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Home > Technology List > A hybrid approach shows great promise in achieving frequency synthesiz

A hybrid approach shows great promise in achieving frequency synthesized microwave signals with low phase both close to and far from the carrier.

Update Time: 2019-12-20 14:06:32

A hybrid approach shows great promise in achieving frequency synthesized microwave signals with low phase both close to and far from the carrier.

Clean, quiet frequency synthesizers are essential for modern communications systems. But the performance of RF/microwave frequency synthesizers is often tied to a lower-frequency reference oscillator, such as an oven-controlled crystal oscillator (OCXO), and great effort is often required to produce a microwave frequency synthesizer with low phase noise. To demonstrate, a 10.24-GHz frequency synthesizer with OCXO reference source was developed, and the design path to that synthesizer will be traced. Phase noise is a vital parameter for oscillators and synthesizers in communications and other systems. It is measured as the ratio between the power density in one phase noise modulation sideband, per hertz, and total signal power.1 Typically, a wideband synthesizer will exhibit more noise than a singlefrequency synthesizer. When low noise is required, a singlefrequency synthesizer can be combined with frequency mixers to build a wideband synthesizer with low noise. The phase noise of the signal produced by the synthesizer is determined by the performance of the oscillator used to build the synthesizer, the performance of the reference, and the transfer characteristic and intrinsic noise of the synchronization method. Major contributors to phase noise are the internal oscillator and the reference source (for noise offset far from and close to the carrier, respectively). An OCXO may be used as the reference for low-noise applications, possibly locked to a rubidium clock or a 1-pulse-per-second GPS source.

The phase noise performance of the oscillators has a fundamental limit imposed by the Johnson-Nyquist theory. A resistor at room temperature (300 K) produces about −173.82-dBm/ Hz noise, with this power level equally split in two sidebands. A signal with 0-dBm power will have a lower phase-noise limit of −177 dBc/Hz, improving upon −177 dBc/Hz only if the signal carries more than 0-dBm power, as with some lownoise OXCOs.2,3 The best low-noise oscillators are capable of approaching this limit at far offset frequencies. For close-in frequency offsets the phase noise will be determined by the quality factor (Q) of the resonator. The frequency offset at which the phase noise approaches the theoretical limit is roughly proportional to the frequency of the oscillator, increasing as the frequency increases. Figure 1 shows typical performance levels for different low-noise oscillators, including a 100-MHz OCXO, a 1-GHz surface-acousticwave (SAW) oscillator, a 10-GHz dielectric-resonator oscillator (DRO), a 10.24-GHz sapphire oscillator (SO), and a 10-GHz opto-electronic oscillator (OEO).4-10

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