Reducing Vibration-Induced Phase Noise - New
Ideas

The reduction of crystal oscillators' vibration sensitivity is
currently receiving the attention of many in the frequency control industry
including both crystal and oscillator manufacturers. Oscillators have reached
the level of performance where a person walking across the room can degrade the
close-in phase noise and a relatively quiet fans can generate sidebands well
above the noise floor of the better oscillators. Present techniques to reduce
vibration sensitivity include the reduction of the crystal's acceleration
sensitivity, vibration isolation
systems, and active compensation. One compensation technique uses an
accelerometer aligned with the crystal's sensitivity vector to generate a
correction signal and another technique uses two crystals oriented such that the
acceleration vectors point in opposite directions. A refinement of the latter
technique scales the contribution by each crystal to the final frequency so that
crystals with different acceleration sensitivities may be used. These techniques
have been successfully implemented but they are somewhat expensive and difficult
to manufacture.

In an effort to simplify the manufacture of vibration compensated
oscillators, three new ideas are being investigated. One technique is an
extension of the two crystal approach mentioned above. In this new
implementation, the crystals are mounted with the acceleration vectors pointing
in the same direction which usually means side-by-side on the PCB - a
far easier mounting scheme, especially if the acceleration vector is not normal
to one of the crystal package axes. Additionally, the crystals are required to
have different acceleration sensitivities, which is usually the case
with randomly chosen crystals. Two oscillators using the crystals are phase
locked and, since the acceleration sensitivities are different, the tuning
voltage contains vibration information. This tuning voltage is scaled and
applied to both oscillators using a bootstrapping technique that avoids
destabilizing the PLL. (For a more detailed description, see: Bootstrapping a Phase
Locked Loop for Better Performance.)

Another technique uses the crystal as its own accelerometer by
detecting audio frequency piezoelectric voltages directly on the crystal plates.
The crystal is operating in a manner similar to an electret microphone and the
audio amplifier must have a very high input impedance. The RF oscillator circuit
is coupled to the crystal through a small value capacitor to reduce loading of
the audio signal and a self-resonant choke isolates the audio amplifier from the
RF signal. In order for this compensation scheme to work the microphonic
sensitivity vector must align with the frequency sensitivity vector. Digital
signal processing may be used to correct frequency response differences.

In certain oscillator topologies, easily detected amplitude
modulation due to vibration has been observed. Again, the vector properties
would need to match the frequency modulation vector but such a match may be
likely if the mechanism of modulation is the same. The AM detector output would
require signal conditioning but a superficial examination of the signals
suggested that a fairly simple "emphasis" circuit may be all that is
necessary.