August 9, 2016

Ultra-stable Oscillator (USO)

USO
PHARAO USO (made by CMAC)

Pharao USOs should have very stringent performances in terms of frequency stability and phase noise, but also in terms of temperature sensitivity and very low G sensitivity.

PHARAO's USO design

The Pharao USOs are based on the second generation of DORIS USOs which were designed from 1990 to 1995 and qualified in 1997 under CNES contracts to CMAC Argenteuil France Company. This USO uses a very high performance quartz crystal resonator associated to an electronics selected to have a very low phase noise near the carrier. To have a very low thermal sensitivity in air and in vacuum, crystal and electronics (oscillator, separator, amplifier, etc.) are placed in a double oven in a dewar and temperature control within 0.01°C in the qualification temperature range. Mechanical analysis was made to ensure robustness, and qualification model EQM 25 @ 10 MHz and EQM 33 @ 5 MHz were tested at 45 g rms.

To have the best phase noise and short term stability, it was necessary to measure several resonators and transistors in noise, using dedicated phase noise benches, and to select the best pieces for Pharao. It was decided that a 5 MHz QHS (Quartz Haute Surtension) resonator would be used, and the best resonators would be used on several criteria (electrical parameters as well as short term stability and ageing).

PHARAO's USO performances

The following Table indicates the main performances obtained on FM 34 and FM 35. Frequency stability for = 1s and 10s and phase noise were measured in air and in vacuum by C Mac using a best USO as reference, and by LNE-SYRTE using a Cryogenic Sapphire oscillator and a very low noise test bench as reference which shows the absolute values of the FM USO performances.

Australian oscillator
World's best USO used to test PHARAO's USO

ItemsSpecificationsGoalsFM 34FM 35
Frequency stability
= 1s
= 10s
 
y() = 1 x 10-13
y() = 1 x 10-13
 
7 x 10-14
7 x 10-14
 
6 x 10-14
6 x 10-14
 
7 x 10-14
7 x 10-14
Phase noise @ 5 MHz in dBrd²/Hz
Distance to the carrier
0.1 Hz
1 Hz
10 Hz
100 Hz
1 kHz
 
 
-98
-128
-144
-153
-153
 
 
-101
-131
-147
-153
-153
 
 
-102
-130
-144
-152
-153
 
 
-100
-130
-144
-152
-154
Thermal sensitivity= 2.5 x 10-12/°Cin 0 to 48.5°C1.9 x 10-13/°C1 x 10-12/°C
G sensitivity= 1.8 x 10-9/g= 1.8 x 10-10/g1.69 x 10-9/g1.87 x 10-9/g
PHARAO FM main performances

PHARAO low G sensitivity USOs

However, due to microvibration disturbances between 0.1 Hz to 50 Hz onboard the ISS, the G sensitivity of the 2 FMs was too high to obtain the Pharao stability performance of 1 x 10-16 versus = 1day in one week continuous measurement period. It was decided that studies on a QAS (Quartz Auto Suspendu, BVA type) resonator at 5 MHz would be conducted. A bench of several crystal QAS resonators was manufactured and measured in G sensitivity first, and then selected over all electrical and phase noise parameters.

The following table indicates the performances obtained on the QAS 5 MHz resonator selected for the FM 45 USO.

ItemsSpecificationsGoalsFM 45 resonator
Frequency stability
= 1s
= 10s
y() = 1 x 10-13
y() = 1 x 10-13
 
7 x 10-14
7 x 10-14
 
7.5 x 10-14
to be measured in FM 45
G sensitivity= 1.8 x 10-9/g 1.76 x 10-10/g
USO FM45 resonator performances (selected for low G sensitivity)

These performances have been confirmed on the FM 45 when it was integrated, and then at microwave source level at 100 MHz and 9.2 GHz.