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