Noise characterization of analog to digital converters for amplitude and phase noise measurements.

Improvements on electronic technology in recent years have allowed the application of digital techniques in phase noise metrology, where low noise and high accuracy are required, yielding flexibility in system implementation and setup. This results in measurement systems with extended capabilities, additional functionalities, and ease of use. In most digital schemes, the Analog to Digital Converters (ADCs) set the ultimate performance of the system; therefore the proper selection of this component is a critical issue. Currently, the information available in the literature describes in depth the ADC features only at frequency offsets far from the carrier. However, the performance close to the carrier is a more important concern. As a consequence, the ADC noise is, in general, analyzed on the implemented phase measurement setup. We propose a noise model for ADCs and a method to estimate its parameters. The method retrieves the phase modulation and amplitude modulation noise by sampling around zero and maximum amplitude, a test sine-wave synchronous with the ADC clock. The model allows discriminating the ADC noise sources and obtaining the phase noise and amplitude noise power spectral densities from 10 Hz to one half of the sampling frequency. This approach reduces the data processing, allowing an efficient ADC evaluation in terms of hardware complexity and computational cost.

Simple-design ultra-low phase noise microwave frequency synthesizers for high-performing Cs and Rb vapor-cell atomic clocks.

We report on the development and characterization of novel 4.596 GHz and 6.834 GHz microwave frequency synthesizers devoted to be used as local oscillators in high-performance Cs and Rb vapor-cell atomic clocks. The key element of the synthesizers is a custom module that integrates a high spectral purity 100 MHz oven controlled quartz crystal oscillator frequency-multiplied to 1.6 GHz with minor excess noise. Frequency multiplication, division, and mixing stages are then implemented to generate the exact output atomic resonance frequencies. Absolute phase noise performances of the output 4.596 GHz signal are measured to be -109 and -141 dB rad(2)/Hz at 100 Hz and 10 kHz Fourier frequencies, respectively. The phase noise of the 6.834 GHz signal is -105 and -138 dB rad(2)/Hz at 100 Hz and 10 kHz offset frequencies, respectively. The performances of the synthesis chains contribute to the atomic clock short term fractional frequency stability at a level of 3.1 × 10(-14) for the Cs cell clock and 2 × 10(-14) for the Rb clock at 1 s averaging time. This value is comparable with the clock shot noise limit. We describe the residual phase noise measurements of key components and stages to identify the main limitations of the synthesis chains. The residual frequency stability of synthesis chains is measured to be at the 10(-15) level for 1 s integration time. Relevant advantages of the synthesis design, using only commercially available components, are to combine excellent phase noise performances, simple-architecture, low-cost, and to be easily customized for signal output generation at 4.596 GHz or 6.834 GHz for applications to Cs or Rb vapor-cell frequency standards.

Doppler-stabilized fiber link with 6 dB noise improvement below the classical limit.

It is known that temperature variations and acoustic noise affect ultrastable frequency dissemination along optical fiber. Active stabilization techniques are adopted to compensate for the fiber-induced phase noise. However, despite this compensation, the ultimate link performances are limited by the delay-unsuppressed noise that is related to the propagation delay of the light in the fiber. We demonstrate a post-processing approach which enables us to overcome this limit. We implement a subtraction algorithm between the optical signal delivered at the remote link end and the round-trip signal. In this way, a 6 dB improvement beyond the delay-unsuppressed noise is obtained. We confirm the prediction with experimental data obtained on a 47 km metropolitan fiber link and propose how to extend this method for frequency dissemination.

A simple method for collection and purification of chicken peritoneal macrophages.

The authors describe a simple method for collection and culture of chicken peritoneal macrophages. The macrophages are collected from peritoneum with polystirol disks on the surface of which they are subsequently cultured in vitro.