacitor of 20pF and the shunt positions C5 and C7 are left open Additionally the RF test point can be rotated 180 degrees which connects the antenna and PI network to the test immurements In this way a Vector Network Analyzer VNA can be used to tune the antenna and adjust the matching network Figure 2 5 Probing the Antenna with RFTP1 Atmel SAMR21 Basic Connections and Wireless Design APPLICATION NOTE 7 Atmel 42391A SAMR21 Basic Connections and Wireless Design ApplicationNote_012015 Transmission Line To efficiently conduct RF power between the ATSAMR21 and the Antenna System this design uses a 500 unbalanced single ended microstrip transmission line Microstrip transmission lines are covered in detail in RF Layout with Microstrip I
25. sories section shows several accessories that have proven useful in practical designs A 4Mb serial flash provides extra storage space for over the air OTA programming and general usage Many wireless standards such as ZigBee Light Link now require OTA programming Additional features include a RESET button one general purpose push button and three LEDs and GPIO headers A 32kHz crystal is also shown in the accessories section of the schematics because it is not required and can be omitted If this crystal is used however it needs to be placed close to the CPU so it is physically located in the required area of the layout One final feature is the low side DC shunt Atmel ATSAMR21 is one of the lowest power solutions on the market the shunt is provided to make accurate line power measurements with grounded test equipment 4 SAMR21 Basic Connections and Wireless Design APPLICATION NOTE Atmel Atmel 42391A SAMR21 Basic Connections and Wireless Design ApplicationNote_012015 2 2 1 Implementation Required Elements Area Figure 2 1 Required Elements Area Antenna System The heart of any radio design is the antenna This design uses an Inverted F Antenna IFA The IFA is a classic PCB antenna that is a good performer with minimal cost The IFA antenna is a quarter wave monopole antenna and requires a ground plane to function This antenna works because it sets up an electric field in the gap between the resonant stub and the ground p
26. th Channel 25 2475MHz and Channel 26 2480MHz Products designed with SAMR21 will have to omit Channel 26 or reduce the TX power to pass this regulatory requirement Designers that need to use Channel 26 may want to consider using ATmega256RFR2 which has special PLL filtering to lower band edge emissions Table 4 5 Radiated Band edge Emissions Frequency Measurement dBuV Limit dBuv Margin dBpuV Result Comments High Frequency Emissions The FCC requires measurements of unintentional emissions with the transmitter operating at full power on high middle and low channels These are radiated tests that include unwanted components from random resonant SAMR21 Basic Connections and Wireless Design APPLICATION NOTE Atmel Atmel 42391A SAMR21 Basic Connections and Wireless Design ApplicationNote_012015 structures on the PCB and errant RF energy that does not make it to the antenna The measurements show microstrip the full RF Ground plane system and the custom balun with its low pass filter are very effective at suppression of out of band spurs The results are shown below Channel 25 was used as the highest frequency because Channel 26 has excessive Band Edge emissions Table 4 6 Spurious Emissions TX Operating on CH11 2405MHz DUT Frequency MHz Level dBuV Limit dBuv Margin dBpV Sie Sei 1850 0 Figure 4 10 Spurious Emissions TX Operating on CH11 2405MHz ey TUV Rheinland of North America 28 Oct 1
27. therwise to any intellectual property right is granted by this document or in connection with the sale of Atmel products EXCEPT AS SET FORTH IN THE ATMEL TERMS AND CONDITIONS OF SALES LOCATED ON THE ATMEL WEBSITE ATMEL ASSUMES NO LIABILITY WHATSOEVER AND DISCLAIMS ANY EXPRESS IMPLIED OR STATUTORY WARRANTY RELATING TO ITS PRODUCTS INCLUDING BUT NOT LIMITED TO THE IMPLIED WARRANTY OF MERCHANTABILITY FITNESS FOR A PARTICULAR PURPOSE OR NON INFRINGEMENT IN NO EVENT SHALL ATMEL BE LIABLE FOR ANY DIRECT INDIRECT CONSEQUENTIAL PUNITIVE SPECIAL OR INCIDENTAL DAMAGES INCLUDING WITHOUT LIMITATION DAMAGES FOR LOSS AND PROFITS BUSINESS INTERRUPTION OR LOSS OF INFORMATION ARISING OUT OF THE USE OR INABILITY TO USE THIS DOCUMENT EVEN IF ATMEL HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES Atmel makes no representations or warranties with respect to the accuracy or completeness of the contents of this document and reserves the right to make changes to specifications and products descriptions at any time without notice Atmel does not make any commitment to update the information contained herein Unless specifically provided otherwise Atmel products are not suitable for and shall not be used in automotive applications Atmel products are not intended authorized or warranted for use as components in applications intended to support or sustain life SAFETY CRITICAL MILITARY AND AUTOMOTIVE APPLICATIONS DISCLAIMER Atmel products are not designed for
28. this PCB design we used two domains D3V3 and A3V3 for digital and analog power rails Both of these rails have ferrite beads to suppress VHF noise 10uF bulk bypass caps for local charge reserve and 20pF caps to shunt RF to ground Clean power is required to keep noise out of mixers and receiver stages To this end ATSAMR21 includes two on board regulators _DOs AVDD and DVDD These LDOs need external compensation caps The recommended values of 0 1uF are used Additionally 20pF caps are added to shunt RF to ground AVDD and DVDD are throttled as the CPU changes sleep states These rails should not be used as external references or supplies Additionally use of large value compensation caps on AVDD and DVDD is not recommended and may delay the PLL ramp up time and destabilize the LDO control loops External Power Connections System designers should connect their power source to the RAW_3V3 bus AC lines should be filtered to reduce conducted emissions from the transmitter The D3V3 and A3V3 busses can be used sparingly for support circuitry like analog signal conditioning and glue logic but the ferrite beads L1 and L2 are limited to 500mA current Programming and UART Headers Atmel strongly recommends preserving programming and UART interfaces for the life of the product UARTs enable Automated Test Equipment ATE for efficient mass production Debugging interfaces are invaluable in development Programming interfaces will significantly reduce t
29. wcases several RF design techniques including controlled impedance PCB antenna interface microstrip layout ground plane segregation star grounding conducted RF test point and several other features that are good practice for RF layout This design is compartmentalized in to two blocks the Required Elements and Recommended Accessories Figure 1 1 Ramen PCBAs Background When Implementing an RF design with new chip set engineers need to know the basic required elements and how to connect them RF designs are risky because the PCB layout is critical and many engineers are not familiar with RF design practice This design identifies the required elements and connections needed for success with Atmel ATSAMR21 This design uses a PCB antenna and 500 unbalanced transmission line with a coax RF test point This combination has proven to be straight forward to implement optimal for regulatory testing and mass production with Automated Test Equipment ATE The minimal design is implemented in a separate region of the PCB and can be cut and pasted into a new system as a whole This design also identifies several recommended accessories such as serial flash memory and 32kHz crystal oscillator that have proven useful in practical systems On PCBs many RF circuits are atypical and trigger CAD tool Design Rule Check DRC errors For example PCB antennas are effectively DC shorts Another example is ground planes The ATSAMR21 uses several independent
30. y for Testing ae ee we rome perita Antenna Tuning One of the first tasks after receiving the PCBs is trimming and tuning the antenna The dimensions of the antenna are roughly correct but adjustments need to be made to compensate for enclosures and variance in fabrication The antenna length is slightly longer than necessary This makes the resonant frequency slightly flat The optimal length can be found by trimming the antenna length in small increments to get the best signal strength This demo board was trimmed using a Vector Network Analyzer VNA Figure 4 2 shows the return loss A return loss of 12 or less indicates better than 90 of the RF energy is dissipated in the antenna After trimming off 50mils of length off the tip the antenna is showing strong resonance in the 2400 2480MHz band If a VNA is not available simple signal RF Signal strength measurements using the Atmel Studio 6 Performance Analyzer can be used Atmel SAMR21 Basic Connections and Wireless Design APPLICATION NOTE 17 Atmel 42391A SAMR21 Basic Connections and Wireless Design ApplicationNote_012015 Figure 4 2 Return Loss of Ramen Antenna g HP7470A Plotter Emulator Version 2 00 C application_notes at08973_basic_atsarmmr21_connections trunk documents rame fo D File View Display Caption Acquire GPIB Help CH1 Sit log MAG LH debe REF H db LPL Bae AB Del START d BEE B HDH MHz STOF 3 HEE BES BEE MHz The
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