Spl Tau Research Articles

Cryocooled sapphire oscillator with ultrahigh stability

We present test results and design details for the first short-term frequency standard to achieve ultrahigh stability without the use of liquid helium. With refrigeration provided by a commercial cryocooler, the compensated sapphire oscillator (10 K CSO) makes available the superior short-term stability and phase noise performance of cryogenic oscillators without periodic interruptions for cryogen replacement. Technical features of the 10 K CSO include use of a two-stage cryocooler with vibration isolation by helium gas at atmospheric pressure, and a new sapphire/ruby resonator design giving compensated operation at 8 K to 10 K with Q=(1-2)/spl times/10/sup 9/. Stability of the first unit shows an Allan deviation of /spl sigma//sub y//spl les/2.5/spl times/10/sup -15/ for measuring times of 200 s/spl les//spl tau//spl les/600 s. We also present results showing the capability of the 10 K CSO to eliminate local oscillator degradation for atomic frequency standards. Configured as local oscillator (L.O.) for the LITS-7 trapped mercury ion frequency standard, the CSO/LITS combination demonstrated a limiting performance of 3.0/spl times/10/sup -14///spl tau//sup 1/2/, the lowest value measured to date for a passive atomic frequency standard, and virtually identical to the value calculated from photon statistics.

Toward an accurate frequency standard at 1.5 μm based on the acetylene overtone band transition

We have made a frequency standard operating at the 1.5 /spl mu/m region using the saturated absorption of the v/sub 1/+v/sub 3/ band of /sup 13/C/sub 2/H/sub 2/. To enhance the moderate power from a laser diode and to saturate a very weak molecular overtone transition, we used a Fabry-Perot (FP) cavity and inside of it we put an acetylene cell. In order to obtain a longer life time for the cell, we designed a new scheme consisting of a cavity with a ULE (ultralow expansion) glass spacer and a baked and sealed-off cell with Brewster windows. We observed the saturated spectrum having a 1 MHz spectral linewidth. The signal obtained using the same cell ten months later had the same signal-to-noise ratio (SNR) and spectral linewidth. We made two versions of the frequency standard and observed a beat note between these two lasers from which we derived the frequency stability, in terms of the square root of the Allan variance /spl sigma/(/spl tau/)=2.4/spl times/10/sup -13/ at an integration time /spl tau/=1000 s.

The design of a temperature-stable stepped-impedance resonator using composite ceramic materials

The authors present a method to design a coaxial ceramic resonator, whose resonance frequency is unchanged with temperature, using a material with a positive temperature coefficient of resonant frequency (/spl tau//sub f/) and a negative /spl tau//sub f/ material. From the resonance conditions according to temperature variation, the length to be filled with each material is calculated to minimize the resonance frequency shift with temperature change. As an example, Ba(Zn/sub 1/3/Nb/sub 2/3/)O/sub 3/ and CaZrO/sub 3/ are selected in this work, and their lengths are obtained at various resonant frequencies. The temperature dependence of the resonant frequency of this resonator is simulated.

Accuracy evaluation of the optically pumped Cs frequency standard at NRLM

The optically pumped cesium (Cs) frequency standard at the National Research Laboratory of Metrology, Ibaraki, Japan (NRLM), NRLM-4, was modified in various ways in order to improve frequency stability and accuracy, We determined the frequency stability by comparing the phase with that of a hydrogen maser: the Allan deviation was /spl sigma//sub y/(/spl tau/)=5/spl times/10/sup -13///spl radic//spl tau/. We estimated the frequency corrections and uncertainties due to various sources, and the total uncertainty was found to be 2.9/spl times/10/sup -14/. The frequency difference between NRLM-4 and the international atomic time (TAI) was also measured.

Frequency stabilization of frequency-doubled Nd:YAG lasers at 532 nm by frequency modulation spectroscopy technique

Frequency-doubled Nd:YAG lasers have been frequency stabilized against the hyperfine structure components of /sup 127/I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> by means of the sub-Doppler frequency modulation technique. A frequency stability of 3x10/sup -13/τ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> 2/ has been obtained for integration times between 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> s < τ < 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> s with an estimated reproducibility of about 1 kHz (2x10/sup -12/). The hyperfine spectroscopy of the rovibrational line P(54)32-0 has been performed and the iodine hyperfine constants have been calculated. The pressure shifts of the hyperfine components α <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> , α <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , α/sub 10/, and α/sub 15/ have been measured.

Optical frequency synthesis with 1 Hz resolution

This paper reports on the development of a second generation coherent optical-to-microwave frequency chain based on the frequency-interval bisection method. We report successful operation of all three key elements of such a frequency chain: efficient nonlinear radiation production, reliable and accurate laser diode phase locking, as well as wide-span optical frequency comb generation. An analysis of the expected frequency stability of a synthesizer based on this type of frequency chain shows that it should be capable of transferring frequency stabilities at the level of 1.3/spl times/10/sup -15///spl tau/.

A model for the multipath delay profile of fixed wireless channels

This paper deals with the measurement and modeling of multipath delay on fixed wireless paths at 1.9 GHz in suburban environments. The primary focus is on the delay profile, which is the normalized plot of received power versus delay in response to an RT "impulse." We describe measurement campaigns in the western suburbs of Chicago, IL, and in suburban north-central New Jersey. Our analysis of the data suggests to us that, for directive terminal antennas, the delay profile can be modeled as having a "spike-plus-exponential" shape, i.e., a strong return ("spike") at the lowest delay, plus a set of returns whose mean powers decay exponentially with delay. This delay profile can be characterized by just two parameters (both variable over the terrain), namely, the ratio (K/sub 0/) of the average powers in the "spike" and "exponential" components and the decay time constant (/spl tau//sub 0/) of the "exponential" component. No such simple structure appears to apply for delay profiles using omnidirectional antennas. For a directive antenna with a 32/spl deg/ beamwidth, we find that: (1) the statistical correlation between the profile parameters K/sub 0/ and /spl tau//sub 0/ is negligible; (2) these parameters are relatively insensitive to antenna height and path length; and (3) over each measured region (Illinois and New Jersey), K/sub 0/ and /spl tau//sub 0/ have median values close to 8 dB and just below 0.2 /spl mu/s, respectively. Moreover, we have found simple probability distributions that accurately portray the variability of K/sub 0/ and /spl tau//sub 0/ over the terrain.

Improved rates and asymptotic normality for nonparametric neural network estimators

We obtain an improved approximation rate (in Sobolev norm) of r/sup -1/2-/spl alpha//(d+1)/ for a large class of single hidden layer feedforward artificial neural networks (ANN) with r hidden units and possibly nonsigmoid activation functions when the target function satisfies certain smoothness conditions. Here, d is the dimension of the domain of the target function, and /spl alpha//spl isin/(0, 1) is related to the smoothness of the activation function. When applying this class of ANNs to nonparametrically estimate (train) a general target function using the method of sieves, we obtain new root-mean-square convergence rates of Op([n/log(n)]/sup -/(1+2/spl alpha//(d+1))/[4(1+/spl alpha//(d+1))])=op(n/sup -1/4/) by letting the number of hidden units /spl tau//sub n/, increase appropriately with the sample size (number of training examples) n. These rates are valid for i.i.d. data as well as for uniform mixing and absolutely regular (/spl beta/-mixing) stationary time series data. In addition, the rates are fast enough to deliver root-n asymptotic normality for plug-in estimates of smooth functionals using general ANN sieve estimators. As interesting applications to nonlinear time series, we establish rates for ANN sieve estimators of four different multivariate target functions: a conditional mean, a conditional quantile, a joint density, and a conditional density. We also obtain root-n asymptotic normality results for semiparametric model coefficient and average derivative estimators.

Weighted voting systems

This paper deals with reliability and cost evaluation of weighted dynamic-threshold voting-systems. The particular voting system studied consists of n units that each provide a binary decision (0 or 1) or abstain from voting (x). The system output is 1 if the cumulative weight of all 1-opting units is at least a pre-specified fraction /spl tau/ of the cumulative weight of all nonabstaining units. Otherwise the system output is 0. Systems of this type are encountered in many areas ranging from target to pattern recognition, safety monitoring and human organization systems. These results for a voting system with a particular structure can easily be modified for other voting systems as well. A general mathematical model of this system is presented. However, its immense combinatorial complexity (even for small-scale and trivial systems) makes the model virtually inapplicable for real systems. By placing two restrictions on the generality of the model parameters, the model can be greatly simplified, which in turn makes the model applicable for evaluating systems of any size. The imposed restrictions are minor, and are fulfilled in most practical situations, so that the derivations in this paper are helpful for the design and analysis of practical, weighted dynamic-threshold voting-systems.

The impact of CSO/CTB distortion on BER characteristics by hybrid multichannel analog/QAM transmission systems

This article describes the measurements of composite second order (CSO), composite triple beats (CTB) frequency spectrum and time-varying distortion in the basic bandwidth (BB) (0-4 MHz) of the 64-QAM digital channels by multichannel AM/64-QAM optical and coaxial transmission systems. For any regular standard frequency plans (Europe, USA, NCTA, etc.) there is, for the I and Q components in the BB of the QAM channels, the harmonic series of the CSO/CTB with the first harmonic f/sub 1/. It has been shown that for all the same laser-optics or coaxial transmission systems the CSO/CTB distortions spectrum form in the QAM BB 60-80 /spl mu/s duration bursts of a series pulse with a pulse period T=1/f/sub 1/ and a pulse duration /spl tau//spl ap/0.4/f/sub m/, where f/sub m/ is the CSO/CTB harmonic with maximum amplitude in the QAM BB. The bursts result in the essential unevenness of error bits in time domain distribution. These bursts can deteriorate essential error correction by the interleaver-deinterleaver and Reed-Solomon (RS) forward error correction (FEC) code of the transmission systems also with a relative low bit error rate (BER) of about 10/sup -5/ before FEC. There is, white Gaussian noise like, intermodulation distortion from interaction of digital-, digital-analog channels too, that have added white Gaussian noise.

New PN code acquisition scheme for CDMA networks with low signal-to-noise ratios

A new approach to PN code acquisition is presented and analyzed. A recirculation loop is used to improve probability of synchro cell detection P/sub D/ in each retrace of the code uncertainty region. To further improve P/sub D/ and probability of false alarm P/sub fa/ simultaneously, code diversity (a number of synchro channels in parallel) is used. This is especially effective in the channel with multiple access interference and near-far effect. Typical applications are networks with very low signal-to-noise ratio. Examples are code division multiple access (CDMA) LEO satellite systems experiencing high Doppler or any CDMA network where the number of users is approaching capacity limits. Even if the carrier Doppler is compensated, in any asynchronous LEO satellite network, compensation of code Doppler is not feasible. For this reason, in order to cope with code Doppler (D) and delay (/spl tau/), a modification based on transforming the two-dimensional uncertainty region (D, /spl tau/) into a new uncertainty region (T/sub c/, /spl tau/) will be introduced. Parameter T/sub c/ is the period of the correlation pulses at the output of a sliding correlator. When Doppler rate is present, the three-dimensional uncertainty region (D, R/sub d/, /spl tau/) is transformed into a new one (T/sub c/, R/sub t/, /spl tau/) where R/sub d/ and R/sub t/ are Doppler rate and correlation pulse period change rate, respectively. The main motivation for this work is to find new algorithms suitable for all digital receiver implementation and operation at low signal-to-noise ratios. These algorithms make CDMA techniques feasible for direct communication between LEO satellite and small ground-based user terminals (handsets). A comprehensive performance study of the new PN code acquisition system is presented and discussed. The results obtained demonstrate that, for low signal-to noise ratios, the acquisition time achieved with the new algorithm is one order of magnitude shorter compared with standard techniques known so far.

Effects of pad and interconnection parasitics on forward transit time in HBTs

A new analytical expression using Y-parameters is derived from the full heterojunction bipolar transistor (HBT) equivalent circuit including probe-pattern parasitics, in order to extract the forward transit time accurately without a conventional de-embedding procedure. The correction procedure of this Y-parameter method does not require the measurement of extra test patterns such as open or short structures, thus being free of the possible de-embedding errors attributed to the inaccuracy of test structures. To compare the de-embedding effect on /spl tau//sub F/ in the Y-parameter method with that in the previous f/sub T/ method, C/sub /spl pi///g/sub mo/ versus 1/I/sub c/ curves are plotted with 1/2/spl pi/f/sub T/ versus 1/I/sub c/ in the cases of de-embedding and no de-embedding. This comparison clearly exhibits that the contribution of the probe-pattern parasitics to /spl tau//sub F/ in the Y-parameter method is much smaller than that in the conventional f/sub T/ method. The smaller contribution to /spl tau//sub F/ may result in the enhancement of the de-embedding accuracy in the Y-parameter method.

Steady-state and transient forward current-voltage characteristics of 4H-silicon carbide 5.5 kV diodes at high and superhigh current densities

Steady-state and transient forward current-voltage I-V characteristics have been measured in 5.5 kV p/sup +/-n-n/sup +/ 4H-SiC rectifier diodes up to a current density j/spl ap/5.5/spl times/10/sup 4/ A/cm/sup 2/. The steady-state data are compared with calculations in the framework of a model, in which the emitter injection coefficient decreases with increasing current density. To compare correctly the experimental and theoretical results, the lifetime of minority carriers for high injection level, /spl tau//sub ph/, has been estimated from transient characteristics. At low injection level, the hole diffusion length L/sub pl/ has been measured by photoresponse technique. For a low-doped n-base, the hole diffusion lengths are L/sub pl//spl ap/2 /spl mu/m and L/sub ph//spl ap/6-10 /spl mu/m at low and high injection levels respectively. Hole lifetimes for low and high injection levels are /spl tau//sub pl//spl ap/15 ns and /spl tau//sub ph//spl ap/140-400 ns. The calculated and experimental results agree well within the wide range of current densities 10 A/cm/sup 2/<j< 4/spl times/10/sup 3/ A/cm/sup 2/. At j>5 kA/cm/sup 2/, the experimental values of residual voltage drop V is lower than the calculated ones. In the range of current densities 5/spl times/10/sup 3/ A/cm/sup 2/<j<2/spl times/10/sup 4/ A/cm/sup 2/, the minimal value of differential resistance R/sub d/=dV/dj is 1.5/spl times/10/sup -4/ /spl Omega/ cm/sup 2/. At j>25 kA/cm/sup 2/, R/sub d/ increases with increasing current density manifesting the contribution of other nonlinear mechanisms to the formation steady-state current-voltage characteristic. The possible role of Auger recombination is also discussed.

An all-digital phase-locked loop (ADPLL)-based clock recovery circuit

A new algorithm for all-digital phase-locked loops (ADPLL) with fast acquisition and large pulling range is presented in this paper. Based on the proposed algorithm, portable cell-based implementations for clock recovery with functions of a frequency synthesizer and on-chip clock generator are completed by standard cell. These modules have been designed and verified on a 0.6-/spl mu/m CMOS process. Test results are summarized as follows: 1) the proposed ADPLL can satisfy full locked bandwidth and fast acquisition within one data transition; 2) the on-chip clock generator can generate any target clock rate f/sub clock/; and 3) the function of nonreturn-to-zero clock recovery has a maximum f/sub clock//4 recovering capability with a locking range of (/spl tau//sub input//spl plusmn//spl tau//sub input//2)) where /spl tau//sub input/ is the input period.

Transferred-substrate HBTs with 254 GHz f

The authors report the simultaneous achievement of record current gain cutoff frequency f/sub /spl tau// and high power gain cutoff frequency f/sub max/ in a transferred-substrate HBT. The device parasitics are examined, and inferences made about the relative value of various approaches toward further improvements in f/sub /spl tau// and f/sub max/. Transferred-substrate HBTs with record f/sub /spl tau// of 254 GHz are reported.

Long-term wavelength stability of 1.55-μm tunable distributed Bragg reflector lasers

To investigate physical mechanisms involved in long-term wavelength drift of tunable distributed Bragg reflector (DBR) laser, the evolution of the tuning characteristics as well as the Bragg section intensity modulation response of several DBR lasers have been simultaneously assessed by current injection in the Bragg section only. Current versus voltage I(V) characteristics under aging have also been recorded to compare the role of leakage current and nonradiative recombination defect evolution. The wavelength drift as well as the carrier lifetime of the tuning section varies following an exponential law A+Bexp(-t/sub A///spl tau/) versus aging time t/sub A/. The time constant /spl tau/ is aging temperature and Bragg current dependant. The carrier lifetime decreases with time indicating a wavelength drift mainly due to nonradiative recombination defect increase. Modeling of the I/sub B/(V) and /spl lambda//sub B/(I/sub B/) characteristics is presented, that fits nicely the experimental data. The exponential form of the wavelength drift is used to propose novel and adequate burning conditions of DBR lasers.

Fast implementation of scale-space by interpolatory subdivision scheme

While the scale-space approach has been widely used in computer vision, there has been a great interest in fast implementation of scale-space filtering. We introduce an interpolatory subdivision scheme (ISS) for this purpose. In order to extract the geometric features in a scale-space representation, discrete derivative approximations are usually needed. Hence, a general procedure is also introduced to derive exact formulae for numerical differentiation with respect to this ISS. Then, from ISS, an algorithm is derived for fast approximation of scale-space filtering. Moreover, the relationship between the ISS and the Whittaker-Shannon sampling theorem and the commonly used spline technique is discussed. As an example of the application of ISS technique, we present some examples on fast implementation of /spl lambda//spl tau/-spaces as introduced by Gokmen and Jain (1997), which encompasses various famous edge detection filters. It is shown that the ISS technique demonstrates high performance in fast implementation of the scale-space filtering and feature extraction.

F&amp;H filter: A novel ultra-low power discrete time filter

A new technique for designing filters with long time constants in the discrete time domain is presented. The F&H (filter and hold) methodology halts the state of a continuous time filter every T seconds resulting in a filter implementation with time constants that can be controlled in three distinct ways: by the sampling period T, the duty cycle k=/spl tau//T or the time constant of the continuous time filter prototype. The final filter can be constructed from a typical Gm-C technique with very low power consumption.

Theoretical analysis of a hybrid AM-VSB/QAM SCM lightwave system with adjustable preclipping levels

A model for the analysis of the performance of a hybrid multichannel amplitude modulation vestigal sideband (AM-VSB)/quadrature amplitude modulation (QAM) subcarrier-multiplexed (SCM) lightwave transmission system with adjustable preclipping levels is presented. In the proposed model, two-stage adjustable clipping effect due to the preclipper and laser are considered. The analytical result shows that in this hybrid system the bit error rate (BER) is improved while the carrier-to nonlinear distortion (C/NLD) ratio is degraded as the value of the preclipping factor, is decreased, and vice versa. The validity of this model is verified by comparing the theoretical optimal value of /spl tau/ which is derived using our approach with the existed experimental result.

Multiple-wavelength quasi-phase-matched nonlinear interactions

Quasi-phase-matching allows one to arbitrarily phase match a single interaction by periodic modulation of the material nonlinear coefficient. A partial extension is obtained by Fibonacci-based quasi-periodic modulation of the nonlinear coefficient. These Fibonacci-based structures allow for simultaneously phase matching two interactions, provided that their wavevector mismatch ratio obeys selection rules, which are governed by the golden ratio /spl tau/=(1+/spl radic/5)/2. In this paper, we present a novel method for simultaneously phase matching any two nonlinear interactions by general quasi-periodic modulation of the nonlinear coefficient. These quasi-periodic structures, which also include the Fibonacci-based structures as a subgroup, provide greater design flexibility. Our method can be useful for various nonlinear devices, such as multiple-peak frequency doublers, frequency triplers, and frequency quadruplers. We show for two specific devices that similar efficiency, compared to a cascaded device, can be obtained. Furthermore, in contrast to some cascaded devices, these structures can be used in double-pass and standing-wave configurations, since they operate with the same efficiency in both directions of propagation.