FORMING RESERVED OUTPUT SIGNAL OF AN ATOMIC CLOCK ENSEMLE

K. Mishagin, I. Chernyshev, V. Soloviov, S. Podogova

6th International Symposium “METROLOGY OF TIME AND SPACE”

«Vremya-CH», Nizhny Novgorod, Russia

MotivationModern atomic clock ensemble etalons are required to produce continuous output signal� long continuous measurements� critical applications: communication with space

stations, etc.

Requirements to reserving system:

� Failure of certain atomic clock must not lead to phase/frequency jumps of the output signal

� Designed system is desired to be full-automatic� It is attractive to get physical signal possessing

frequency stability of ensemble weighted average

1 PPSCLOCK-1

CLOCK-2

CLOCK-n

Sw

itch

Computer

Phase comparator

orTime interval

counter

5 MHz 1 PPS

5 MHzAOG-1

AOG-2

AOG-n

1 PPS

5 MHz

1 PPS

5 MHz

Standard approach to output signal reserving

Standard approach to output signal reserving

Disadvantages:

• Long time for input signal analysis and commutation • Unavoidable phase/frequency shift during commutation• System complexity

Alternative approach∆f1,i

∆f2,i

∆f3,i

CLOCK-1

CLOCK-2

CLOCK-n

CLOCK-3M

ulti-

cha

nne

l pha

se (

fre

que

ncy)

com

para

tor

Pro

cess

or Voltage controlled quartz

oscillator5 MHz

DAC

Former of output signals

f1

fn

5, 10, 100 MHz, 1 Hz

1 PPS synch.

f2

f3

Scheme for output signal reserving in atomic clock ensemble

Real-time atomic clock

combinerVCH-317

Advantages:

� Fast detection and program detaching of failure signal� No phase/frequency shifts due to attaching/detaching

input signals� Can be realized in one device and reserved additionally � Frequency stability of output signal can be improved (it

can be better than the stability of the best reference standard)

PID-control algorithm for frequency stabilization

( ) ( )( )

∆

−=

+−−−−−∆+=

−

−−−+

1

2111

1

2DACDAC

nnn

nnnd

nnp

ni

nn

xxy

yyykyykyk

τ– relative frequency difference

– programmed frequency offset Loop sample time τ = 10 ms

Output frequency calculation

∑=

++=N

nn

Nn fwtf

1out νδ

ki, kp, kd are optimized Introduced phase noise and AVAR are minimized

Frequency instability introduced by the system

100

101

102

103

10-17

10-16

10-15

10-14

10-13

τ, s

σ y(τ

)HROG-5

AOG-110

VCH-317

Time TechClean-up osc.

No temperature stab.

in the room

No shifts in phase and frequency after adding/removing reference signal

Optimal weights selection problem

min2317

1

222out →+=∑

=σσσ

N

iiiw

∑=

−

−=

N

kk

iiw

1

2

2

)(

)()(

τσ

τστ

Output signal frequency instability is minimized for single averaging time only!Possible solutions:

1) Virtual atomic time scale + control of auxiliary oscillator 2) Multi-scale control

Frequency stability of the output signal

∑=

=N

nnn fwf

1out

∑=

−=∆N

n

Sn

Snk ywU

1

One-time-scale control algorithm:

Two-time-scale control algorithm:

( )∑ ∑= =

−−−=∆

N

n

N

m

Lm

Ln

Lm

Sn

Snk yywywU

1 1

Addition long averaging time estimation of frequency differences is required

( )∑ ∑= =

−−−=∆

N

n

N

m

Lm

Ln

Lm

Sn

Snk yywywU

1 1

Weights estimation for two-scale frequency control:

∑=

−

−=

N

k

Sk

SnS

nw

1

2

2

)(

)(

τσ

τσShort time:

Long time: ∑=

−

−

= N

k

Lk

LnL

nw

1

2

2

)~(

)~(

τσ

τσ

Estimation of frequency stability of reference signals is needed

Modeling

τS = 1 secτL = 20000 sec

Two types of reference signals: 4 + 4

∑=

−

−=

N

k

Sk

SnS

nw

1

2

2

)(

)(

τσ

τσ

∑=

−

−

= N

k

Lk

LnL

nw

1

2

2

)~(

)~(

τσ

τσ

Results of modeling. Dynamics of weights

Results of modeling

Two-scale frequency control

Summary

� Atomic clock combiner system seems to be effective for output signal reserving (fast detection and exclusion of invalid reference signals, no phase/frequency jumps)

� Simple modification of the algorithm allows to obtain output signal frequency stability better than the best input signal has for all τ

THANK YOU FOR YOUR ATTENTION!

WELCOME TO THE POSTER SECTION

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