allan deviation j. mauricio lópez r. time and frequency division centro nacional de metrología,...
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Allan DeviationAllan Deviation
J. Mauricio López R.
Time and Frequency Division
Centro Nacional de Metrología, CENAM
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Real Clocks
There not exist the perfect clock, all the real clocks are unestable. The output frequency of a clock changes with time. A correct mathematical tool is needed to characterise the frequency instability of oscillators.
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)2sin()( tAtV
A: amplitude: frequencyt: time
+A
-A
Mathematical model for ideal frequency signal
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Quartz oscillators are a very common equipment to produce frequency signals but their frequency output could be more or less instable as function of many parameters
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V(t) = V0 sen(20t)
V(t) =[V0 + (t)] sen[20t + (t)]
tiempo
tiempo
Mathematical model for real (instable) frequency signals
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The Time – Frequency relation
Tf
1
Frequency
Period
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Frecuencia estable (oscilador ideal)
Frecuencia inestable (Oscilador real)
Tiempo
(t)
Tiempo
(t)
V1-1
T1 T2 T3
1-1
T1 T2 T3
V(t) = V0 sin(20t)
V(t) =[V0 + (t)] sin[20t + (t)]
(t) = 20t
(t) = 20t + (t)
V(t) = salida del oscilador, V0 = Amplitud nominal pico-a-pico(t) = amplitud de ruido, 0 = frecuencia nominal(t) = Fase , (t) = ruido de fase
t d)t(d
21=
t d)t(d
21 = )t( 0
π
Φπ
V
The time – frequency relation
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Ruido de amplitud
Inestabilidad en frecuencia
Ruido de fase
-V
olt
age
+0
Tiempo
Voltage de salida de un oscilador
V(t) =[V0 + (t)] sen[20t + (t)]
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Precisión sin exactitud
Ni exactitud ni precisión
Exactitud sin precisión
Exacto y preciso
Tiempo TiempoTiempoTiempo
Estable de baja exactitud
Inestable de baja exactitud
Alta exactitud a largo tiempo e
inestable a corto tiempo
Alta estabilidad y alta exactitud
0
f fff
Exactitud y Estabilidad
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5 10 15 20 25 Time / Days
Short term instability
f/f
(p
pm
)
30
25
20
15
10
Aging and short term stability
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An example of noise in measurements
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4-23
0.1 s average time3 X 10-11
0
-3 X 10-11
f
f
100 s
1.0 s average time3 X 10-11
0
-3 X 10-11
f
f
100 s
Frequency noiseFrequency noise
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Las graficas muestran las fluctuaciones de la variable z(t), la cual puede ser, por ejemplo, la salida de un contador (f vs. t), o la medición de fase ([t] vs. t). Los gráficos muestran tanto la dependencia temporal como la dependencia en frecuencia; h es el coeficiente de amplitud.
Sz(f) = hf
= 0
= -1
= -2
= -3
name
White
Flicker
Randomwalk
Time dependence
Frequency noiseFrequency noise
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22y 2
1σ iy
222
2y 2
1σ ix
Fase
iii yyy 1
iiii xxxx 122 2
Concepto de la Varianza de Allan
ii
i
xxy
1
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Allan varianceAllan variance
1N
1i
2i1i
2y yy
1N21
σ
donde:donde:
Allan varianceAllan variance
Number of measurementsNumber of measurements
Frequency measurementFrequency measurement
2y
N
iy
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mN
iimimiy xxx
mN
2
1
222
2 222
1
donde:donde:
Allan varianceAllan variance
Númber of measurementsNúmber of measurements
Time window = mTime window = m00
Phase measurementPhase measurement
2y
Nix
Allan varianceAllan variance
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Barras de IncertidumbreBarras de Incertidumbre
Distribución Distribución 22
Para df < 100Para df < 100 2
2
2 )(y
ysdf
2ys
donde:donde:
Estimado de la Varianza de AllanEstimado de la Varianza de Allan
df Número de grados de libertadNúmero de grados de libertad
Varianza de Allan verdaderaVarianza de Allan verdadera2
y
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Distribución X2
Barras de IncertidumbreBarras de Incertidumbre
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Barras de IncertidumbreBarras de Incertidumbre
025,0975.0 2
22
2
2
dfsdfs y
yy
Tablas XTablas X22
Barra InferiorBarra InferiorBarra SuperiorBarra Superior
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Tabla XTabla X22
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Para df > 100Para df > 100
Barras de incertidumbreBarras de incertidumbre
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Barras de incertidumbreBarras de incertidumbre
22 96,12
1025,0 h
Para df > 100Para df > 100
22 96,121
975,0 h
12 dfhdonde:donde:
Barra InferiorBarra Inferior
Barra Barra SuperiorSuperior
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Número de Grados de LibertadNúmero de Grados de Libertad
White Phase ModulationWhite Phase Modulation
Flicker Phase Flicker Phase ModulationModulation
White Frquency White Frquency ModulationModulation
mN
mNNdf
2
21
4
112ln
2
1lnexp
Nm
n
Ndf
54
422
2
132
2
m
m
N
N
m
Ndf
NBS Technical note 679NBS Technical note 679
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Flicker Flicker Frequency Frequency ModulationModulation
Random-Walk Random-Walk Frequency Frequency ModulationModulation
234
5
19,43,2
22
2
mparamNm
Ndf
mparaNN
df
2
22
3
41312
N
mNmNm
Ndf
NBS Technical note 679NBS Technical note 679
Número de Grados de LibertadNúmero de Grados de Libertad
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Por debajo del ruido “fliker”, los cristales de cuarzo tipicamente tienen una dependencia -1 (white phase noise). Los patrones atómicos de frecuencia muestran una dependencia del tipo -1/2 (white frequency noise) para tiempos de promediación cercanos al tiempo de ataque del lazo de amarre, y -1 para tiempos menores del tiempo de ataque. Tipicamente los ’s para el ruido flicker son: 1 s para osciladores de cuarzo, 103s para relojes de rubidio y 105s para Cesio.
y()-1
-1
0
Tipo de ruido:
Whitephase
Flickerphase
Whitefreq.
Flickerfreq.
Randomwalk freq.
-12 12
Time window dependence of the Time window dependence of the yy(())
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Example of Allan deviation calculationExample of Allan deviation calculation
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Example of Allan deviation calculationExample of Allan deviation calculation
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ff
y
)(
tt
ff
Example of Allan deviation calculationExample of Allan deviation calculation
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Allan DeviationAllan Deviation
J. Mauricio López R.
Time and Frequency Division
Centro Nacional de Metrología, CENAM