Transcript Embedded Meets RF Evan Lojewski RF Primer Terminology / Intro

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
Standards Power RX (idle) Throughput
XBee
802.15.4
40 mA
250Kbps
GS1011MI
P
802.11b
150mA (150uA)
11Mbps
RN-171
802.11b/g
120mA (15mA)
54Mbps
nRF24L01
Proprietary 11.3mA (22uA)
+
2Mbps
RN-42
Bluetooth
30mA (26uA)
240Kbps
BluetoothL
BR-LE4.0
E
27mA (0.9uA)
1Mbps
RF Modules: 2.4Ghz
Module
Interface
XBee
UART
GS1011MI
P
UART / SPI
Custom
90dB
28
RN-171
UART / SPI
Custom
93dB
30
nRF24L01
+
SPI
Packet
82dB
20
RN-42
UART / SPI
Passthrough 84dB
16
UART / SPI
AT
Commands
BR-LE4.0
API
DNR
$
Passthrough 99dB
23
96dB 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
• ε : Affects thickness of PCB
• Ground Plane
• Controlled Impedance
• Manufacturer Recommendations
r
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%20Docume
nts/68xx/BTHub/41104402RP1%20Part%2015.247.pdf