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The speed of conventional antimicrobial susceptibility testing (AST) is intrinsically limited by observation of cell colony growth, which can extend over days and allow bacterial infections to advance before effective antibiotics are identified. This report presents an approach for rapidly sensing mechanical fluctuations of bacteria and the effects of antibiotics on these fluctuations. Bacteria are adhered to a quartz crystal resonator in an electronic bridge that is driven by a high-stability frequency source. Mechanical fluctuations of cells introduce time-dependent perturbations to the crystal boundary conditions and associated resonant frequency, which translate into phase noise measured at the output of the bridge. In experiments on nonmotile E. coli exposed to polymyxin B, cell-generated frequency noise dropped close to zero with the first spectra acquired 7 minutes after introduction of the antibiotic. In experiments on the same bacterial strain exposed to ampicillin, frequency noise began decreasing within 15 minutes of antibiotic introduction and proceeded to drop more rapidly with the onset of antibiotic-induced lysis. In conjunction with cell imaging and post-experiment counting of colony-forming units, these results provide evidence that cell death can be sensed through measurements of cell-generated frequency noise, potentially providing a basis for rapid AST.

This paper primarily addresses the usefulness of phase-modulation (PM) noise measurements versus noise figure (NF) measurements in characterizing the merit of an amplifier. The residual broadband (white PM) noise is used as the basis for estimating the NF of an amplifier. We have observed experimentally that many amplifiers show an increase in the broadband noise of 1 to 5 dB as the signal level through the amplifier increases. This effect is linked to input power through the amplifier's nonlinear intermodulation distortion. Consequently, this effect is reduced as linearity is increased. We further conclude that, although NF is sometimes used as a selection criteria for an amplifier for low-level signal, NF yields no information about potentially important close-to-carrier 1/f noise of an amplifier nor broadband noise in the presence of a high-level signal, but a PM noise measurements does. We also have verified experimentally that the single-sideband PM noise floor of an amplifier due to thermal noise is -177 dBc/Hz, relative to a carrier input power of 0 dBm.

We report exceptionally low PM noise levels from a microwave oscillator that uses a conventional air-dielectric cavity resonator as a frequency discriminator. Our approach is to increase the discriminator's intrinsic signal-to-noise ratio by use of a high-power carrier signal to interrogate an optimally coupled cavity, while the high-level of the carrier is suppressed before the phase detector. We developed and tested an accurate model of the expected PM noise that indicates, among other things, that a conventional air-dielectric resonator of moderate Q will exhibit less discriminator noise in this approach than do more esoteric and expensive dielectric resonators tuned to a high-order, high-Q mode and driven at the dielectric's optimum

We discuss a novel design of a self-starting regenerative divider that permits division by 3, 4, 5, 6 ... instead of the usual 2. This is accomplished by having the loop oscillate simultaneously at two harmonically related conjugate frequencies, e.g., at nu/4 and 3nu/4. A prototype of the divide-by-four circuit has been constructed for an input frequency of 400 MHz. This divider exhibits very low phase noise, Fourier frequency (1 kHz) = -162 dBc/Hz and Fourier frequency (100 kHz) = -170 dBc/Hz, which is approximately 9 dB lower than that of its constituent parts. Simple modifications of the feedback loop of this circuit enabled it to divide by 3, 5, and 8. Operation at higher division ratios appears feasible under certain conditions.

In this work we 1) analyze and give a theoretical explanation for the anomalously low cross-spectral density of voltage fluctuations that is observed when two thermal noise sources with matched intensities are coupled to the inputs of two-channel phase modulation (PM) or amplitude modulation (AM) noise measurement systems (NMS), 2) empirically evaluate spectral resolutions of different types of measurement systems, and 3) discuss noise measurement techniques involving cross-correlation signal processing. Our work shows that the statistical uncertainty, which sets the ultimate spectral resolution in the thermal noise limited regime, is approximately the same for both systems. However, in practical terms, the non-stationary nature of the noise, the temporal separation of calibration and measurement, and the difficulty of reproducing the calibrations for two measurements make it extremely difficult to resolve noise that is more than 10 dB below the noise floor in a single channel NMS. In a two-channel NMS, however, the calibrations of the two channels are carried out simultaneously, and one can take full advantage of a large number of averages and make reproducible noise measurements with resolution 10 dB below the noise floor of a single channel NMS.

We present experimental results on intrinsic 1/f frequency modulation (FM) noise in high-overtone thin-film sapphire resonators that operate at 2 GHz. The resonators exhibit several high-Q resonant modes approximately 100 kHz apart, which repeat every 13 MHz. A loaded Q of approximately 20,000 was estimated from the phase response. The results show that the FM noise of the resonators varied between Sy (10 Hz) = -202 dB relative (rel) to 1/Hz and -210 dB rel to 1/Hz. The equivalent phase modulation (PM) noise of an oscillator using these resonators (assuming a noiseless amplifier) would range from [symbol: see text](10 Hz) = -39 to -47 dBc/Hz.

The development of atomic frequency standards at NIST is discussed and three of the key frequency-standard technologies of the current era are described. For each of these technologies, the most recent NIST implementation of the particular type of standard is described in greater detail. The best relative standard uncertainty achieved to date for a NIST frequency standard is 1.5×10(-15). The uncertainties of the most recent NIST standards are displayed relative to the uncertainties of atomic frequency standards of several other countries.

The paper describes a new approach to synthesizing the Cs hyperfine frequency of 9.192 GHz that is designed to be sufficiently rugged for use in space, specifically for the Primary Atomic Reference Clock in Space (PARCS) planned for the International Space Station, as well as ground applications. This new approach requires no narrow band filters or frequency multiplication, and the primary source of cooling is conduction. Instead of frequency multiplication, it uses a custom regenerative divider stage followed by two commercial binary dividers and several mixing stages. A fractional frequency step of 2x10(-17) is achieved by mixing the output of a 48-bit numerically controlled oscillator with the microwave signal. Preliminary tests on the new synthesizer design indicate an internal fractional frequency stability of 1x10(-15) at 10 s and 1x10 (-18) at 1 d, dominated by the daily room temperature variations. The phase and amplitude noise are similar to our previous designs that used frequency multiplication and narrow band filters. The temperature coefficient is less than 0.2 ps/K.

Phase noise measurements of an optoelectronic oscillator (OEO) at frequencies less than 10 Ha from the carrier (10.6 GHz) as well as the measured Allan variance are presented for the first time. The system has a measured single-side-band (SSB) phase-noise of -123 dB/Hz at 10 kHz from the carrier and a sigma(y)(tau)=10(-10) for an integration time between 1 and 10 seconds. The importance of amplifier phase-noise and environmental fluctuations in determining the noise of the oscillator at these low Fourier frequencies is verified experimentally and analyzed using a generalized model of noise sources in the OEOs. This analysis then allows prediction of the oscillator performance from measured parameters of individual components in the system.

The performance of a simple microwave frequency reference based on Raman scattering in an atomic vapor is examined. This reference has the potential to be compact, low-power, and insensitive to acceleration. Several design architectures have been evaluated with a table-top experiment in order to guide the future development of a compact system. Fractional frequency deviations of

We demonstrate regenerative divide-by-two (halver) circuits with very low phase modulation (PM) noise at input frequencies of 18.4 GHz and 39.8 GHz. The PM noise of the 18.4 to 9.2 GHz divider pair was L(10 Hz)=-134 dB below the carrier in a 1 Hz bandwidth (dBc/Hz) and L(10 MHz)=-166 dBc/Hz, and the PM noise of the 39.8 GHz to 19.9 GHz divider pair was L(10 Hz)=-122 dBc/Hz and L(10 MHz)=-167 dBc/Hz.

In this paper we report the results of extensive research on phase modulation (PM) and amplitude modulation (AM) noise in linear bipolar junction transistor (BJT) amplifiers. BJT amplifiers exhibit 1/f PM and AM noise about a carrier signal that is much larger than the amplifiers thermal noise at those frequencies in the absence of the carrier signal. Our work shows that the 1/f PM noise of a BJT based amplifier is accompanied by 1/f AM noise which can be higher, lower, or nearly equal, depending on the circuit implementation. The 1/f AM and PM noise in BJTs is primarily the result of 1/f fluctuations in transistor current, transistor capacitance, circuit supply voltages, circuit impedances, and circuit configuration. We discuss the theory and present experimental data in reference to common emitter amplifiers, but the analysis can be applied to other configurations as well. This study provides the functional dependence of 1/f AM and PM noise on transistor parameters, circuit parameters, and signal frequency, thereby laying the groundwork for a comprehensive theory of 1/f AM and PM noise in BJT amplifiers. We show that in many cases the 1/f PM and AM noise can be reduced below the thermal noise of the amplifier.

In this paper we discuss guidelines for designing linear bipolar junction transistor amplifiers with low 1/f amplitude modulation (AM) and phase modulation (PM) noise. These guidelines are derived from a new theory that relates AM and PM noise to transconductance fluctuations, junction capacitance fluctuations, and circuit architecture. We analyze the noise equations of each process for a common emitter (CE) amplifier and use the results to suggest amplifier designs that minimize the 1/f noise while providing other required attributes such as high gain. Although we use a CE amplifier as an example, the procedure applies to other configurations as well. Experimental noise results for several amplifier configurations are presented.

We have studied the amplitude modulation (AM) and phase modulation (PM) noise in a number of 5 MHz and 100 MHz oscillators to provide a basis for developing models of the origin of AM noise. To adequately characterize the AM noise in high performance quartz oscillators, we found it necessary to use two-channel cross-correlation AM detection. In the quartz oscillators studied, the power spectral density (PSD) of the f(-1) and f(0) regions of AM noise is closely related to that of the PM noise. The major difference between different oscillators of the same design depends on the flicker noise performance of the resonator. We therefore propose that the f(-1) and f(0) regions of AM and PM noise arise from the same physical processes, probably originating in the sustaining amplifier.

The frequency, amplitude, and noise of the output signal of a quartz oscillator are affected by a large number of environmental effects. The physical basis for the sensitivity of precision oscillators to temperature, humidity, pressure, acceleration and vibration, magnetic field, electric field, load, and radiation is examined. The sensitivity of quartz oscillators to radiation is a very complex topic and poorly understood. Therefore, only a few general results are mentioned. The sensitivity to most external influences often varies significantly both from one oscillator type to another and from one unit of a given type to another. For a given unit, the sensitivity to one parameter often depends on the value of other parameters and on history. Representative sensitivity to the above parameters is given.