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Embedded Meets RF
Evan Lojewski
RF Primer
•Terminology / Intro
•RF Options
•Certification
Terminology
•Decibel - logarithmic scale
•Channel - subdivided portion of spectrum
•Carrier - center frequency of channel / transmission
•DNR - dynamic range
Decibel
•dB is unit-less, denotes logarithmic
•value dB = 10log10(value)
•value = 10^(value dB / 10)
•Standard exponential rules apply
•Cannot multiply value together
db Suffixes
•dBW: Watts
•dBm: miliWatts
•dBi: forward gain of antenna
•dBc: relative to carrier
Gain / Loss•Gain is the Amplification of signal
(unitless). 3db gain is an amplification of 2x.
•Linear: Power * gain(amp) = final power
•dB: Power + gain(amp) = final power
•Linear: Gain = 1/Loss
•dB: Gain = - Loss
Sample Calculations
•5dB + 10dBw = 15dBw
•-3dBw = 27dBm
•-3dBm + 3dBm = ????
•10dBm = 10 mW = 0.01 W
More Examples
•3dB ~= 2
•6dBm ~= 4 mW
•13dB ~= 20
•20dB = 100
Test Equipment
•Spectrum Analyzer
•Verify emissions
•demodulate packet
•Network Analyzer
•Impedance Matching
•Power Meter
•Anechoic Chamber
RF Options
•Don’t do it -> go wired (really)
•Modules
•PDB Design
Wires: The easy option
•Batteries are annoying
•Reliability
•Throughput
•Don’t have to mess with the FCC... much
RF Frequencies: ISM
•2.45GHz: Wifi, Bluetooth, 802.15.4, Cordless Phones
•1W peak power
•915 MHz: UHF RFID, Cell Phones
•1W peak power
Classic Bluetooth
•Interoperability
•Android
•Power hungry
•3mA connected
•30mA connecting
Classic Bluetooth
•AT Command Set
•Control
•UART passthrough (SPP)
•Transparent to software
Bluetooth Smart•Interoperability
•iOS
•Window 8
•Low Power
•~40uA - 150uA connected
•18mA transmitting (max ~300us burst)
XBee
•Serial passthrough
•Pair two devices
Wifi
•Interoperability
•Anything with network access
•web pages
•telnet
•Complex - TCP/IP
•High Power
Proprietary
•Non-Interoperability
•Vendor lock in
•Minimal overhead
•Depending on implementation, can be very low power.
RF Modules: 2.4Ghz
Module StandardsPower RX
(idle)Throughput
XBee 802.15.4 40 mA 250KbpsGS1011
MIP802.11b
150mA (150uA)
11Mbps
RN-171 802.11b/g120mA (15mA)
54Mbps
nRF24L01+
Proprietary
11.3mA (22uA)
2Mbps
RN-42 Bluetooth 30mA (26uA) 240Kbps
BR-LE4.0BluetoothL
E27mA (0.9uA) 1Mbps
RF Modules: 2.4Ghz
Module Interface API DNR $
XBee UARTPassthroug
h99dB 23
GS1011MIP
UART / SPI Custom 90dB 28
RN-171 UART / SPI Custom 93dB 30nRF24L0
1+SPI Packet 82dB 20
RN-42 UART / SPIPassthroug
h84dB 16
BR-LE4.0 UART / SPIAT
Commands96dB 119
Custom PCB vs Module
•Lower Cost than modules in volume
•Full control over design
•Smaller size
•Must go through complete certification process
PCB Considerations
•εr: Affects thickness of PCB
•Ground Plane
•Controlled Impedance
•Manufacturer Recommendations
FCC Regulations
•Covers Unintentional Radiator
•And Intentional radiators
Unintentional Radiators
•Digital Device operating at 9kHz+
•Must be Verified.
•Manufacture makes measurements.
•Submittal of measurements / sample unit only required if FCC asks.
Intentional Radiators
•Digital Device operating at 9kHz+
•Must have a Declaration of Conformity
•You declare part complies with FCC regulations.
FCC Certification
•Form 731
•$1265.00 Fee
•Measurement Results / Report
•System Block Diagram
FCC Exceptions:Subassemblies
•Subassemblies
•Do not need to be certified / verified
•End user will need to FCC certify
•Home-built devices
•Up to 5 of any non-marketed, non-kit
•Personal use only.
FCC Requirements: 15.247 (2.4GHz)
•1W peak Power
•spurious emissions: 20dB below carrier
•And many more...
FCC Requirements:
47 CFR 2•Must retain description of faciility
•List of equipment used
•Must retain original drawings, records of procedures and measurements made.
•Retain all data required to show conformity.
PCB Considerations
•Software?
•Module vendors usually give you their stack and examples
•Chip vendors don’t support small companies often.
PCB Considerations
•Antenna
•Chip Antenna? (expensive)
•Trace Antenna? (cheap)
•RF Connector? (expensive)
•Impedance Matching
•balun?
•Tuning components
References•FCC 47 CFR 15: Radio Devices
•FCC 47 CFR 2 Subpart J: Equipment Auth
•http://www.sparkfun.com/tutorials/398
•Example FCC report: http://faenl.msi.com/ftp/CE%20Documents/68xx/BTHub/41104402-RP1%20Part%2015.247.pdf