Phase noise is random phase modulation which is present in quartz crystal oscillator. The modulation is caused by a combination of the noise figures of the crystal and the transistors used in the circuit. Phase noise is a critical factor in radar, ECM and communications applications.
Phase noise is measured in the frequency domain and is measured by beating two oscillators of like design in a double balanced mixer in quadrature. A low frequency wave analyzer or spectrum analyzer is swept along the carrier envelope and measures the power of the noise relative to the peak power of the carrier in a very narrow bandwidth, typically 1 KHz.
Phase noise that is close-in to the carrier, (5 Hz. To 500 Hz.) is usually a function of the crystal. Further out, typically 5 KHz. and beyond, phase noise is a function of the signal-to-noise ratio if the oscillator transistors and the drive level of the crystal.
Phase noise performance is degraded by frequency multiplication at the rate of 20 log (N) whereas N being the multiplication factor. A doubler in an oscillator will degrade the phase noise by 6 dB. Times 10 multiplication will degrade phase noise by 20 dB equating to about 6 dB per octave.
Phase noise can be degraded by environmental conditions such as shock and vibration. Vibration in particular can induce unwanted side-bands (see section on vibration). Both the crystal and the oscillator designs can be optimized to minimize the effects of shock and vibration through the inclusion of ruggedized mounts and bonding of components to the substrate. Crystal mounting structures for minimizing the effects of shock and vibration on phase noise include designs using four point mounts and using TO-type crystal holders: HC-35, HC-37 and HC-40.
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