Ultralow-noise programmable voltage source

To avoid introducing additional noise sources while making low-frequency noise measurements, batteries are normally used instead of electronic power supplies. This paper presents an alternative solution by describing the design, construction, and testing of an ultralow-noise voltage source. Such a power supply can be computer controlled and has a typical noise level two orders of magnitude below that of similar commercial instruments. Some typical values of the spectral density of the voltage fluctuations at its output are: (10^-12, 10^-15, 10^-16) V²/Hz at (0.01, 0.1, and 1) Hz, respectively. These noise performances are almost independent of the supplied current, with a degradation of less than 3 dB up to 400 mA. A special algorithm for digital-to-analog conversion, using passive devices with 1% tolerance, ensures a resolution of 2.5 mV and an accuracy better than ±1.5 mV over the entire output range from 0 to 8 V     

Investigation of Environmental Noise in Small Electrical Conductors

Large excess resistance noise has been observed in small conductors surrounded by air. The conductor resistance was found to have a well-defined average power spectral density over the observed frequency range from 10^-4 to 200 Hz and the dependence of the noise was measured as a function of bias current, temperature, temperature coefficient of resistance (TCR), spatial correlation, and state of the surrounding air. In this paper, three different mechanisms were identified that produce the noise. The room-temperature fluctuations were measured and found to have a spectral density nearly proportional to f^-2 over the observed six-order-of-magnitude frequency range. The lowest frequency noise around 10^-4 Hz could be predicted from the measured temperature fluctuations using the TCR. Above 10^-3 Hz and below 1 Hz, enclosing the wire in a box greatly reduces the noise, and placing the wire in a vacuum eliminates the predominant noise. This noise was directly related to the temperature of the conductor, somewhat proportional to the TCR, independent of bias current, and has a correlation length smaller than the specimen size. The highest frequency noise does not depend on the conductor temperature, TCR, or the presence of air. It had a very strong dependence on bias current and had a long spatial correlation. The mechanism that generates this noise is not understood     

Secondary standard for PM and AM noise at 5, 10, and 100 MHz

A practical implementation of a portable secondary standard for phase modulation (PM) and amplitude modulation (AM) noise at 5, 10, and 100 MHz is described. The accuracy of the standard for both PM and AM noise is +0.14 dB, and the temperature coefficient is less than 0.02 dB/K. The noise floor S_φ (10 kHz) of the standard for PM noise measurements is less than -190 dBC relative to 1 rad²/Hz at 5, 10, and 100 MHz. The noise floor for AM measurements depends on the configuration. A calibrated level of PM and AM noise of approximately -130±0.2 dB relative to 1 rad² /Hz (for Fourier frequencies from approximately 1 Hz to 10% of the carrier frequency) is used to evaluate the accuracy versus Fourier frequency. Similar PM/AM noise standards are under test at 10 GHz. This new standard can also be used as an alternative to the normal method of calibrating the conversion sensitivity of the PM/AM detector for PM/AM measurements. Some types of time-domain measurement equipment can also be calibrated     

Ion chamber system for neutron flux measurements

A helium-filled ion chamber detector for intensity measurements of high-intensity epithermal neutron bursts with instantaneous rates as high as 10¹¹ Hz is presented. This system consists of an ion chamber to detect a portion of the neutron beam, a current-to-frequency converter and CAMAC scalers to readout the chamber. The chambers and readout electronics have a small temperature sensitivity and have high noise immunity. The statistical precision of the system is measured to be 10⁻³ for each neutron beam pulse.     

High spectral purity CO2 laser stabilized using amolecular frequency reference

A CO₂ laser has been frequency stabilized to a molecular absorption line with a bandwidth of 10 kHz, yielding a laser noise spectral density reduction below 1 Hz/√ Hz and a relative accuracy of 10^-10. In the time domain, this corresponds to an Allan standard deviation of σ(2, τ)=3·10^-14 τ^-1/2, for τ>100 μs. This source allows experiments of very high resolution spectroscopy as well as the realization of accurate frequency standards in the 10-μm region     

Measurement of weak magnetic fields using zero-field parametric resonance in optically pumped He4

A vector magnetometer has been built utilizing then = 0, p = 1parametric resonance associated with zero-field level crossings in the optically pumped 2³S1level of He⁴. The principles of operation are discussed and performance is described. The parametric resonance signal-to-noise ratio is 3.5 × 10⁴in a 0.5 Hz noise bandwidth. The linewidth is 9.7 × 10^-4G. Techniques for optimizing the sensitivity are discussed, and the sensitivity of the instrument is demonstrated by placing the sensor inside a superconducting magnetic shield and applying calibration signals, The peak-to-peak noise level is 1 × 10^-8G. The use of parametric resonance instruments to measure interplanetary and geomagnetic fields is discussed.     

Anomalous elastic response of amorphous alloys suggesting collective motions of many atoms

The dynamic Young’s modulus, E, of amorphous (a-) Zr₆₀Cu₃₀Al₁₀ (numbers indicate at.%) alloy was measured as a function of frequency, f, with a strain amplitude, _t, of 10⁻⁶, E(10⁻⁶,f), and also as a function of _t for f near 10² Hz, E(_t,10² Hz), by means of the vibrating reed methods. The elasticity study under the passing of electric current (PEC) was carried out too. E(10⁻⁶,f) is lower than E₀ for f between 10 and 10⁴ Hz showing local minima near 5×10, 5×10² and 5×10³ Hz, which are indicative of the resonant collective motion of many atoms, where E₀ is the static Young’s modulus. E(_t,10² Hz) increases showing saturation with increasing _t. Qualitatively, the outlines of E(10⁻⁶,f) and E(_t,10² Hz) observed for a-Zr₆₀Cu₃₀Al₁₀ are similar to those reported for various a-alloys. Quantitatively, a change in E(_t,10² Hz) for a-Zr₆₀Cu₃₀Al₁₀ is smallest among that reported for various a-alloys, presumably reflecting that the crystallization volume, (ΔV/V)_x, is smallest for a-Zr₆₀Cu₃₀Al₁₀. The effective charge number, Z^*, estimated from the change in E(10⁻⁶,10² Hz) due to PEC is 3.0×10⁵, which is comparable with Z^* reported for various a-alloys. We surmise that the number of atoms in the collective motions excited near 10² Hz is similar among various a-alloys. The E(10⁻⁶,f) data suggest that the spatial sizes of the density fluctuations may show a distribution.     

High-Q thermoelectric-stabilized sapphire microwave resonators forlow-noise applications

Two low-noise high-Q sapphire-loaded cavity (SLC) resonators, with unloaded Q values of 2×10⁵ and very low densities of spurious modes, have been constructed. They were designed to operate at 0°C with a center frequency of 10.000000 GHz. The cavity was cooled with a thermoelectric (TE) Peltier element, and in practice achieved the required center frequency near 1°C. The resonator has a measured frequency-temperature coefficient of -0.7 MHz/K, and a Q factor which is measured to be proportional to T^-2.5. An upper limit to the SLC residual phase noise of ℒ (100) Hz=-147 dBc/Hz, ℒ (1 kHz)=-155 dBc/Hz, and ℒ (10) kHz=-160 dBc/Hz has been measured. Also, we have created a free-running loop oscillator based on one of the SLC resonators, and measured a phase noise of ℒ(f)~-10-30log [f] dBc/Hz between f=10 /Hz and 25 kHz, using the other as a discriminator     

Electrical conduction mechanisms in thermally evaporated thin films of 2-(4-biphenylyl) -5-(4-tert-butylphenyl) -1,3,4-oxadiazole

The AC electrical behaviour of sandwich devices fabricated entirely in vacuum using thermally evaporated thin films of 99+% pure 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4 oxadiazole (Bu–PBD) and gold electrodes, is observed over the frequency range 10⁴ to 10⁷ Hz and the temperature range 297 to 398 K. The capacitance and dissipation factor decrease with increasing frequency and at high frequency (>2×10⁶ Hz) the capacitance and dissipation factor are not temperature dependent. The results obtained show the relative importance of the variable range hopping model and band theory in describing the film conductivity.     

Direct comparison of two independent Josephson voltage standards at1 V and precision measurements up to 10 V

Two Josephson voltage standards have been compared using a room-temperature electronic nanovoltmeter with a peak-to-peak noise of about 2 nV at the 1-V level corresponding to a RMS uncertainty of 4×10^-10. The excellent stability in maintaining the desired voltage steps makes it possible to obtain recorder traces comparing Nb/Al₂O₃/Nb Josephson standards with Weston cells and Zener reference standards at 1 V and 10 V. At 10 V the best result shows a peak-to-peak noise of 250 nV corresponding to a RMS uncertainty of 5×10^-9 for a Zener reference and 50 nV corresponding to 1×10^-9 for a series connection of nine Weston cells. As an example for the application of the Josephson standard as a potentiometer the deviation in the linearity of a digital voltmeter is confirmed to be on the order of 0.1 p.p.m. in the range from -10 V to +10 V     

Design and calibration of a noise measurement system

The setup principle and calibration method of a noise measurement system for frequencies from dc to 10 kHz are described. This system measures the current noise power spectral density of some device, and consists of a low-noise current preamplifier, a voltage preamplifier, and a dynamic signal analyzer which implements the fast Fourier transform (FFT). The noise aspects of the whole system can be modeled as a serial noise voltage generator and a parallel noise current generator at the input port, plus a system free of noise. The cross correlation of the two noise generators is an imaginary number because the system input stage is composed of some junction field effect transistors (JFET's). Via the thermal noise measurement of several resistors, we derive the magnitudes of the noise generators in addition to the input impedance and the total system gain. The imaginary cross correlation is obtained by the noise measurement of pure capacitance. With a well-calibrated procedure, we can measure the noise power down to 2×10^-27 A²/Hz. Two systems with different input stages were calibrated with the same procedure and the noise measurement results of the various resistance values with the two systems all agree well with theoretical values. One of these with an input stage which has a much smaller noise current generator shows great improvement in the noise measurement of the high-impedance device     

The water purification system for the low background counting test facility of the Borexino experiment at Gran Sasso

The Borexino experiment, for the study of solar neutrino physics, requires radiopurity at the level of 5 × 10⁻¹⁶ g/g ²³⁸U equivalent (or 6 × 10⁻⁹ Bq/kg) on a detector mass of many tons of scintillator. Feasibility studies are performed in a counting test facility now operating at LNGS, which consists of 4 t of liquid scintillator viewed by 100 photomultipliers and shielded by 100 t of water. The accomplishment of this goal requires the shielding liquid, water, to be at the 10⁻¹³ g/g contamination level (1.2 × 10⁻⁶ Bq/kg) or better. This paper describes the water purification system; it consists of a combination of several purification processes to remove particulate, radioactive ions, dissolved gases and other impurities. Residual contaminations are measured by analytical or direct-counting techniques. For radon measurement, particularly challenging at this low activity levels, a low background counting method has been developed.     

HBAR-Based 3.6 GHz oscillator with low power consumption and low phase noise

We have designed and built 2 oscillators at 1.2 and 3.6 GHz based on high-overtone bulk acoustic resonators (HBARs) for application in chip-scale atomic clocks (CSACs). The measured phase noise of the 3.6 GHz oscillator is -67 dBc/Hz at 300 Hz offset and -100 dBc/Hz at 10 kHz offset. The Allan deviation of the free-running oscillator is 1.5 × 10^-9 at one second integration time and the power consumption is 3.2 mW. The low phase noise allows the oscillator to be locked to a CSAC physics package without significantly degrading the clock performance.     

A cesium fountain frequency standard: preliminary results

An atomic frequency standard based on a fountain of cold cesium atoms has been in operation in our laboratory for a few months. Ramsey fringes as narrow as 0.7 Hz have been obtained. The short-term frequency stability, measured against an H-maser, is 3 10^-13 τ^-1/2, probably limited by the local oscillator frequency noise. A preliminary evaluation of some systematic effects shows that cold collisions will be the limiting effect for long-term stability and accuracy. Measurements of this shift presented here, using a magnetooptical trap, show that at low density an uncertainty of about 10 ¹⁵ is obtained. Under these conditions, the number of atoms is large enough to allow a potential stability of 10^-13 τ ^-1/2 to be reached. When optical molasses is used, the initial density is lower and collisional effects are reduced by a factor of three     

Comparative study on the high-bandgap material (GaN and SiC)-based impact avalanche transit time device

The dynamic characteristics of GaN and SiC-based impact avalanche transit times (IMPATTs) are reported at D-band. The device properties are compared at the same operating conditions and frequency of operations. A noise analysis model is employed to assess the performance studies. The noise of SiC-based IMPATTs is found to be higher than that of GaN-based IMPATTs. It is shown that the high-bandgap semiconductor material-based IMPATTs are potential candidates for replacing traditional IMPATTs at high frequency of operation. The computed power densities are found to be, respectively, 2.48times10⁷, 2.78times10⁷ and 0.07times10⁷ W/cm² for SiC, GaN and GaAs-based IMPATTs at the same operating conditions.     

An ultralow noise microwave oscillator based on a high-Q liquidnitrogen cooled sapphire resonator

Two liquid nitrogen-cooled sapphire loaded cavities (SLC's) operating at about 80 K have been successfully constructed, Both cavities were designed to operate on the whispering gallery (WG) E_{12, 1, δ} mode at a resonant frequency of 8.95 GHz. The first SLC was used as the frequency-determining element in a loop oscillator, while the second was used as a frequency discriminator to measure oscillator phase noise. The single sideband phase noise of a free running loop oscillator incorporating the first SLC was measured as -133 dBc/Hz at an offset frequency of 1 kHz, and was limited by the SLC Q-factor and the amplifier flicker phase noise. By using specially designed feedback electronics the oscillator phase noise was reduced to -156 dBc/Hz and -162 dBc/Hz at 1 and 10 kHz offset, respectively. This measurement was shown to be limited by the electronic flicker noise imposed by the phase detector in the feedback electronics, To our knowledge the phase noise and resonator Q-factor of 6×10⁷ represent the best results ever measured at liquid nitrogen temperatures or above     

基于SQUID的直流电阻电桥指零仪设计

设计了一款基于超导量子干涉仪(SQUID)的直流电阻电桥指零仪电路,该电路可用作高精度电阻电桥中低值电阻(0.1~100Ω)测量的指零仪电路。通过自制SQUID外围电路并优化电路其输入耦合线圈的互感值,实现微弱电桥不平衡电流的精密测量。与传统电阻电桥基于测量不平衡电压的纳伏计指零仪相比,其检测灵敏度在中低值电阻测量范围有数量级的提升。设计的作为指零仪的SQUID磁通锁定环1/f转折频率在2Hz左右,磁通白噪声水平在3.5μΦ₀/左右,并且输入线圈与SQUID互感系数高达10nH,其等效的电流白噪声密度0.7pA/。将其用作电阻电桥指零仪测量100Ω标准电阻(工作电流为10mA)的分辨率可达到10^-11量级。     

Calculated frequency characteristic of GR1482 inductance standardsbetween 100 Hz and 100 kHz

The Physikalisch-Technische Bundesansalt (PTB) calibrates inductance standards in the frequency range from 100 Hz to 100 kHz and beyond. The relative uncertainties achieved range from less than 5×10^-5 to 5×10^-4 (2σ) depending on the frequency and the value of the standards. Because of the strong frequency dependence of the inductance standards it is of interest to use a suitable equivalent circuit to calculate the most accurate approximation for the frequency variation. A model is presented which allows the values of a GR1482 inductance standard to be calculated over the whole frequency range from 100 Hz to 100 kHz within the measurement uncertainties     

1/f frequency noise of 2-GHZ high-Q thin-film sapphire resonators

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 20000 was estimated from the phase response. The results show that the FM noise of the resonators varied between S_y (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 L(10 Hz)=-39 to -47 dBc/Hz     

锻态TB6钛合金β相区压缩变形行为和动态再结晶

使用圆柱形试样在Thermecmaster-Z型热模拟试验机上进行锻态TB6钛合金β相区的热压缩实验(变形温度950~1100℃,应变速率0.001~1 s^-1),研究了合金的高温压缩变形和动态再结晶行为。结果表明,这种合金在β相区的变形激活能为246.7 kJ/mol,其热变形机制是动态再结晶,动态再结晶新晶粒的主要形核机制是弓弯形核。当应变速率为0.01~0.1 s^-1、变形温度为<1000℃时动态再结晶的发展比较充分,变形组织明显细化;当变形温度高于1000℃、应变速率低于0.001 s^-1时,动态再结晶的晶粒明显粗化。在动态再结晶的晶粒尺寸D与Z参数之间存在着相关性,其函数关系为D=6.44×10²·Z^-0.1628。