Ultra-stable Oscillator (USO)
PHARAO USO (made by CMAC)
PHARAO’s USOs must meet very stringent performance requirements in terms of frequency stability and phase noise, as well as temperature sensitivity and very low G sensitivity.
PHARAO's USO design
PHARAO’s USOs are based on the second generation of DORIS USOs designed from 1990 to 1995 and qualified in 1997 under CNES contracts to CMAC Argenteuil France. This USO uses a very-high-performance quartz crystal resonator with electronics selected to ensure a very low phase noise near the carrier. To have a very low thermal sensitivity in air and in vacuum, the crystal and electronics—oscillator, separator, amplifier, etc.—are placed in a double dewar and temperature is controlled to within 0.01°C in the qualification temperature range. Mechanical analysis was conducted 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 for noise, using dedicated phase noise benches, and to select the best parts for PHARAO. It was decided that a 5-MHz high-overvoltage quartz 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 USO performance
The table below indicates the main performance parameters obtained on FM 34 and FM 35. Frequency stability for 
= 1 s and 10 s 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 a reference, which shows the absolute performance values of the FM USO.
World's best USO used to test PHARAO's USO
| Items | Specifications | Goals | FM 34 | FM 35 |
| Frequency stability = 1s = 10s |  y() = 1 x 10-13y() = 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/°C | in 0 to 48.5°C | 1.9 x 10-13/°C | 1 x 10-12/°C |
| G sensitivity | = 1.8 x 10-9/g | = 1.8 x 10-10/g | 1.69 x 10-9/g | 1.87 x 10-9/g |
PHARAO low G sensitivity USOs
However, due to micro-vibration disturbances between 0.1 Hz and 50 Hz onboard the ISS, the G sensitivity of the two FMs was too high to obtain the required stability performance of 1 x 10-16 where = 1 day over a one-week continuous measurement period. It was decided that studies on an auto-suspended quartz (QAS, BVA type) resonator at 5 MHz would be conducted. A bench of several crystal QAS resonators was manufactured and measured for G sensitivity first, and then selected over all electrical and phase noise parameters.
The table below indicates the performance values obtained on the QAS 5-MHz resonator selected for the FM 45 USO.
| Items | Specifications | Goals | FM 45 resonator |
| Frequency stability = 1s = 10s | y() = 1 x 10-13y() = 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 |
These performance values were confirmed on the FM 45 when it was integrated, and then at microwave source level at 100 MHz and 9.2 GHz.
