MICROCONTROLLER-BASED VIBRATION COMPENSATION

Figure 1

Figure 1 VXO payload module.

A recent development at Quantic Wenzel uses a microcontroller-based, vibration-compensated OCXO called a VXO. This VXO combines an active, low phase noise, vibration-compensated ovenized oscillator and a multi-stage multiplier with a sub-Hz digital phase-locked loop that is locked to a high-stability atomic-based reference. This approach includes a low phase noise OCXO and it combines active and passive vibration-compensation techniques in a mixed-signal design to achieve broad frequency range compensation in multiple applications. The microcontroller circuitry consumes less than 150 mA and can accurately estimate the complex conjugate of the vibration-induced phase noise. The circuitry can cancel the noise in real-time, in effect becoming the “brain” of the system. Passive isolators may also be added for improved performance if space permits. The VXO payload may also “learn” its aging characteristics when locked to an Rb or Cs standard. This allows for a long-term holdover time that is useful when the external reference is disconnected or lost. A prototype incorporating this technique is shown in Figure 1. This unit is built and tested using 0.3E-9/g crystal sensitivities, indicating an effective sensitivity per axis of 0.3E-11/g from 8 Hz to 1 kHz.

Figure 2 shows the static (PN) and dynamic (DPN) SSB phase noise results from an OCXO using active vibration-compensation techniques when subjected to a random vibration profile of 0.002 g2/Hz from 8 Hz to 1 kHz. Repeated dynamic phase noise measurements indicate a consistent improvement of 20 dB for all axes from 10 to 100 Hz and greater than 15 dB improvement from 100 to 1000 Hz. When combined with a passive isolation system, this improvement becomes significant even above 100 Hz.

Figure 2

Figure 2 OCXO static and dynamic phase noise with active vibration compensation.

Figure 3

Figure 3 VXO static and dynamic phase noise with active vibration compensation.

Figure 4

Figure 4 Ruggedized conduction-cooled ATR chassis.

Similarly, Figure 3 shows the Allan Deviation plot of PN and DPN results for an 80 MHz active vibration-compensated VXO that has been subjected to 0.002 g2/Hz random vibration. In this case, the real-time, active vibration-compensation techniques are very effective in reducing the vibrations in the 0.01 to 1 second range. These techniques can be deployed in a host of systems, providing critical performance improvements in different vibration environments. Tying this all together, the VXO payload has been integrated into a VPX module and into a ruggedized ATR chassis as shown in Figure 4.

CONCLUSION

This paper presented some of the challenges and solutions that led to the development of a compact multiplied crystal oscillator with active and passive vibration compensation. Using these techniques, the crystal sensitivity has been reduced by a factor of 100. The performance improvements achieved are as follows:

  • Effective g-sensitivity of less than 0.3E-11/g
  • Size of 4 × 3 × 1 in.
  • Includes digital PLL, aging compensation and ×30 multipliers
  • Automatic calibration of all 3-axes in production
  • Vibration compensation of more than 20 dB
  • Available in a passively vibration-isolated or hard-mounted single module or a 3U VPX plug-in module with multiple low and high frequency outputs.

References

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