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Broadband PXI Local Oscillator Modules with Low Phase Noise and Fast Switching Speed

19th October 2011

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Whitepaper

This paper presents two local oscillator modules developed for PXI instrumentation. The modules are based on a novel, patented PLL design that combines fast switching speed, low phase noise, and low spurious characteristics. The modules cover 3-10 and 2-20 GHz frequency ranges respectively. The step size is 0.1 Hz for both modules. The measured phase noise at 10 kHz offset is about –121 dBc/Hz at 10 GHz and -115 dBc/Hz at 20 GHz respectively. The switching time is less than 500 microseconds to be within ±50 kHz from the final frequency for any frequency step within the entire operating range. Spurs do not exceed the –70 dBc level. Both developed modules occupy two standard 3-U PXI slots and can be used in a variety of synthetic instruments based on the PXI platform.

PXI synthetic instrumentation offers a cost-effective modular approach for building complex test-and-measurement equipment [1-2]. The performance of synthetic instruments primarily depends on technical characteristics of their core modules. The frequency synthesizer is one of the key elements
of virtually any RF/microwave system. It generates a stimulus signal or is used as a local oscillator (LO) in a variety of up- and down-conversion schemes. Thus, the industry feels persistent pressure to deliver higher-performance synthesizer designs.

The ideal synthesizer should be broadband with a fine frequency resolution that allows addressing a wider range of potential applications. Aside from frequency coverage and resolution, phase noise and spurious content are critical parameters that impose the ultimate limit in the system’s ability
to resolve signals of small amplitude. This is an important consideration in spectrum analysis applications. Another key parameter of the synthesizer that impacts overall system performance is the frequency switching speed [3-4]. The time spent by the synthesizer transitioning between frequencies
becomes increasingly valuable since it cannot be used for data processing. While many systems still work adequately with millisecond switching speeds, newer requirements demand microsecond operation along with the good spectral purity of traditional lower-speed designs.

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