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The DWE USOs are designed to withstand the Cassini/Huygens launch and cruise phase, as well as the Huygens atmospheric entry and descent on Titan. The TUSO on the Huygens Probe is exposed to higher mechanical loads during the Huygens mission than the RUSO on the Cassini Orbiter. The most critical factor for the TUSO is the peak deceleration of 16.5 g during the Huygens entry phase and was the major driver for the selection of an ultrastable oscillator based on rubidium technology. It could not be guaranteed that the required frequency stability of +/- 2E-10 could be met after the probe entry into Titan's atmosphere with a state-of-the-art quartz oscillator. The high mechanical load during entry might cause a deformation of the internal quartz fastening system in combination with an unpredictable frequency offset and an unknown frequency relaxation time. A similar problem with continuously varying mechanical stresses on the quartz box was foreseen in the subsequent pressure variation from 0 to 1.5 bar during the Huygens descent phase. These adverse effects can be averted with a rubidium oscillator, for which the frequency source is also a quartz, because the nominal output frequency is locked to the very stable frequency of the rubidium ground-state hyperfine transition.
The basic principle of a rubidium oscillator is to utilize the two ground-state hyperfine levels A and B and a much higher optical level C of rubidium atoms to produce an error signal for the control circuit of a voltage-controlled quartz oscillator VCXO (for a scheme [9 Kb] look here). Infrared light from a rubidium lamp is filtered and passes through a heated rubidium resonance cell with a frequency f(C,A) = f(C) - f(A), exciting transitions to state C of the rubidium gas. After a very short time atoms in level C drop back either to state A or, with less probability, to state B. Because atoms in state A are continuously re-excited to C, the population of level B steadily increases. When state A is depopulated, a maximum photocurrent is produced in the photocell since the light is no longer attenuated by excitation processes from level A to C. The HF-signal of the syntheziser, which is upconverted from the quartz output signal, also irradiates the rubidium resonance cell. The syntheziser is calibrated to generate the exact resonance frequency f(B,A)=f(B)-f(A)=6.835 GHz if the quartz has its nominal output frequency. At this frequency atoms in level B de-excite to state A thereby inducing a minimum photocurrent, because the rubidium vapor is no longer transparent to the frequency f{CA}. Deviations from the minimum current condition are produced by deviations in the nominal VCXO output frequency. The current dip, however, is very small (0.1% of the total photocurrent) and not suitable for a DC detection. To circumvent this shortcoming, the synthesizer frequency is modulated at an audio frequency f(m) = 127 Hz. The similarly modulated photocurrent can now be AC detected by synchronous current demodulation. There is a positive correlation between the sign of the current error signal (positive or negative) and the offset from the nominal quartz output frequency. This error signal is now used to lock the nominal quartz signal to the rubidium resonance frequency by varying the quartz control voltage.
The USO consists of the Physics Package (rubidium resonance cell and lamp) and several printed circuit boards, which contain discrete electronic elements as well as integrated circuits. The USO electronics and the Physics Package are integrated into an aluminium box, which is constructed as a Faraday cage to avoid electromagnetic contamination of the USO environment. The USO box is attached to the experiment platform via 4 mounting studs, thereby providing a thin air buffer for better insulation from the mounting platform. This reduces the USO conductive heat loss, saving power that would otherwise be needed for heating.
The box surface is plated with nickel and coated with Chemglaze Z 306. The total mass of the rubidium oscillator is 1.9 kg, including the mass of the radiation shielding of the electronics and physics package. This shielding is required to minimize the effects of changes in the external magnetic fields, ranging from more than 10E4 nT on Earth to essentially zero at Titan. Variations in the ambient magnetic field induce a change in the Rb hyperfine resonance frequency and thus a frequency shift in the USO output signal. The most radiation sensitive USO components such as transistors and analogue IC's are radiation hardened up to a standard dose of 10 krad. While the maximum radiation dose for the probe TUSO is 5 krad, the RUSO on the orbiter will be exposed to about 18 krad in the event of a 1999 launch. In order to provide additional radiation protection for the RUSO, the critical components of both USO units are shielded with tantalum caps (thickness 1 mm). Radiation shielding contributes about 10% to the total USO mass budget.
A block diagram [17 Kb] of the USO unit can be found here. The Huygens supply voltage of 28 V is transformed down to 5 V and 17 V by the DC/DC converter. The converter control signal is provided by the syntheziser, synchronized to the quartz output signal. The VCXO quartz, located on the Oscillator Board, provides the 10 MHz output through a buffer amplifier. The same signal is upconverted by the Syntheziser to the rubidium resonance frequency. The photocurrent of the Physics Package is routed to the Servo Board, which generates the error signal for the voltage control of the VCXO. The heater control of the rubidium lamp within the Physics Package is located on a seperate Lamp Board. Three analogue sensors monitor the temperatures of the rubidium lamp, rubidium cell and VCXO, respectively. A bi-level lock indicator flips from "0" to "1" whenever the quartz output signal is in lock with the rubidium resonance frequency. The temperatures and lock indicator signal are part of the Huygens HK data.
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