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PLO fundamentals
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Understanding Low Phase Noise
Phase noise is the measure of the spectral purity of a signal and can be a critical attribute for many systems and applications.
PLO fundamentals
All microwave oscillators exhibit noise spectra and the mechanisms producing these are well understood. In general, the dominant effect impacting on applications is FM noise (or phase noise, to which it is physically related) which may limit sensitivity, bit rate and channel spacing (amongst other things) in a typical communication system.
There are external sources of FM noise such as mechanical vibration and radiation from nearby digital circuits and power supply lines. Elimination of all of these spurious sources still leaves the fundamental noise mechanisms in the oscillator such as flicker noise and thermal effects which exhibit a generally continuous spectrum whose amplitude is affected by the Q of the oscillator circuit.
The purpose of phase-locking the oscillator is to improve the close to carrier noise of an oscillator which has a good noise floor by superimposing a reference oscillator which has good close to carrier performance. In a typical PLO a crystal having excellent short-term stability is used as a reference for a DRO which has relatively poor short-term stability but an excellent far from carrier noise floor.
The advantage of ultra-low phase noise
Phase noise standards are implemented for good reason. By meeting or exceeding their specifications, you can optimize and maximize usage of the satellite resource. Well-designed earth stations with ultra-low phase noise have the advantages of:
- Improved availability (less susceptibility to other noise sources, even atmospheric/rain)
- Increased effective coverage area
- Reduced operating costs
- Reduced antenna size Lower civils/capital costs
- Higher throughput (combination of data rate, modulation and coding rate)
High Q Designs Improves Phase Noise Dielectric Resonators exhibit very High unload Q Factors at microwave frequencies.
Dielectric Resonator Oscillators (DROs) are microwave oscillators that use a dielectric resonator (DR) as the frequency stabilizing element in order to achieve excellent frequency stability, high Q and very low micro-phonics. The DR, when used as part of the resonating circuit of any active microwave device, produces a steady state oscillation under the right conditions at the resonant frequency of the DR.