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lecture-7-cho_pte587_sensornet1-feb-24-2011

# Offsets from ref usc 51 pte587 frequency compensaon

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Unformatted text preview: Sink 3/2/11 USC  ­ PTE587 44 Time Synchroniza;on 14.85 13.86 12.87 11.88 10.89 9.90 8.91 7.92 6.93 5.94 4.95 3.96 2.97 1.98 0.99 0 •  •  •  •  9.9 9.9 11 14 9.9:11:14 0 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 14.975 T2 = T1 + Propaga;on Delay + Timer Oﬀset1 T4 = T3 + Propaga;on Delay – Timer Oﬀset2 Propaga;on Delay ≈ ( ( 12  ­ 9.9 ) + ( 14.85  ­ 14 ) ) / 2 = 0.975 T1 T1 T4 T3 T1 T1 T4 14 Timer Oﬀset ≈ ( ( 12  ­ 9.9 )  ­ ( 14.85  ­ T3 ) ) / 2 = 0.125 0.125 3/2/11 USC  ­ PTE587 45 Example: Ranging 15.475 14.85 15 15 15.5 0.125 14.975 • T2 = T1 + P = (Time Arrived – T Oﬀset1 •   Distance ropaga;on Delay + Timer ime Sent) x Speed of the Signal •  T4 = T3 + Propaga;on Delay – Timer Oﬀset2 •   If Acous;c, Distance 1 (15.475 (–1 4.85  ­x 14 ) ) / 2 m 0.975 = T 15) 340.29 = eters/sec ≈ • Propaga;on Delay ≈ ( ( 12  ­ 9.9 ) + T4 T3 T 1 162 Oﬀset ≈ (+ E11  ­ f9rom T ime S ync ) ) / 2 = 0.125 meters ( T T.9 )  ­ ( 1 4.85  ­ T 4 rror T4 13 •  Timer 2 1 3/2/11 USC  ­ PTE587 46 Sources of Errors •  Local Clock –  Environmental –  Oscillator behavior •  Pla{orm conﬁgura;on Blink { } implementa;on { components Main, BlinkM, SingleTimer, LedsC; Main.StdControl  ­> BlinkM.StdControl; Main.StdControl  ­> SingleTimer.StdControl; BlinkM.Timer  ­> SingleTimer.Timer; BlinkM.Leds  ­> LedsC; } –  Hardware latencies –  Sooware processing •  Propaga;on –  Environmental –  Distance 3/2/11 USC  ­ PTE587 47 Oscillators and Frequency Stability   Temp vs. Freq drio curves •  Frequency Tolerance •  Frequency Stability F +25C 3/2/11 USC  ­ 48 PTE587 Exploi;ng the Diﬀerences •  Two Inexpensive Crystals –  –  –  –  Exploit diﬀerences in the stability Diﬀeren;al Frequency Error (DFE) Variety of inexpensive b...
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