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# Frequency electronics, inc. Rb and quartz oscillators for space applications

## 1. FREQUENCY ELECTRONICS, INC.

Rb and Quartz Oscillators for Space ApplicationsMartin Bloch

1

## 2. Frequency Electronics, Inc.

• FEI was started 51 years ago with amission to bring precision timing

technology from the laboratory to the

real world

• FEI is a Long-Recognized Technology

Leader

High

Precision Timing, Space Applications

Low Phase Noise Microwave Sources and

Synthesizers

Low “G”- Sensitivity Clocks

Timing/Frequency Systems for Severe

Environments

2

## 3. Development of Rb and Quartz Oscillators for Space Applications

1.2.

3.

Quartz Clock Performance in Space

Rb Atomic Clocks in Space – The Results So Far

Next Generation Rb Atomic Clock for Space

Applications

A.

B.

Design, Rb Atomic Frequency Standard (RAFS)

Performance Results, RAFS

3

## 4. Introduction

• For the past 50 years, most satellite systems have usedprecision quartz oscillators as the time and frequency

reference

– Reliable

– Low Power

– Light Weight

• Atomic Frequency Standards have demonstrated

reliable operation in space during the last 25 years on a

limited number of programs

–

–

–

–

GPS

Glonass

Milstar/AEHF

Galileo

4

## 5. Technology Trade-offs

• Quartz oscillators are the most reliable, lowest costtechnology for providing precision frequency and time in

space applications

• Quartz is inherently sensitive to nuclear radiation

• Atomic frequency standards cost more, are less reliable,

but provide improved frequency stability, and are

relatively immune to radiation

Frequency Stability (Allan deviation)

10 sec

100 sec 1000 sec

10,000 sec

100,000 sec Drift per day

Technology Power

Weight 1 sec

Quartz

2 Watts < 1 kg

1.00E-13 1.00E-13 1.00E-13

3.00E-13

1.00E-12

3.00E-12

1.00E-12

Rubidium 15 Watts 2 - 5 kg

1.00E-12 3.00E-13 1.00E-13

3.00E-14

1.00E-14

1.00E-14

7.00E-15

5

## 6. Development of Rb and Quartz Oscillators for Space Applications

1. Quartz Clock Performance in Space2.

3.

Rb Atomic Clocks in Space – The Results So Far

Next Generation Rb Atomic Clock for Space

Applications

A.

B.

Design, Rb Atomic Frequency Standard (RAFS)

Performance Results, RAFS

6

## 7. State of the Art Quartz Performance in Space

• Usage of “Premium Q Swept Quartz” or radiationhardened quartz material.

• SC-cut crystals (SC-cut crystals stabilize faster than ATcut crystals. The retrace of SC-cut crystals is orders of

magnitude better than AT-cut crystals).

• 5th overtone resonators (aging is significantly affected

by the thickness of the resonator, hence, the thickest

quartz blank should be used at the highest practical

overtone for best aging performance).

• Crystals exhibiting monotonically-positive aging slope

(radiation offsets the positive aging trend of quartz as

further explained below).

7

## 8. Radiation Effects on Quartz

Quartz sensitivity to most radiation

has been shown to be

approximately:

-1E-12 per Rad

Typical background radiation for

geosynchronous orbit is:

6 Rads/day

Typical frequency aging on earth

is ~1E-11/day for high precision

quartz oscillators

On-orbit compensation resulting in

frequency aging of ~1E-12/day is

possible for positive aging quartz

resonators

Quartz sensitivity to protons is

similar magnitude, but less

predictable; and can result in

positive or negative frequency

changes

Even with compensation for background

radiation:

• best performance observed in space is

~2E-13/day

• effect of solar flare is 1E-10 to 4E-10 over

several days

8

## 9. State of the Art Quartz Oscillator

-133Lᵠ(f) = -133 dBC/Hz @ 1 Hz offset

ADEV = 7E-14 @ 10 sec

9

## 10. Development of Rb and Quartz Oscillators for Space Applications

1.Quartz Clock Performance in Space

2. Rb Atomic Clocks in Space

– The Results So Far

3.

Next Generation Rb Atomic Clock for Space

Applications

A.

B.

Design, Rb Atomic Frequency Standard (RAFS)

Performance Results, RAFS

10

## 11. Overview

• Compared to crystal oscillators, atomic clocks are inherently insensitiveto space radiation.1

• Compared to other types of atomic clock (e.g., Cs),

Rb clocks have the advantage of size, weight, and power,

Rb clocks have the (believed) disadvantage of relatively large frequency

aging.

Parameter

3

Rb Clock

size (in )

100

weight (kg)

2.5

power (W)

20

frequency aging (day-1) 10-12 to 10-13

Cs Clock

1000

20

50

< 10-14

• To better understand the utility of Rb clocks for long-term space

missions, we have investigated Milstar Rb clocks (on-orbit) over the past

two decades.

• Contrary to popular mythology,

Rb clocks can have exceptionally long life with stable operation,

Rb clocks can have extremely low frequency-aging rates: 10-14 to 10-15/day

1) J. Camparo, S. Moss, & S. LaLumondiere, Space-system timekeeping in the presence of solar flares, IEEE

Aerospace and Electronic SYSTEMS Magazine, 19(5), 3-8 (2004).

11

## 12. Milstar Rb Clocks

• The Milstar Rb atomic clocks are manufactured by FrequencyElectronics, Inc. (FEI)

• The clock is a classical Rb vapor-cell design, with optical

excitation using an rf-discharge lamp.2

– The clock has a 10 year design life.

– The clock has a weight of 2.3 kg (i.e., 5 lbs.).

– The clock frequency can be tuned with a resolution better than 1

10-12.

Solenoid Windings

Microwave Cavity

To Generate VCXO

Correction Signal

87Rb

rf-Discharge

Lamp

85Rb

6,834.7 MHz

Filter Cell

Resonance Cell:

87Rb

Photodiode

& Buffer Gas

2) T. McClelland, I. Pascaru, M. Meirs, Development of a Rb Frequency Standard for the MILSTAR Satellite

System, 41st Annual Symposium on Frequency Control, 1987, p. 66

12

## 13. On-Orbit Data: Rb Satellite Clocks Functional Fit: y(t) = Ae-gwt + Be-get + Dt

• Empirically, we find that as a family the data is well fit by a biexponential plus linear term:g

t

g

t

w

e

y

t

A

e

B

e

D

t

• Clock “warm-up”

– Ae-gwt

– This term may be due to movement of the liquid Rb pool in the discharge

lamp.

• Clock Equilibration4

– Be-get

– The mechanism of equilibration is not yet understood.

– Nevertheless, helium permeation remains an open possibility.

Linear Frequency Aging

– Dt

– The mechanism of linear frequency aging is not yet understood.

13

## 14. Milstar Rb Satellite Clocks Summary

• Six clocks have operated on orbit (data for three presentedhere)4

• Longest operating time on-orbit is 14.8 years

• Including estimates for ground operation, 3 of these clocks

have logged more than 18 years of operation!

Clock ID

A

B

C

D

E

F

A term

time constant

(1/γw)

(in months)

A

-10

-2.91 10

-10

-2.12 10

-10

-2.00 10

-10

-15.3 10

-10

-0.91 10

1.4

0.2

3.4

4

0.7

B term

time constant

(1/γe)

(in years)

B

-10

-7.62 10

-10

-0.00 10

-10

-7.44 10

+9.96 10

-10

-5.64 10

-10

N/A

-2.43 10

-10

1.62

-2.20 10

-0.00 10

averages -3.87 10

-10

1.13

Operating time (years)

Ground

total

On-Orbit (estimate) (estimate)

D term

(1/day)

-14

-(2.7 ± 0.2) 10

N/A

+(0.07 ± 0.11) 10

1.49

-14

0.36

0.76

-10

0.59

-10

0.72

12.7

5.9

18.6

6.1

2.4

8.5

14.8

3.4

18.2

-14

+(2.3 ± 0.2) 10

-14

8.4

2.2

10.6

-14

13.9

4.3

18.2

-14

5.8

1.9

7.7

10.3

3.3

13.6

+(0.24 ± 0.16) 10

-(3.6 ± 0.1) 10

-(7.3 ± 2.2) 10

-1.83 10

-14

4) J. Camparo, T. McClelland, and J. Hagerman, “Long Term Behavior of Rb Clocks in Space,” European

Frequency and Time Forum, 2012.

14

## 15. Development of Rb and Quartz Oscillators for Space Applications

1.2.

Quartz Clock Performance in Space

Rb Atomic Clocks in Space – The Results So Far

3. Next Generation Rb Atomic Clock

for Space Applications

A.

B.

Design, Rb Atomic Frequency Standard (RAFS)

Performance Results, RAFS

15

## 16. Design Goals

• Precision Rubidium Atomic Frequency Standard(RAFS) for Space Applications

– Best possible frequency stability

– 20 year operating life

• Emphasis on frequency stability rather than size, weight

and power (SWAP)

• “Classical” Rb vapor frequency standard

– Optical pumping using Rb lamp to create ground state hyperfine

population difference

– Temperature controlled filter cell with Rb85

– Temperature controlled resonance cell, with Rb87, in microwave

cavity

• One prototype, 3 engineering models

16

## 17. Development of Rb and Quartz Oscillators for Space Applications

1.2.

3.

Quartz Clock Performance in Space

Rb Atomic Clocks in Space – The Results So Far

Next Generation Rb Atomic Clock for Space

Applications

A. Design, Rb Atomic Frequency Standard

(RAFS)

B.

Performance Results, RAFS

17

## 18. Rb Atomic Frequency Standard (RAFS)

• 15 x 4.6 x 5.2 inches(381 x 117 x 132 mm)

• 16.5 Lbs (7.5 kg)

• 28 VDC Input

• 30 Watts

• 10 MHz Output

18

## 19. Block Diagram

• Temperature controlled chassis (±1°C) operatesfrom -34°C to +25°C

• Modular Design

• Digital Rb control loop

19

## 20. Digital Rb Control Loop

• Digital processing of analog signal from physicspackage

• Space qualified FPGA

• Direct Digital Synthesizer (within FPGA) to tune

output frequency (1 x 10-14 tuning resolution)

20

## 21. 6.8 GHz Frequency Synthesizer

• Phase locked CRO at2.278 GHz

• Output signal at 6.8

GHz has no

sidebands within a

±2.278 GHz window

21

## 22. Radiation Hardening

• Component selection• FPGA

– Frequency setting stored on select resistors connected to input

pins

– Fuse programmed (write once)

– Hardware triple redundant logic, with three way voting to

minimize single event effects

– Software triple redundant logic with 3 way voting of critical

values (digital output to DAC (quartz oscillator control voltage))

• Radiation shields

– Chassis, covers (material and thickness)

– Spot shields for critical components

22

## 23. Development of Rb and Quartz Oscillators for Space Applications

1.2.

3.

Quartz Clock Performance in Space

Rb Atomic Clocks in Space – The Results So Far

Next Generation Rb Atomic Clock for Space

Applications

A.

Design, Rb Atomic Frequency Standard (RAFS)

B. Performance Results, RAFS

23

## 24. Relative Frequency vs Time prototype unit in vacuum

24## 25. Allan Deviation Prototype Unit in vacuum

25## 26. Allan Deviation multiple units, in vacuum

1.00E-11Allan Deviation

1.00E-12

Performance Goal

SN 00, Feb 2010

SN 02, Sept 2010

SN 02, Aug 2010, corrected

SN 03, Sept 2010

maser 20 vs maser 73

1.00E-13

1.00E-14

1.00E-15

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

Averaging Time (seconds)

26

## 27. Warm-Up and Stabilization SN 01 in vacuum, -4°C

Warm-Up and Stabilization1.0E-09

70

0.0E+00

65

-1.0E-09

60

-2.0E-09

55

-3.0E-09

50

-4.0E-09

45

-5.0E-09

40

-6.0E-09

35

0

1

2

3

Input Power (Watts)

Relative Frequency

SN 01 in vacuum, -4°C

Frequency

DC Power

4

time (hours)

27

## 28. Initial Warmup SN 01 in vacuum, -4°C

1.E-0970

Rb loop locked

(13 min.)

65

-1.E-09

60

-2.E-09

55

-3.E-09

50

-4.E-09

45

-5.E-09

40

-6.E-09

35

0

0.25

0.5

0.75

Input Power (Watts)

Relative Frequency

0.E+00

Frequency

DC Power

1

time (hours)

28

## 29. Frequency vs Temperature SN 01, in vacuum

1E-14 per °C29

## 30. Power Consumption SN 01 in Vacuum

SN 01 Power Consumption (Vacuum)Base-plate

Temp (˚C)

71

61

9

5

-4

-5

-20

-24

-34

Power

Consumption

(W)

19.91

20.52

30.85

31.61

36.06

37

42.62

44.41

48.22

30

## 31. Frequency vs Magnetic Field SN 01 in vacuum

Optical Axis (X - Axis)magnetic field

Δ

df/dH

(gauss)

(freq - fit line) (1/gauss)

3

-2.2742E-13 -7.5807E-14

-3

2.7864E-13 -9.2880E-14

Optical Axis (X - Axis)

4

3,E-12

3

Relative Frequency

4,E-12

2,E-12

2

1,E-12

1

0,E+00

0

-1,E-12

-1

-2,E-12

-2

-3,E-12

y = -9,3134E-17x + 4,1915E-13

-4,E-12

0

1800

3600

-3

5400

7200

Magnetic Field (Gauss)

mean sensitivity: -8.4344E-14

frequency

frequency (100 sec

avg)

frequency (1800 sec

avg)

magnetic field

-4

9000

time (seconds)

31

## 32. Allan Deviation SN 03 SN 03, vacuum

Unit Name:fit function

a Ln b t

start slope

1

c, with a

SN 03

1.69609 10 11 , b

1.14892 10 13 day

end slope

0.00677393, c

1.01406 10 9

8.67013 10 14 day

1.0 10 13

GOAL

5.0 10 14

Allan Deviation

START: Sun 19 Sep 2010 23 : 58 : 20

STOP: Sat 6 Nov 2010 23 : 58 : 20

7.0 10 14

SN 03

3.0 10 14

2.0 10 14

1.5 10 14

1.0 10 14

100

1000

104

105

Averaging Time seconds

106

107

32

## 33. Summary Next Generation Rb Atomic Clock

• Demonstrated performance over environments–

–

–

–

Temperature (-35°C to +71°C) in vacuum

Warmup from OFF condition (-4°C) in vacuum

Magnetic Field (-3 gauss to +3 gauss)

Continuous extended operation at 8°C in vacuum

• Performance at constant temperature in vacuum:

– Demonstrated: y( ) < 9 x 10-13 / + 2 x 10-14

for between 1 and 1,000,000 seconds

– Target:

y( ) < 7 x 10-15 at = 100,000 sec

33