1 / 18

A Low Cost Disciplined Sampling Clock Using the WWVB (60-kHz) Broadcast CASPER 2012 Workshop

A Low Cost Disciplined Sampling Clock Using the WWVB (60-kHz) Broadcast CASPER 2012 Workshop Green Bank, WV August 17, 2012 Bogdan Vacaliuc, James J. Nutaro , Daniel B. Koch, Benjamin E. Huey and Stephen F. Smith. Motivation – A Next Generation Grideye.

ash
Download Presentation

A Low Cost Disciplined Sampling Clock Using the WWVB (60-kHz) Broadcast CASPER 2012 Workshop

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. A Low Cost Disciplined Sampling Clock Using the WWVB (60-kHz) Broadcast CASPER 2012 Workshop Green Bank, WV August 17, 2012 Bogdan Vacaliuc, James J. Nutaro, Daniel B. Koch, Benjamin E. Huey and Stephen F. Smith

  2. Motivation – A Next Generation Grideye Frequency Disturbance Recorder (FDR) • Record fluctuation in power grid frequency from consumer line voltage measurements • ~80 “FDR” sensors deployed today • $2500/unit BOM cost • FY12 Project Developing New Sensor Design • Time Sync – VLF radio vs. GPS • Detector – Fourier vs. Sinusoid Correlation • Cost – $1000/unit BOM cost • Security – Hardware encryption, Open-Source • Identification – Electronic Signature Analysis

  3. Challenges for Time Synchronization • Sensor data needs to have accurate timestamps • Current state-of-practice is to use Global Positioning System (GPS) • Often inoperative inside buildings • Can be affected by clouds [1] and terrestrial interference [2] • Typically do not need position *after* we have provisioned the sensor • IEEE 1588 (PTPd, http://ptpd.sourceforge.net) • We are a networked sensor, so this makes sense • We need a grandmaster • Can we revisit WWVB? [1] L. Vanfretti, U. Aliyu, J.H. Chow, J.A. Momoh, "System frequency monitoring in the Nigerian power system," IEEE Power & Energy Society General Meeting, Jul 2009; http://dx.doi.org/10.1109/PES.2009.5275936 [2] FCC, “Status of Testing in Connection with LightSquared’s Request for ATC Commercial Operating Authority,” IB Docket No. 11-109, Sep 2011; http://apps.fcc.gov/ecfs/document/view?id=7021711881

  4. WWVB • 70kW peak power • Reduce 17dB @second • Restore @0.2t or @0.5t

  5. Traceability Chain for NIST Time Services [3] M. Lombardi, "NIST Time and Frequency Services," NIST Special Publication 432, 2002 Edition, Apr 2003; http://tf.nist.gov/general/pdf/1383.pdf

  6. Technical Issues • Evaluation of WWVB Receiver (RadioSync RB2 from Beagle Software) • Did not go particularly well • Serial connectivity issues • Would not acquire signal inside Engineering Technology Facility building (my office) • WWVB signal may be problematic inside buildings • NIST provides much guidance on analog design principles • Most implementations do not seem to follow these for various reasons… • We have a powerful digital signal processor, can we use it?

  7. Time-Domain Representation of WWVB(not to scale) [4] • Accurate determination of the On-Time Marker (OTM) • Provides the ‘time synchronization’ point for all sensors • Problem: Skywave vs. Groundwave • Problem: Path Length Delay Estimation [4] M. Lombardi, “The Evolution of Time Measurement, Part 5: Radio Controlled Clocks” IEEE Instrumentation & Measurement Magazine, Apr 2012;http://tf.nist.gov/timefreq/general/pdf/2536.pdf

  8. WWVB Signal Propagation Direct Wave Ground Wave • Basic signals are OTH (over-the-horizon) ground-wave propagated • Basic time measurements are ground-wave based • “3-D” WWVB receiver processes sky-wave components to minimize sky-wave contributions (and errors) via DSP of received waveforms Ground Target Ionosphere Main+ Diff. RXs Sky Wave f = 60 kHz TX1 Height ~ 70-110 km

  9. Ground-wave Propagation Issues • WWVB OTM timing shifts due to multipath over complex terrain • Carrier phase shifts due to multipath contributions TX Ridge RX RX RX (Ideal) TX Amplitude time

  10. ORNL Software-Defined PLL Design [5] M. Lombardi, “How Accurate is a Radio Controlled Clock?” Horological Journal, Mar 2010, pg. 108-111; http://tf.nist.gov/general/pdf/2429.pdf

  11. Design Characteristics • Use a sampling clock that is locked to the OTM • Reduce the skywave contribution using an H-field antenna • Reduce multi-path interference using the FIR Multipath Equalization technique [6] for skywaveand groundwave • The filter coefficients are kept in a database indexed by location (approx 72 regions), time-of-day (hourly) and season-of-year (monthly) • Track 60 kHz received signal power by estimating signal/noise • Compensate for exponential decay in Ft. Collins transmitter • Use location information to estimate path-length delay, indexed to season-of-year (for Groundwave propagation) • Use the OTM to calculate the number of cycles we have clocked • >>> drive the sample rate error to 0 [6] R.L. Fante, J.J. Vaccaro, "Evaluation and reduction of multipath-induced bias on GPS time-of-arrival," IEEE Transactions on Aerospace and Electronic Systems, vol.39, no.3, pp. 911- 920, July 2003; http://dx.doi.org/10.1109/TAES.2003.1238745

  12. New H-field Antenna • Offers much lower noise levels than standard E-field whip antenna • Developed as offshoot of ORNL internal R&D program to solve high E-field noise pickup noted during Theater Positioning System (TPS) onsite testing • Smaller package than whip but requires some additional electronics (two preamps vs. one) • Prototype works well but needs optimization (for 60 kHz) • Antenna can also be integrated with GPS antenna if desired • Can be modified to provide TPS vectors using DSP

  13. LF H-Field Antenna Details • Quadrature-mounted ferrite-rod antennas are sampled and separately amplified • The two are vector-combined for desired H & V patterns; no phase shift versus orientation (after calibration) • Patterns reject sky-wave and off-axis signals

  14. ORNL Clock R&D Progress • Developed new ultra-stable oscillator topology (IT cut, 5th overtone mode) • Simulation & characterization efforts started; 3 test chips done • Initial circuit results positive: very low noise & drift • Full-range temperature testing: very stable waveform • Dual-mode oscillator design, simulation, and testing in process • Patent application on new ORNL Multimode Oscillator for Wide-Range Temperature Stability submitted

  15. Precision Crystal Differential Oscillator (Main Mode) Differential Oscillator (T-Mode) ORNL Dual-Mode Quartz Oscillator Crystal Mode Frequencies …. • Quartz crystal is operated in multiple vibration modes • Advanced oscillator circuit reduces LF phase noise & improves stability • Main mode is corrected and used as final output • “T” mode(s) are used for temperature measurement • Very high-stability TCXO replaces ovenizedunits X Feedback Mixer Signal Processor Final Output Ext. Ref. Key advantage: Very low power + stability Thermometric Difference Frequency Main-Mode Frequency T-Mode Frequency Final Corrected Frequency

  16. Prototype Oscillator Performance 35-dB Improvement @ 1 mHz Single-Ended Differential

  17. Key Component List • Fully finished evaluation boards represent reasonable estimate of custom development

  18. Summary – Leveraged Efforts at ORNL • Revisting WWVB as timing reference is possible due to advances in low power high performance computing capability • Novel H-Field Antenna & Magnetometer R&D • High-performance oscillators optimized for RF and timing • Ground-wave Propagation Studies (M.S. thesis at UTK) • Advanced signal-processing techniques directly applicable to accurate WWVB time reception

More Related