Understanding Low Phase Noise Signals Presented by: Riadh Said Agilent Technologies, Inc.
Introduction Instabilities in the frequency or phase of a signal are caused by a number of different effects. Each type of noise process has distinct characteristics that can be measured using time domain and frequency techniques. In many cutting edge radar and communication systems, phase noise is the characteristic that limits the system performance. In radar systems, phase noise degrades the ability to process Doppler information in radar. And, in digitally modulated communication system, phase noise degrades error vector magnitude. Frequency Instability Effects I I Q
Phase Noise can Matter: A Lot Some Not At All Getting the Best Possible Performance Phase noise may be limiting parameter in measurement/sys. performance Balancing Phase Noise with Cost, Other Factors How to identify, quantify effects Identifying When it s Not Important Phase Noise Tradeoffs Acquisition cost Switching speed Frequency and offset noise 3
Agenda Frequency Stability Terminology & representation Architecture implications When Phase Noise Matters Example: Doppler Radar Example: Phase noise and EVM in OFDM signals Degrading phase noise performance for real-world signal substitution Signal Generator Choices, Performance & Tradeoffs Getting the Most from the Signal Generator You Have 4
Frequency Stability: Frequency & Time Domain Measurements Random perturbations that appear as instabilities in frequency can be represented in either the frequency domain or the time domain. Frequency Domain - Frequency Offsets of a few Hz to tens of MHz Time Domain - For close-in noise (<<1 Hz) - Short-Term Stability 5
Frequency Stability: Short & Long Term Terms, Measurements, Displays Short Term Stability Short term = seconds Terminology: Phase Noise, Jitter L(f) curves, integrated totals, spot measurements, jitter (p-p) Can be a function of both signal generator and frequency reference Long Term Stability Long term = minutes - years Terminology: Accuracy, drift, aging Often determined by frequency reference Understand Your System and its Sensitivity vs. Frequency Detail References Phase Noise Measurement Methods and Techniques 6
Pedestals, Slopes & Bumps: Signal Generator Architecture & Phase Noise Example: Agilent PSG Microwave Signal Generator Reference Section Detail References Reducing Phase Noise at Microwave and RF Frequencies Synthesizer Section YIG Oscillator Output section 7
Agenda Frequency Stability Terminology & representation Architecture implications When Phase Noise Matters Example: Doppler Radar Example: Phase noise and EVM in OFDM signals Degrading phase noise performance for real-world signal substitution Signal Generator Choices, Performance & Tradeoffs Getting the Most from the Signal Generator You Have 8
When Phase Noise Matters Matters More Doppler RADAR OFDM Oscillator substitution ADC testing Matters Less OFDM (matters more and less, depending on offset) Wideband single-carrier modulation Harmonic distortion testing Wideband ACPR testing Amplifier gain testing 9
Example: Phase Noise Doppler RADAR 10
Phase Noise and RADAR Applications Doppler Example STALO phase noise sidebands with no target clutter P t Detail BW = 1/t References Reducing Phase Noise at Microwave and RF Frequencies PA STALO F 0 IFA LNA F 0r + F d1 F 0 F d1 P r 11
Doppler Frequency Shift and Phase Noise Offset Frequencies R Doppler Frequency f d : 2 2 f c f d v R 0 v R Close-In (example: Airport Surveillance Radar) 100 mph, S-band (3 GHz): f d = 900 Hz Wider Offsets (example: Military Radar) Mach 1.5, X-band (10 GHz) f d = 33,000 Hz Detail References Radar & Electronic Warfare Threat Simulation 12
Consider Noise Contributions from Amplifiers, Including Broadband Noise PA Pulse Mod BW = 1/t Additive noise of the transmitter is present in the large reflected signal from the clutter and can mask the targets response STALO STALO PA F 0r F d2 13
Also Consider Spurious Performance for RADAR and EW Applications Noise and Spurious Effects System Spurious Signals within the Receiver BW BW = 1/t PA STALO F 0,F s1,f s2 IFA LNA F 0 + F d1 F 0 F d1 F s1 F d2 F s2 14
Estimating Required Phase Noise Performance STALO PA PULSE MOD Transmitter phase noise sidebands with clutter BW = 1/t Example: 1 GHz Doppler radar Doppler BW = 10 khz Target = 1 m 2 Range = 100 km Min Det Vr = 80 knots Clutter visibility = 80 db STALO PA Transmitter Noise Sidebands: L(f) < -120 dbc/hz @ 200 Hz offset F 0r F d2 Detail References Introduction to Radar Systems Skolnik 15
Example: Phase Noise and OFDM 16
OFDM and Pilot Tracking Pilots Shown in Time (I/Q) and Frequency 17
OFDM and Pilot Tracking BPSK Pilots: Demodulation Reference, No Data Transmitted Pilot Tracking Tracks Out Some Freq/Phase Instability Instability Not Just Constellation Rotation, also FFT Leakage or Inter-Carrier Interference No Pilot Tracking Pilot Tracking Enabled Common Pilot Error: Phase 18
Translating Phase Noise to EVM in OFDM Assumptions Estimating effects of only phase noise and broadband (wide offset) noise Spurious and other nonlinearities are not significant Equalization effective in removing linear errors Pilot tracking is effective to ~10% of subcarrier spacing Common phase error removed in demodulation Example: WiMAX 10 khz subcarrier spacing 10 MHz channel bandwidth 19
Example: Phase Noise Contribution to EVM in OFDM Error power calculated on log scale: -95 dbc/hz integ. over ~100 khz & Convert SSB to DSB: add 3 db -95 dbc/hz + 10log(100 khz) + 3 db EVM = -95 db + 50 db + 3 db EVM = -42 db (conservative) 10 MHz Channel Bandwidth 10 khz Subcarrier Spacing 95 dbc/hz pedestal EVM a function of integrated phase noise beyond tracking BW and inside channel BW (and correct SSB to DSB) Phase Noise Tracked Out Integrate Power for Total EVM Phase Noise Filtered Out 20
Signal Generation and Signal Analysis for Design & System Integration Q: Generate with no phase noise or representative amount? A: Yes, both Understand ultimate performance and residual error (error budget) Understand phase noise tolerance, design for just good enough performance Q: Measure with pilot tracking enabled or disabled? A: Yes, both Even when tracked out, phase noise can reduce demodulation margins 21
Adding Known Phase Noise Using FM Signal Modulated with Uniform Noise Simple Technique for -20 db/decade Detail References Phase Noise Measurement Methods and Techniques Example: Agilent PSG Set up an signal generator for FM modulation, by selecting: FM path 1 FM on FM deviation, as specified FM waveform to noise, uniform Ensure that noise of the PSG in FM off is at least 10 db less than the desired calibrated noise at a desired offset frequency, to ensure accuracy. 22
Degrading Phase Noise for Accurate Signal Substitution Simulate VCOs, Lower-Performance Synthesizers When Representative is Better than Perfect Use Baseband Real-Time Processing 23
Selectively-Impaired Phase Noise Performance L low L mid L high User sets f1, f2, and Lmid Slope -20dB/decade below f1 and above f2 f 1 f 2 f1=5khz, f2=500khz, Lmid=-80dBc f1=10khz, f2=1mhz, Lmid=-90dBc 24
Agenda Frequency Stability Terminology & representation Architecture implications When Phase Noise Matters Example: Doppler Radar Example: Phase noise and EVM in OFDM signals Degrading phase noise performance for real-world signal substitution Signal Generator Choices, Performance & Tradeoffs Getting the Most from the Signal Generator You Have 25
Sig. Gen. Choices: Frequency Stability Narrowing the Hardware & Software Choice CW or Analog vs. Digital Modulation or Vector Phase noise performance choices may interact with other capabilities Internal and/or external digital modulation Pulse modulation Internal/external software Memory and/or real-time baseband signal generation Power, distortion Single-Loop vs. Multiple Loop Phase Noise Performance Levels as Options VCO (voltage-controlled oscillator) vs. YIG (yttrium iron garnet) Switching speed, phase noise, cost 26
New X-Series Single- & Multiple-Loop Synthesizers Single Loop (example: EXG) Less complex, less expensive Moderate performance, can provide very good ACPR Simpler to design, optimize Multiple Loop (example: MXG) Typically fine loop + offset/step loop + sum loop Better phase noise Lower spurious More optimization choices and flexibility EXG N5172B RF Vector MXG N5182B RF Vector EXG N5173B Microwave Analog MXG N5183B Microwave Analog 27
Single vs. Multiple-Loop Architecture New Agilent X-Series (MXG, EXG) SSB Phase Noise @ 10 GHz EXG MXG standard MXG enhanced low phase noise option 1Hz 10Hz 100Hz 1kHz 10kHz 100kHz 1MHz 10MHz 100MHz L(f) [dbc/hz] vs. Frequency 28
MXG N5183B Analog, N5182B Vector Spurious and Broadband Noise May be as Important as phase noise MXG std. -134 dbc/hz typ. (20 khz offset @ 1 GHz) MXG opt. UNY -146 dbc/hz typ. (20 khz offset @ 1 GHz) Spur/Non-Harm. -96 dbc typ. (opt. UNY @ 1 GHz) khz Broadband Noise -163 dbc typ. (10 MHz offset @ 1 GHz) 29
Phase Noise (dbc/hz) Phase Noise vs. Frequency Generally Worse with Increasing Frequency Delta = 20*log(N), where N = multiplication factor 0 Agilent PSG Microwave/Millimeter Signal Generator -20-40 -60 PSG (15 GHz) 60 GHz (15 GHz x4) 90 GHz (15 GHz x6) 120 GHz (15 GHz x8) 180 GHz (15 GHz x12) 270 GHz (15 GHz x18) 450 GHz (15 GHz x30) -80-100 -120-140 -160 1 10 100 1,000 10,000 100,000 1,000,000 10,000,000 Frequency Offset from Carrier (Hz) 30
Phase Noise vs. Frequency Not always a Simple Relationship Agilent MXG RF Signal Generator (opt. UNY) 100 Hz 10 khz 1 MHz 10 MHz 31
Oscillator and PLL Technologies Affect SSB Curve Shape, Tradeoffs New MXG vs. EXG vs. ESG @1GHz 100Hz 1kHz 10kHz 100kHz 1MHz 10MHz 100MHz 32
Tradeoffs Switching Speed Generally faster with VCOs (vs. YIGs) Faster with wider loop bandwidths But wider loop BW may move noise to undesirable offset region Phase Noise YIGs generally better than VCOs Multi-loop significantly better than single-loop Move problem energy to a non-problem region Optimize for wide offsets vs. optimize for close-in 33
Other Capabilities, Tradeoffs vs. Phase Noise Performance Modulation Capability Maximum modulation bandwidth Modulation quality In-band: EVM or MER, etc. Out-of-band: ACPR Modulation inputs & outputs (bandwidths can be different) Pulse Capability Complex pulse scenarios, pulse performance Modulation on pulses, Doppler, PRT Software/application support Multi-Channel Synchronization 34
Agenda Frequency Stability Terminology & representation Architecture implications When Phase Noise Matters Example: Doppler Radar Example: Phase noise and EVM in OFDM signals Degrading phase noise performance for real-world signal substitution Signal Generator Choices, Performance & Tradeoffs Getting the Most from the Signal Generator You Have 35
Methods for Improving Signal Generator Phase Noise Connect to a Low-Noise External Frequency Reference Advanced Architectural Improvements to the Reference and Synthesizer Design Adjustment of the Reference Oscillator & VCO PLL Bandwidth Use of Dividers at Lower Frequencies Instead of Heterodyne Mixing Detail References Reducing Phase Noise at Microwave and RF Frequencies 36
Optimizing Signal Generator Phase Noise Choice of External Reference Oscillator Choosing External Reference Bandwidth Optimizing for Close-In or Wide Offset Noise Optimizing for Broadband Noise Optimizing for low frequency coverage Noise Subtraction, Including Phase Noise Subtraction Read Documentation, Specs for Product-Specific Optimization Choices 37
Residual Phase Noise: Comparisons with Perfect Reference Using perfect external10 MHz reference up to 20 db improvement @ 1 Hz 1Hz 10Hz 100Hz 1kHz 10kHz 100kHz 1MHz 10MHz 100MHz L(f) [dbc/hz] vs. Frequency 38
Residual Phase Noise: Comparisons with Perfect Reference Agilent PSG UNY up to 20 db improvement @ 1 Hz 39
Optimize Frequency Reference Choice and Settings: Signal Analyzer Example Quality of external 10 MHz ref. oscillators can make a big difference in phase noise measurements (& signal generation) Detail References Phase Noise Measurement Methods and Techniques External reference lock narrow External reference lock wide If an external reference oscillator has better close-in phase noise than the PXA the wide locking bandwidth can be used to obtain better closein (< 30 Hz) measurement performance. If higher offset frequencies are more important the narrow setting can be used. 40
Reference Oscillator PLL Bandwidth Adjustment: PSG Signal Generator The 10 MHz reference signal can be internally generated or externally supplied Detail References Reducing Phase Noise at Microwave and RF Frequencies Adjusting the PLL integrator bandwidth 41
PLL Bandwidth Adjustment: PSG Microwave Signal Generator Example 25 Hz 55 Hz 125 Hz 300 Hz 650 Hz 42
Optimizing Pedestal Phase Noise: PSG Microwave Signal Generator Example 43
Optimizing Signal/Noise vs. Phase Noise 10 khz 1 MHz 10 MHz 44
Generating Lower Frequencies: Freq. Division vs. Heterodyne Mixing Frequency division reduces the phase noise sidebands of the signal by a factor of 20 db/decade or 6 db/octave Example: Dividing by 4 reduces phase noise by 12 db. (20*log(1/4) ) Divider tradeoff: Maximum FM and PM deviations are reduced by the same factor as the division number Heterodyne mixing provides fine frequency adjustment while retaining full bandwidth FM and PM Heterodyne mixing tradeoff: Heterodyne mixing to lower frequencies provides no phase noise reduction 45
Subtracting Signal Generator Phase Noise Power In some scalar (power) measurements the phase noise contribution from the signal generator can be subtracted from the measured result Raw measurement Reference measurement Result after subtraction 46
Subtracting Signal Generator Broadband Noise Power T R A C E A : F 1 P S D 1 / K 1 A M a r k e r 4 4 3 0 5 9 0.3 8 1 Hz - 1 2 3. 6 d B m / H z B M a r k e r 4 4 3 0 5 9 0.3 8 1 Hz - 1 3 5. 7 7 5 d B m / H z - 5 0 d B m / H z L o g M a g 1 0 d B / d i v - 1 5 0 d B m / H z C e n t e r : 4. 4 2 0 7 7 7 8 8 1 M H z S p a n : 5 0 k H z 47
Noise Sidebands May Not Be Entirely Phase Noise AM Noise may be Significant Fraction of Noise Sideband Power AM Noise may Affect a System or Not Example: Pilot Tracking Usually Removes Close-In AM Noise 48 AM Rejection on AM Rejection off AM noise is included in direct noise sideband measurements X-Series phase noise meas. app (N9068A) can reject AM noise < 1 MHz AM rejection can improve phase noise measurements Understand system sensitivity, signal generator AM noise Detail References Phase Noise Measurement Methods and Techniques
Phase Noise can Matter: A Lot Some Not At All Getting the Best Possible Performance Phase noise may be limiting parameter in measurement/sys. performance Balancing Phase Noise with Cost, Other Factors How to identify, quantify effects Identifying When it s Not Important Phase Noise Tradeoffs Acquisition cost Switching speed Frequency and offset noise 49
References, More Information Techniques for Improving Noise and Spurious in PLLs Eric Drucker, Microwave Journal, May 2012 Reducing Phase Noise At Microwave and RF Frequencies Agilent A/D Symposium 2011 Signal Generators Provide Perfect and Precisely Imperfect Signals Ben Zarlingo, Microwave Journal, May 2012 Testing Low-Noise Components in Pulsed or Moving Target Radars Agilent A/D Symposium 2009 Phase Noise Measurement Methods and Techniques Agilent A/D Symposium 2012 Radar & Electronic Warfare Threat Simulation Agilent A/D Symposium 2011 Introduction to Radar Systems by Merrill Skolnik, 2002, ISBN 978-0072881387 50
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