RF Basics June 2010 WLS 04

Transcription

1 RF Basics June 2010 WLS 04

2 Agenda Basic link parameters Modulation Types Datarate Deviation RX Baseband BW Crystal selection Frequency error compensation Important t radio parameters Regulatory Standards (time allowing) 2 Silicon Laboratories Confidential

3 Modulation Modulation is the mechanism by which data is transferred Unmodulated carriers do not contain any information Information is represented by: Amplitude (OOK, ASK) Frequency (FSK) Phase (PSK, QPSK, etc.) Etc. 3 Silicon Laboratories Confidential

4 Modulation Types FSK/GFSK Frequency Shift Keying (FSK) Frequency modulation Logical 1 TX signal frequency is above carrier frequency Gaussian Frequency Shift Keying (GFSK) Frequency modulation Same as FSK but data is Gaussian filtered 4 Silicon Laboratories Confidential

5 FSK Spectrum Unmodulated Carrier GFSK Modulated Spectrum 5 Silicon Laboratories Confidential

6 FSK vs. GFSK 20kbps, 20kHz modulated signal Measured with EZRadioPRO 6 Silicon Laboratories Confidential

7 Modulation Types - OOK OOK Modulation Logical 1 PA ON transmitting at carrier frequency Logical 0 PA OFF ASK Modulation Logical 1 PA ON transmitting at carrier frequency Logical 0 PA ON transmitting at a lower power level than a logical 1 7 Silicon Laboratories Confidential

8 OOK Spectrum Unmodulated Carrier OOK Modulated Spectrum 8 Silicon Laboratories Confidential

9 OOK vs. FSK/GFSK GFSK/FSK is a more robust modulation technique Most new applications are GFSK/FSK OOK used primarily in legacy systems Operates in the same way noise works OOK can be challenging for some RFIC s at high TX power level due to pulling of the internal oscillator RKE applications use OOK because it is cheap to design with SAW based resonator SAW based resonator frequency accuracy is not precise Most RX channel bandwidths >300KHz USE Si4010 & Si4312 Less power consumption in OOK mode 9 Silicon Laboratories Confidential

10 Supported modulation types Modulation EZRadio EZRadioPRO OOK Si4312, Si4010 All family members FSK All, except Si4312 All family members GFSK None All family members 10 Silicon Laboratories Confidential

11 Datarate Datarate is how fast the modulation waveform is switching between a logical 1 and a logical 0 Datarate is independent of FSK, GFSK, or OOK Modulation Datarate = 1/Bit Time Bit Time 11 Silicon Laboratories Confidential

12 Datarate Design Considerations Design considerations: Lower data rate is better sensitivity longer range Higher data rate results shorter packet time longer battery life time Sensitivity vs Datarate -95 tivity (dbm) Sensit Note: Measured with EZRadioPRO (The RX BW was optimized for each data rate!) Datarate (kbps) Chip EZRadio (except Si4010 & Si431x) Si4010 Si431x EZRadioPRO Max. Datarate 115.2kbps 50kbps 10kbps 256kbps 12 Silicon Laboratories Confidential

13 GFSK / FSK Deviation FSK / GFSK deviation is the peak change in frequency for a logical 1 or 0 from the carrier frequency 16:14:48 Jun 2, 2010 Ref 20 dbm Peak Log 10 db/ Atten 30 db -Δf +Δf V1 S2 S3 FC AA Center 913 MHz Span 200 khz #Res BW 3 khz VBW 3 khz Sweep ms (401 pts) NOTE: using old EZRadio products the deviation has to be higher than the datarate! 13 Silicon Laboratories Confidential

14 RX Baseband Bandwidth Filters the real data out from the noise The required Baseband bandwidth (BB BW) is defined by datarate (DR), deviation( f ) in case of FSK/GFSK-, and crystal tolerance 16:15:44 Jun 2, 2010 Ref 20 dbm Peak Log 10 db/ Atten 30 db RECEIVER RECEIVES 0 RECEIVER RECEIVES 1 M1 S2 S3 FC AA Center 913 MHz Span 500 khz #Res BW 3 khz VBW 3 khz Sweep ms (401 pts) 14 Silicon Laboratories Confidential

15 RX BW vs. sensitivity Carson s rule can be used for an estimate of the BB BW BW = 2* f + DR Sensitivity = log10(BW) + SNR + NF Smaller BW (smaller deviation) better sensitivity Link budget: TX output power + RX sensitivity ~6 9dB more link budget typically doubles the outdoor range ~9 12dB more link budget typically doubles the indoor range Sensitivity vs Datarate -95 tivity (dbm) Sensit Datarate (kbps) 15 Silicon Laboratories Confidential

16 Receive bandwidth Chip EZRadio (except Si431x) Si4311 Si4312 EZRadioPRO Receive bandwidth 67kHz 400kHz 200kHz (600kHz if AFC used) 420kHz 2.6kHz 620kHz 16 Silicon Laboratories Confidential

17 Crystal Tolerance Crystal tolerance directly effects the output frequency accuracy: F centre ~ F XTAL Crystal inaccuracy may require an increase in BW to compensate Crystal tolerance is defined as parts per million (ppm) All the initial iti accuracy, temperature t accuracy and aging has to be taken into account Both RX and TX side crystal accuracy has to be considered To convert to TX-RX offset (in khz): F [ khz ] = ( X + X )* XTAL _ FREQ * err[ ppm _ TX + ppm _ RX Freq XTAL _ FREQ Example: assuming 20-20ppm XTAL tolerance at both side of the link: Alltogether 40ppm error = 900MHz 17 Silicon Laboratories Confidential

18 Crystal tolerances, selection Crystal load capacitance: To get accurate oscillation frequency, the crystal needs a specified amount of load capacitance Load capacitance consists of: oscillator output capacitance PCB parasitics Capacitance bank is programmable Crystal selection: For datarates less than 40kbps a crystal with less than 20ppm error is required to achieve optimum sensitivity The lower the ppm XTAL more $ For low datarate or narrow channel (12.5 / 25kHz) applications a temperature compensated crystal oscillator must be used (TCXO) In those system the maximum allowed frequency error is typically <5ppm 18 Silicon Laboratories Confidential

19 RX Baseband Bandwidth and Crystal Tolerance No frequency offset from crystal Crystal offset causes the desired modulated signal to be shifted outside of the RX baseband bandwidth. 19 Silicon Laboratories Confidential

20 How to compensate? Crystal tuning during manufacturing: Measure a statistic number of boards: which crystal load capacitance gives accurate centre frequency Define the average value for the capacitance bank Use the average for all the boards Automatic Frequency Control (AFC) It can help automatically re-center the desired modulated signal inside of the RX baseband bandwidth up to a certain limit. If the offset is outside of this limit then the RX BW needs to be increased. AFC settling time requires longer preamble to be transmitted 20 Silicon Laboratories Confidential

21 AFC in EZRadio family Old EZRadio products (Si432x, Si442x) It can tolerate up to 0.8 * RX BW The AFC settling time is 1byte time AFC limit has to be set properly! Si4311 As long as one of the FSK tone is in the RX BW, the AFC can compensate the offset It requires 3 preamble bit + 700us fix delay (worst case 10kbps) 200ms hold time after packet reception 21 Silicon Laboratories Confidential

22 AFC in Si4312 Si4312 Scans 3 frequency bins: 3 x 140kHz = 420kHz Sweeps the gain also to avoid saturation. The scan is always enabled. It stops if OOK modulated signal found. The frequency is held as long as the desired signal present + threshold time Threshold h time is programmable: ms 22 Silicon Laboratories Confidential

23 AFC in EZRadioPRO Modem frequency compensation Part of the digital modem, works continuously It does not adjust the PLL It can tolerate up to ± 0.25*RX BW Does not require longer preamble! AFC It adjust the PLL It can tolerate up to ± 0.35*RX BW AFC settling time requires 8bit additional preamble NOTE: most of the cases the Modem Frequency Compensation is sufficient! 23 Silicon Laboratories Confidential

24 Summary Basic link parameters Modulation Types Datarate Deviation RX Baseband BW Crystal selection Frequency error compensation Important radio parameters 24 Silicon Laboratories Confidential

25 Selectivity and Blocking Measure the robustness of the radio to perform in the presence of interferers Selectivity results can change greatly depending on the test conditions and RF parameters Sometimes hard to get an apples to apples comparison between datasheet values because conditions are different It defines as a C/I db (dbc) value ETSI and some competitors define as the interferer level in dbm Desired 3dB above ETSI sensitivity (-92.8dBm) Raise interferer until sensitivity degrades to 20% PER Carrier/Interferer (C/I) Ratio with BER still <0.1% Desired 3dB above Sensitivity Interferer Interferer Frequency Offset 25 Silicon Laboratories Confidential

26 Selectivity example (EZRadioPRO) Adjacent Channel Selectivity at 50 kbps Measured at RX SMA Connector Input 10 db 0 db AGC Enabled -10 db C/I -20 db -30 db -40 db -50 db -60 db MHz MHz MHz MHz 0.00 MHz 0.25 MHz 0.50 MHz 0.75 MHz 1.00 MHz Interferer Frequency Offset Example: With the desired signal 3dB above sensitivity an interferer 750kHz away at a level 50dB stronger than the desired signal, the chip will still achieve BER less than 0.1% 26 Silicon Laboratories Confidential

27 Occupied bandwidth The bandwidth containing 99% of the total integrated power of the transmitted spectrum 16:15:44 Jun 2, 2010 Ref 20 dbm Peak Log 10 db/ Atten 30 db M1 S2 S3 FC AA Center 913 MHz Span 500 khz #Res BW 3 khz VBW 3 khz Sweep ms (401 pts) 27 Silicon Laboratories Confidential

28 Adjacent channel power The amount of modulated RF signal falls within an adjacent channel Typically important for narrow band application 28 Silicon Laboratories Confidential

29 Regulatory standards that apply to EZRadio / EZRadioPRO

30 Frequency Band(s) by Country / Region Each geographical area in the world has authorized different bands A general high-level overview is shown below 30 Silicon Laboratories Confidential

31 Summary FCC Part : MHz FCC Part : MHz FCC Part 90: Various ETSI: 868 MHz Japan: MHz, MHz China AMR: MHz 31 Silicon Laboratories Confidential

32 Overview EZRadioPRO chips must comply with many regulatory standards Different standards are required for: Different countries Different frequency bands Different end-use applications Regulatory standards primarily differ in the areas of: Maximum allowed output power Maximum allowed spurious emissions Harmonics Modulation bandwidth Minimum required adjacent channel selectivity and blocking Minimum required sensitivity Standards also vary in terms of methods of testing Peak power vs. average power Radiated vs. conducted power etc. 32 Silicon Laboratories Confidential

33 FCC Part ( MHz) This FCC section covers wide bandwidth applications Frequency Hopping Spread Spectrum (FHSS) Wideband digital modulation (BW MOD_6dB > 500 khz) Direct Sequence Spread Spectrum (DSSS, not applicable to PRO) Applicable primarily to MHz ISM band LOTS of general customers use this band and fall under Weather station reporting Automatic meter reading I.e., ANYONE who wants to use higher power Because signal is wideband spread out, greater TX power is allowed under this part Wideband digital modulation 1 watt (+30 dbm) max FHSS (> 50 hopping channels) 1 watt (+30 dbm) max FHSS (> 25 hopping channels) 0.25 watt (+24 dbm) max 33 Silicon Laboratories Confidential

34 FCC Part and Spurious Emissions Limits on Spurious Emissions fall into two basic categories Strict limits apply to emissions in restricted bands Restricted bands are frequencies where other important radio services exist Restricted frequency bands defined in FCC Part Radiated emission limits in these bands defined in FCC Part Spurious falls below 960 MHz = -49 dbm max Spurious falls above 960 MHz = -41 dbm max Very loose limits apply everywhere else Spec is only -20 dbc RX Requirements under FCC Part = NONE 34 Silicon Laboratories Confidential

35 FCC Part ( MHz) This FCC section covers much lower power applications Applicable primarily to MHz band Our customers mainly use 315M, 390M, and 433M LOTS of applications that use this band fall under Garage door openers Alarm systems Remote Keyless Entry (RKE) Control links RX Requirements under FCC Part = NONE This FCC section limits duration and type of transmit periods Regularly-spaced periodic transmissions are NOT PERMITTED Continuous (CW) transmission i is NOT PERMITTED Except for emergency situations alarms Basic limit on transmission duration is 2 seconds per hour 35 Silicon Laboratories Confidential

36 FCC Part and TX Power Limits Limits on emissions specified in terms of field strength (μv/meter) For both fundamental as well as spurious (harmonics) Specified at a distance of 3 meters Specified limits vary as a function of frequency Limit on desired fundamental ranges from: 3,750 uv/m at 260 MHz, to 12,500 uv/m at 470 MHz Limit on spurious emissions range from: 375 uv/m at 260 MHz, to 1,250 uv/m at 470 MHz Field strengths th may be converted to equivalent conducted d limits it You must choose (or assume) a type of antenna Assuming isotropic radiator (0 db gain), we get (for the fundamental) dbm at 315 MHz dbm at 433 MHz dBm at 470 MHz 36 Silicon Laboratories Confidential

37 FCC Part 90 (various frequencies) FCC Part 90 applies to narrowband licensed applications Customers must apply for (and receive!) license approval to operate under this section Advantage is your own private band (at least within your geographic area) Private band potentially less interference Also, higher output power is allowed Allowed bandwidths are narrow 12.5 khz or 25 khz Some modulation emission masks in Part 90 are TOUGH! E.g., Mask D for M Difficult, if not impossible, to meet Mask D with EZRadioPRO Due to our current phase noise performance Mask B for 900M band is easier, can be met with EZRadioPRO PRO2 is being specified to comply with FCC Part 90, Mask D Because of these difficult requirements, Silicon Labs is not aggressively pursuing Part 90 customers with EZRadioPRO 37 Silicon Laboratories Confidential

38 ETSI EN (or just ETSI ) (868 MHz) ETSI European Telecommunications Standards Institute Many, many regulatory standards under ETSI governance ETSI standard most applicable to us is: ETSI EN , Short Range Devices (SRD) Radio Equipment to be used in the 25 MHz to 1000 MHz Frequency Range with Power Levels up to 500mW 38 Silicon Laboratories Confidential

39 ETSI sub-bands Frequency range we are most interested in is MHz band Most common applications fall in smaller sub-bands around 868 MHz These sub-bands bands are for non-specific or general use Thus they have been given names using the prefix g I.e., sub-band g1, g3, etc. 39 Silicon Laboratories Confidential

40 ETSI Applications and Specs Typical customer applications Control links Alarms ETSI specs are much more comprehensive than FCC specs TX Power Spurious Emissions and Harmonics Occupied Modulation Bandwidth Including splatter due to transient performance Receiver performance Sensitivity Adjacent Channel Selectivity Blocking Spurious Response Rejection 40 Silicon Laboratories Confidential

41 ETSI and TX Output Power Most ETSI sub-bands limit the TX output power to: 10 mw (+10 dbm), or 25 mw (+14 dbm) Thus Si4431 chip (+13 dbm) is popular choice for ETSI applications g3 sub-band allows higher power 500 mw (+27 dbm) over to MHz g3 sub-band Can use Si4432 chip at +20 dbm, or external PA ETSI specs TX output power in two different ways Conducted measurement, for devices with antenna connectors Radiated measurement, for devices with integral antennas Limits are the same for both methods Test methods obviously differ Conducted method is easy, just hook up a power meter or spectrum analyzer Radiated method measures Effective Radiated Power (ERP) Basically determines how much power you need to apply to a reference dipole antenna, to produce same radiated field strength as the DUT + Integral Antenna 41 Silicon Laboratories Confidential

42 ETSI and Spurious / Harmonics Spurious Emission limits are both easier and tougher Depending upon relevant part of spectrum Harmonics are generally easy to meet -30 dbm max limit above 1 GHz Other spurs may be tougher -36 dbm max limit from 870 MHz to 1 GHz -54 dbm max limit from 470 MHz to 862 MHz EZRadioPRO may have some spurs of concern ±10 MHz Reference Sideband spurs E.g. spur at 858 MHz when transmitting at 868 MHz Typically better than -75 dbc can meet spec up to +21 dbm output power Sampling Spurs Somewhat variable frequency offset, depending upon Synthesizer frequency May limit our TX output power to no more than ~ +14 dbm 42 Silicon Laboratories Confidential

43 ETSI and TX Modulation Bandwidth ETSI spec limits the bandwidth of TX modulated signal Modulated signal must fit within the emission mask shown below EZRadioPRO complies with emission mask At normal TX output power levels (+14 dbm), with considerable margin But may be limited in g3 sub-band to ~ +21 dbm, due to phase noise PRO2 phase noise is being spec ed to improve upon this 43 Silicon Laboratories Confidential

44 ETSI and Categories of Receivers ETSI defines three categories of receivers Cat-1, Cat-2, & Cat-3 (duh!) Categories differ primarily in AdjChSel, Blocking, and Spurious Response requirements EZRadioPRO does not comply with Category-1 Neither do chips from our competitors Neither will PRO2 Virtually impossible to meet without external filters or heterodyne system 44 Silicon Laboratories Confidential

45 ETSI and important RX Requirements RX Sensitivity is easy Basic spec is: Prx = -107 dbm + 10*log(BW IF / 16 khz) Spec is so relaxed, it s trivial RX Adjacent Channel Selectivity is TOUGH! Measures ability to receive desired signal, with interferer on adjacent channel But applies ONLY to Category-1 receivers Thus not applicable to EZRadioPRO RX Blocking Measures ability to receive desired signal, with interferer much further away Basic limits (for Category-2 receivers) are Interferer at > -69 dbm at ±2 MHz offset Interferer at >-44 dbm at ±10 MHz offset We meet this easily (with margin) We fail much tougher Category-1 limits There are other less important parameters as well 45 Silicon Laboratories Confidential

46 Japanese Applications Typical customer applications Control links Alarms Remote Keyless Entry (RKE) Narrowband channelized systems ( MHz) Typically 12.5 khz to 25 khz channels, well-defined channel frequencies ARIB specs both RX and TX TX specs are concerned primarily with Spurious Emissions and Harmonics Relatively easy to meet Adjacent Channel Leakage Power Can be tougher to meet RX Specs mostly concerned with Adjacent Channel Selectivity IIP3 Spurious Response Rejection MHz new emerging AMR band 46 Silicon Laboratories Confidential

47 China AMR Band ( MHz) Chinese Automatic Meter Reading (AMR) MHz band Very skimpy spec documents exist (in English, anyways!) TX output power = 50 mw (+17 dbm) max Our Chinese customers are still requesting +20 dbm Apparently, this spec is routinely ignored! TX Harmonics -36 dbm below 1 GHz -30 dbm above 1 GHz Note that 2 nd harmonic of 470 MHz falls below 1 GHz But 2 nd harmonic of 510 MHz falls above 1 GHz Thus 2 nd harmonic limit of -36 dbm for +20 dbm output power can be tough Occupied bandwidth < 200 khz No RX specs 47 Silicon Laboratories Confidential

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