Output Noise Spectrum Research Articles

Noise spectra of semiconductor optical amplifiers: relation between semiclassical and quantum descriptions

The paper presents a comparison between a semiclassical and a quantum description of the output noise spectra of semiconductor optical amplifiers. The noise sources are represented by Langevin noise functions or operators and by the vacuum field operator, and the analysis takes into account the effects of increasing gain saturation along the amplifier waveguide. It is shown that the difference between the semiclassical and quantum descriptions sums up to a shot noise term and to a correction to the semiclassical autocorrelation relation for the carrier noise. A discussion is given of the shapes of the noise spectra, the relative importance of various noise contributions, and their dependence on input power.

A micropower CMOS, direct-conversion, VLF receiver chip for magnetic-field wireless applications

A micropower CMOS, direct-conversion very low frequency (VLF) receiver is described for receiving low-level magnetic fields from resonant sensors. The single-chip, phase locked loop (PLL)-synthesized receiver covers a frequency range of 10-82 kHz and provides both analog and 9-b digital baseband I and Q outputs. Digital I and Q outputs are accumulated in a companion digital chip which provides baseband signal processing. Emphasis is plated on the receiver micropower RF preamplifier which uses a lateral bipolar input device because of the significant increase in flicker noise illustrated for PMOS devices in weak inversion. Lateral bipolar transistors are also utilized in the mixer and IF stages for low flicker noise and low dc offsets. Special attention is given to isolating the internal local oscillator signals from the low-level RF input (0.3 /spl mu/V noise floor in 300 Hz BW), and local oscillator feedthrough is indiscernible in the RF preamplifier output noise spectrum. The 100% duty-cycle receiver, intended for miniature, battery-operated wireless applications, operates approximately four months at 80 /spl mu/A from a 6-V, 220-mA-hr battery.

Simple model of optical amplifier chains to evaluate penalties in WDM systems

A simple model to evaluate the penalty due to amplifier gainshape in a wavelength division multiplexing (WDM) optical system is proposed. The model requires only knowledge of the overall gain of the amplifier chain for all channels and provides signal-to-noise ratio (SNR) penalties for given launched powers. It also provides the input power distribution required for SNR equalization, and the associated SNR penalty, by solving an eigenvalue/eigenvector problem. The penalty predicted by the model agrees well with recent field measurements on a transoceanic cable. The model can also be applied to predict the gainshape of the amplifier chain from a measurement of the output noise spectrum with no input signal. The capacity of a long amplifier chain can thereby be determined from the receiving terminal only.

Optical noise coherence effects in a Mach-Zehnder system

In this paper we present an exact, quantitative theoretical study of coherence effects in the optical noise spectrum of a Mach-Zehnder system coupled to a random-phase source. A simple, analytic formula for the output noise spectrum is derived. This formula is exact and valid for arbitrary source coherence time. We demonstrate two new facts concerning coherence effects: (1) These effects give rise to quasiperiodic features in the noise spectrum with period 1//spl tau/, where /spl tau/ is the time-of-flight difference between the two interferometer arms. (2) When the source coherence time is of the order of magnitude of /spl tau/ or larger, the noise spectrum is sensitive to changes in /spl tau/ on the scale of the optical period. It is suggested that this phenomenon can be used for optical RF interferometry. The characteristics of the noise spectrum are studied analytically, and illustrated by several numerical calculations. The results can be used to estimate the optical noise in any interferometric system in which the main contribution to the output field comes from two optical paths.

Quantization noise spectrum of double-loop sigma-delta converter with sinusoidal input

An exact formula for the output noise spectrum of a double-loop sigma-delta modulator, under the no overloading assumption and with a sinusoidal input, is derived without the use of a white-noise model. In the case of a sinusoidal input with irrational input amplitude and digital frequency, the result agrees with the exact formula derived by ergodic theory for two-stage modulators. In addition, the present method also provides an exact formula for sinusoidal inputs with rational frequency and amplitude. Furthermore, the period of the output with rational initial conditions and DC input is also calculated. The results are of primary interest to multibit sigma-delta modulators, which do not overload over the entire input amplitude range. The ergodic theory method for calculating the exact noise spectrum involves explicitly determining the autocorrelation of the internal quantization error with ergodic theory techniques, and then determining the noise spectrum from the correlation function. The present method, however, directly determines the quantization noise spectrum by using an open-loop model for the coder and applying a Fourier series representation of the quantization error function. The result of both of these methods is that the output noise spectrum for a sinusoidal input is composed of discrete spectral lines shaped by a sin /sup 4/(w/2) envelope. >

Quantum theory of nonlinear loop mirrors.

We formulate a continuous-mode quantum theory of nonlinear loop mirrors. We exploit the ``conservation of squeezing'' property at a beam splitter to provide a particularly simple description of the output noise spectra from such devices. We also derive a general condition for which one may obtain a squeezed vacuum from one of the output ports of the loop mirror.

Variable bit rate adaptive predictive coder

An adaptive predictive coder providing almost toll quality at 16 kb/s and minimal degradation when the bit rate is lowered to 9.6 kb/s is described. The coder can operate at intermediate bit rates and can also change bit rate on a packet-by-packet basis. Variable bit rate operation is achieved through the use of switched quantization, thus eliminating the need for buffering of the output. A noise shaping filter provides flexible control of the output noise spectrum. The filter, in conjunction with an enhanced way to adapt the quantizer step size, which tries to accommodate the quantization noise feedback, accounts for the toll quality. By quantizing the residue with more than one quantizer, the effective number of bits per sample can be controlled in a deterministic way regardless of the entropy residue. The lower limit of operation is at 9.6 kb/s. Performance of the coder under random bit errors is also presented. It has been found that only at error rates of 10/sup -2/ and higher does the degradation becomes objectionable. >

A modular 550 watt, 25 watts per cubic inch power supply for next generation aircraft

The development of the HVS-550 high-performance power supply is described. The HVS-550 was designed and built utilizing state-of-the-art surface mount construction, low-profile magnetics, and aggressive thermal management techniques. This combination allowed the module height to be only 0.58 in. The unique requirements of the military system that bounded the power-supply parameters are discussed. Cooling is provided either by liquid circulating through the chassis or by conduction through the card edges. The measured power supply efficiency is in excess of 85%, with 11-V output. The output noise spectrum is excellent because of noise-cancelling techniques utilized to achieve the level of performance required by the system.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Nonlinear static characteristics of monolithic active integrated-optic waveguides

A travelling optical flux analysis is carried out on a GaAs monolithic active integrated-optic waveguide (AIOW) to calculate both the nonlinearity of its static input/output characteristics and the spontaneous noise generated. From this model, the output signal-to-noise ratio of the guide, as well as its quasistatic harmonic distortion characteristics are found. Features of the model are the inclusion of the wavelength dependence of the stimulated gain and the spontaneous emission characteristics, and the variation of the carrier density and photon flux in the longitudinal direction of the waveguide. This enables the effect of the input wavelength of the light source on the transmission characteristics to be studied, as well as the output noise spectrum to be calculated under different conditions of forward path gain along the guide.

Noise analysis of switched capacitor networks

Noise generated in switched capacitor (SC) networks has its origin in the thermal fluctuations of charged particles in the channels of the MOS switch transistors on one hand, in the operational amplifiers on the other hand. Using a SC integrator as vehicle, it is shown that the output noise spectrum consists in general of a broad-band component due to a continuous-time noise signal and of a narrow-band contribution predominating in the baseband of the SC network resulting from a sampled-data noise signal. The ratio of undersampling is introduced and it is shown that the latter noise contribution can be evaluated by sampled-data techniques using the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</tex> -transform transfer function. These results are applied to the SC integrator and excellent concordance to the measurements made on a laboratory model is established.

Output noise spectrum from demodulator in an optical PFM system

An expression relating the input and output noise spectra of a PFM demodulator used in an optical transmission system has been derived and confirmed experimentally.

Erratum: Output noise spectrum from demodulator in an optical PFM system

Erratum: Output noise spectrum from demodulator in an optical PFM system

Subjective improvements in DPCM-AQ performance based on adaptive noise shaping

This paper reports subjective improvements in DPCM coders with adaptive quantizers (DPCM-AQ coders). The improvements were observed in simulations with real speech. An adaptive feedback filter shaped the output noise spectrum, and subjective quality increased despite a decrease in signal-to-noise ratio. The noise spectrum in the best case was shaped like the speech spectrum but was below it at almost all frequencies by 20 dB. This result is consistent with work on adaptive noise shaping in ADPCM-AQ systems. The improvements were enough to make 32 kbits/s DPCM-AQ systems sound equivalent to 40 kbits/s systems. Subjective improvements were observable, with diminished effectiveness, down to 24 kbits/s. Since the feedback filter does not have a counterpart at the receiver, transmission of the adaptive filter coefficients is unnecessary.

Impulse response estimation with underwater explosive charge acoustic signals

A procedure is presented for estimating the impulse response of a linear system from noise contaminated measurements of random input and output signals. The estimation requires ensemble averaged estimates of input and output noise spectra and a priori assumed spectra of the system function and input signal. A minimum mean-square-error estimator is derived and its theoretical signal-to-noise ratio is shown to be a slowly increasing function of the measured signal-to-noise ratios. The procedure is applied to a set of measured underwater explosive charge acoustic signals. For this application additional techniques are presented for synthesizing source replica signals and for accurately determining bubble pulse periods. The result of the deconvolution is a single sharp spike with at least 20-dB bubble pulse suppression. With approximately 20-dB signal-to-noise ratio in the measured signals the processed signal-to-noise ratios range from 10 to 13 dB and are all within 1 dB of theoretical values. Peak-signal to rms-noise ratios are 41 and 38 dB before and after processing, respectively.

The output signals and noise from a nonlinearity with amplitude-dependent phase shift

On the assumption that each input signal represents only a small part of the total input power, the output signals and output noise spectrum are found by extending the Barrett-Lampard diagonal expansion to the case of the quadrivariate distribution of input amplitude and phase at two different times. This expansion yields a decomposition of the nonlinearity into a summation of generalized Laguerre polynomials with uncorrelated outputs, whose spectra are simply autoconvolutions of the input spectrum. A slight generalization of the notion of memoryless nonlinearity allows this result to he applied very simply to devices, such as limiters and TWT amplifiers, that cause AM-to-PM conversion.

The influence of biasing circuits on photomultiplier tube output noise spectra and transfer functions

An ITT type FW118 photomultiplier tube biased by using a high-voltage power supply and a resistor chain was found experimentally to have output noise spectra decreasing in magnitude as frequency increases, resembling in shape the 1/ <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</tex> noise spectrum. This lowering of the noise level as frequency increases was explained in terms of: 1) the shunting effect of the interelectrode impedance and the stray capacitances of the last stage, and 2) the gain fluctuation of the photomultiplier tube caused by the noise voltage developed across each biasing resistor. A method of flattening the noise spectra and improving the frequency response of the tube is also given in this paper.

Output Noise Characteristics of Vacuum Photodiode Circuits

The effect of the biasing circuit configuration on the output noise spectra of the vacuum photodiode was investigated. It was found that under certain biasing circuit conditions, the output noise spectra were frequency dependent, resembling the 1/f noise spectrum at moderate frequencies. This similarity and its cause should be noted to avoid possible misinterpretation of data or erroneous measurements. Also, the conditions for obtaining white output noise spectra (and thus flat frequency-response curves) are presented. The measured white noise levels were found to be slightly in excess of that predicted by shot noise considerations alone.

Noise in phototransistors

The investigation reported here presents both analytical and experimental results on the noise performance of phototransistors. The noise performance of phototransistors depends on fluctuations traceable to two main sources: 1) the random fluctuations in the rate of photon arrival; and 2) mechanisms inherent in the device such as fluctuations in the generation of free carriers, diffusion and recombination fluctuations, and 1/f mechanisms, to cite a few. On the basis of corpuscular arguments, it is shown that the important fluctuations may be represented by simple, partially correlated shot noise current generators in parallel with the device junctions for a wide range of frequencies. The hybrid-pi transistor model and these noise generators have been used to derive an expression for the noise current to the short-circuited output of a phototransistor. Attention has been primarily focused on the low-frequency expression since correction terms due to high-frequency effects are negligible. The output noise spectra of commercially available silicon phototransistors and silicon units fabricated at the Microelectronics Laboratory of the University of Florida have been measured. The measurements are in good agreement with the model. The current gains and beta cutoff frequencies of these devices have been computed by interpreting the measured spectra with the aid of the analytical results. Independent ac and dc measurements of these parameters are in satisfactory agreement with values determined from noise measurements. It is thereby ascertained that noise measurements on phototransistors can be used to characterize them in terms of current gain and beta cutoff frequency.

An FM Detector for Low S/N

Conventional frequency-modulation (FM) detectors show a marked decrease in output <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S/N</tex> below an input <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S/N</tex> of 5 to 10 dB. This threshold effect is caused by "clicks" or phase slips of multiples of π. This effect is related to a normalization (clipping) which is very beneficial at high <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S/N</tex> . A detector is described which has no normalization and is superior to the ideal detector below zero to 10 dB input <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S/N</tex> . The noise at the output of the detector is additive for all input <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S/N</tex> ratios and in this sense is comparable to synchronous detection of AM. Plots of output noise spectrum and output <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S/N</tex> with and without sine-wave modulation are given. Another detector is also proposed that is asymptotic to the ideal detector at high <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S/N</tex> and the aforementioned detector at low <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S/N</tex> . Comparison is made with conventional detectors. The results indicate that this detector may be especially useful in the detection of frequency-shift keying and phase-shift keying in telemetry systems.

Hilbert transform analysis of the output noise spectrum of an ideal AM demodulator and FM discriminator

Hilbert transform analysis of the output noise spectrum of an ideal AM demodulator and FM discriminator