Spectral Analysis of Synthesized Signals in the mm Wavelength Region

In this paper some experimental results on the spectral purity along a multiplier chain from 5 MHz up to 600 GHz are reported. The measured points are compared to the numerical data which can be obtained from simple calculations considering the phase noise spectrum of the crystal oscillators used to drive the multiplier chains.     

One-port noise model of a crystal oscillator

This paper presents a one-port noise model of a crystal oscillator combined with equivalent impedances of a resonator and linearized feedback amplifier. Based on the noise conversion technique, we translate the thermal additive and flicker noise of both the resonator and amplifier into the oscillator signal amplitude and phase. The generic transformation coefficients for the noise are derived, and the power spectral density (psd) function of the oscillator signal phase is analyzed in detail. The remarkable property of the model is demonstrated by determining the separate contribution of each noise source to the oscillator performance. Some important rules for shaping the phase psd are noted. The consistency with Leeson's model also is reported.     

AM noise impact on low level phase noise measurements

The influence of the source AM noise in microwave residual phase noise experiments is investigated. The noise floor degradation problem, caused by the parasitic detection of this type of noise by an imperfectly balanced mixer, is solved thanks to a refinement of the quadrature condition. The parasitic noise contribution attributable to the AM to PM (phase modulation) conversion occurring in the device under test is minimized through the development of a dedicated microwave source featuring an AM noise level as low as -170 dBc/Hz at 10 kHz offset from a 3.5 GHz carrier     

Spectrally pure excitation signals: only a dream?

For accurate testing of a nonlinear device-under-test (DUT), the spectral purity of the input signal is utterly important. This paper describes a method to create very pure signals, such as sine waves, dual-tones, or multitones, using only power measurements. The signals are generated with an arbitrary waveform generator. Any unwanted spectral line is removed, independent of its exact origin. This allows to apply the correct signal to the DUT, even if the signal source is not perfect or is followed by filters or amplifiers with a small nonlinear distortion. Measurements are used to demonstrate the capabilities of the method.     

Sub‐syllable segment‐based voice conversion using spectral block clustering transformation functions

Abstract

This paper presents a novel framework for voice conversion based on sub‐syllable spectral block clustering transformation functions. The speech signal is first transferred to a spectrum by Fast Fourier transform. A sonority measure is used to extract sub‐syllable segments from input utterances by computing the energy concentration measure among frequency components. According to the syllable structure of Mandarin, Hidden Markov Model based syllable clustering is used to deal with the variety among different syllables. Dynamic programming is applied to align the spectral blocks of the parallel corpus to constrain the mapping between the spectral unit of the source speaker and that of the listener speaker under the constraint that mapped unities should be constrained to the same sub‐syllable and sub‐band in the Mel‐scale filter bank. A content based image retrieval algorithm is employed to find the target spectral block in the transformation phase. This paper illustrates voice conversion by spectral block transformation that transfers the speech signal of the source speaker to that of the listener. Experimental results show that the proposed method is effective in voice conversion, and the discrimination with regard to speaker identification is better than with traditional approaches. However, there remain additional noises, especially in high frequency components, which reduce the signal quality carried in the transformation phase, due to the fact that speech is not smooth.     

Direct digital synthesizer with jittered clock

Since the direct digital synthesizer (DDS) can potentially be used as a flexible clock source, it is of interest to study its spectrum purity as well as jitter characteristic. In this paper, we investigate the jitter transfer characteristic of the DDS clock driven by a jittered digital-to-analog converter (DAC) clock. We first derive the dosed form expressions of the spectrum of the DAC output signal with jittered driving clock. These expressions are then used to investigate the spectral structure of the DDS clock. Equations are derived for the calculation of the SNR. For a small phase noise power in the driving clock, the DDS clock SNR is obtained in a simple closed form and is shown to be lower than that of the input driving clock by the amount of 20 log(f_s/d_d) dB, where f_s is the nominal driving clock frequency and f_d is the desirable DDS output clock frequency     

RF Spectrum of a Signal after Frequency Multiplication; Measurement and Comparison with a Simple Calculation

A novel experimental technique is introduced and used to measure the effect of frequency multiplication on the RF spectrum of an oscillator. This technique makes it possible to produce the RF spectrum at X band?where measurements are relatively straightforward?that would have been produced by frequency multiplication of the 5-MHz source to any frequency from 9.2 GHz to 100 THz (1014 Hz). A simplified theory is developed and shown to reproduce the experimental results for the relative power in the carrier and noise pedestal, and the shape and the width of the carrier and noise pedestal, to within the measurement uncertainty of 2 or 3 dB, from 5 MHz to 10 THz. The calculations are easily made using analytical techniques from the measurement of the spectral density of phase fluctuations of the source, the effective input spectrum density and the bandwidth of the multiplier chain, and the frequency multiplication factor. It is shown that present 5-MHz-crystal-controlled oscillators are useful as a precision source to ~500 GHz. Suggestions for extending their range to ~100 THz are made.     

A simple analysis of the Dick effect in terms of phase noise spectral densities

In cold-atom frequency standards based on the Ramsey double interaction method, the phase noise of the interrogating signal appears as a random "end-to-end phase difference", thereby introducing frequency noise in the loop. This phenomenon is analyzed in this paper in the Fourier frequency domain, using phase noise power spectral densities S(phi)(f). In continuously operated standards, the excess noise thus introduced is servoed out in the long term to become eventually smaller than the atomic shot noise, whereas in standards with pulsed operation the phase noise around even harmonics of the pulse rate is down-converted by aliasing to base band. This latter mechanism is referred to in the literature as Dick effect. In this paper, a model of the frequency control servo system is proposed, in which the input signal is the (known) local oscillator (LO) phase noise S(phi)(f) and the output signal is the (unknown) phase noise S(phi)(f) of the standard in closed loop operation. The level of excess white frequency noise added by aliasing on the stabilized LO through the Dick effect can be related analytically to the characteristics of the free LO phase noise. From this, the stability limitation (with slope tau(-1/2)) typical of the Dick effect can then be obtained by the usual conversion formulas based on the power law model.     

Microprocessor-Based Walsh?Fourier Spectral Analyzer

A microprocessor-based Walsh-Fourier spectral analyzer is described. It includes the sampling of the incoming signal at 64 times the input signal frequency, using a special purpose frequency multiplier module (FMM), storing the digital data in a permuted sequence in the system memory under the control of a direct memory access (DMA) controller, and performing a fast Walsh-Hadamard transform (FWHT) of the permuted data sequence. The system uses a single board computer SBC80/10 in an Intel System 80/10 and a special purpose board which includes FMM, DMA, and A/D conversion circuits. Fourier coefficients are obtained via a Walsh to Fourier conversion algorithm; the total process is then faster than the Cooley-Tukey FFT algorithm for a data length of 64 or less.     

A nonlinear dynamic model for performance analysis of large-signal amplifiers in communication systems

A new nonlinear dynamic model of large-signal amplifiers based on a Volterra-like integral series expansion is described. The new Volterra-like series is specially oriented to the modeling of nonlinear communication circuits, since it is expressed in terms of dynamic deviations of the complex modulation envelope of the input signal. The proposed model represents a generalization, to nonlinear systems with memory, of the widely-used amplitude/amplitude (AM/AM) and amplitude/phase (AM/PM) conversion characteristics, which are based on the assumption of a practically memoryless behavior. A measurement procedure for the experimental characterization of the proposed model is also outlined.     

Resolution investigation of a ratiometric wavelength measurement system

Theoretical analysis and experimental investigations are presented on the resolution of a ratiometric wavelength measurement system. Theoretical modeling indicates that the resolution of a ratiometric wavelength measurement system is determined by the signal-to-noise ratio of the input signal and the noise of the photodetectors associated with optical-to-electronic conversion. For experimental verification, a ratiometric system employing a macrobending standard single-mode fiber is developed and corresponding results are in good agreement with the theoretical prediction.     

Short-Term Frequency Stability of Relaxation Crystal Oscillators

Due to the need for stable frequency rectangular wave signals, various relaxation quartz crystal oscillators were designed. Therefore it is of interest to have data on their short-term frequency stability. The generally accepted definitions of measures for short-term frequency stability and measurement procedures are reviewed in this paper. Measurement results for the short-term frequency stability of quartz crystal multivibrators in time and frequency domains show a high spectral purity of the multivibrator output signal. The single-sideband-to-carrier phase noise has values lower than -90 and -120 dB on the offset frequencies of 1 and 10 Hz, respectively. The white phase noise is about -160 dB. The power law spectral density model of fractional frequency fluctuations for the quartz multivibrators is established and a discussion on the noise sources is given.     

Phase fluctuations in a heterodyne multistage frequency multiplier

Conclusions The inherent phase instability and error of these multipliers are not inferior (as regards frequency stability) to those found in other methods of frequency multiplication.Multiplication with heterodyning is especially justified in frequency comparison of atomic clocks, where the mutual deviation and output frequency instability are much less than the limiting values permissible for undistorted multiplication. A transistorized multiplier is simple, so one can provide reliability over long periods with continuous comparison.The main component in the phase fluctuations in a multiplier is due to noise in the coupling and transformation circuits, so in principle it is possible to improve the stability further via advances in this part of the device.The input stages are of broad bandwidth and have low temperature coefficients, so one can compare frequencies over periods up to 100 sec without thermostatic control without appreciable loss of accuracy.Translated from Izmeritel'naya Tekhnika, No. 11, pp. 41–43, November, 1972.     

HF and VHF Source Stabilization Technique Incorporating Q-Multiplied Quartz Crystal Resonator

A new approach is described for the desiga of HF/VHF crystal-controlled frequency sources exhibiting theoretical short-term stability unattainable through the use of conventional quartz oscillator design. The signal generator design uses the concept of AFC stabilization of a conventional quartz oscillator (VCXO) by means of a crystal-controlled highly selective active frequency reference. The AFC reference is a phase-shift type frequency discriminator that employs a product detector and an active Q-multiplied quartz crystal resonator. The extremely selective transmission response, large group delay, and power gain exhibited by the resonator, together with resonator phase noise levels comparable to that exhibited by the oscillator-maintaining circuit, provide the principal means for prediction of superior output signal spectral purity. Models of the resonators have been designed and constructed at 30 and 80 MHz, exhibiting 3-dB bandwidths of 30 and 160 Hz, respectively. Based on actual measurement of VHF Q-multiplied crystal resonator performance characteristics, approximately 16 dB improvement in VHF crystal-controlled frequency source spectral purity at low and moderate modulation rates is possible, compared to that attainable using the best available VHF quartz oscillator circuit designs.     

Design of a Simple Current-Mode Multiplier Topology Using a Single CCCII+

In this paper, an analog current-mode (CM) multiplier circuit employing a single plus-type second-generation current-controlled conveyor (CCCII+) and a grounded resistor is proposed. The developed circuit can provide two-quadrant multiplication, four-quadrant multiplication, and frequency doubling all from the same topology with the selection of the applied input currents. In addition to the signal limitations of the multiplier configuration, the nonideality effects of the CCCII+ and the nonideal gain and parasitic impedance effects on the proposed circuit are investigated. The realized circuit is simulated with SPICE to exhibit its performance. In addition, a CM multiplier derived from the proposed multiplier is set up with commercially available active components to easily perform its experimental test.     

Measurement of AM to PM conversion in medium wave AM transmitter

A method for measuring AM (amplitude modulation) signal distortion caused by AM to PM (phase modulation) conversion at the output of an AM radio transmitter is presented. A novel type of measuring instrument is also presented for testing the phase distortion which may occur in an AM radio transmitter when an AM modulation signal is applied. The down-converted AM signal is first divided into the inphase and orthogonal-phase components, and these signals are processed by an operational circuit to obtain phase distortion, which is the angle between the orthogonal-phase and inphase components. A phase angle of 10° or more is detected in the prototype version     

A Wide-Band Digitally Controlled Frequency and Phase Multiplier

A method is considered for designing a wide-band digitally controlled frequency and phase multiplier based on nonlinear polynomial transformation of Chebyshev type applied to the initial harmonic signal. The method provides an output signal with a frequency a multiple of the input signal 1. Computer calculations are used to examine the output signal.     

Dependence of the channel electron multiplier noise spectral functions on the parameters of its operation

The channel electron multiplier (CEM) current flicker noise was examined for different parameters of its operation. In particular, the dependence of CEM power spectral density function on its output current value has been verified. Also the dependence of the CEM power spectral density function on the input electron energy was investigated.     

Interpreting anomalously low voltage noise in two-channel measurement systems

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     

组合声源测试消声器传递损失的噪声信号修正方法

针对某些传递损失较大的消声器,现有白噪声测试设备的单一噪声源发生器无法满足消声量的测试需求。通过采用低频和中频两种声源发生器,利用过渡管道与测试管道垂直连接的方式,实现了两种声源组合发声对消声器声学性能的测试。为了解决过渡管道与组合声源系统连接处声阻抗变化,导致输出的噪声信号频谱特性随机波动的问题,提出了一种噪声信号的修正方法。该方法基于四传感器法测量过渡管道声阻抗,根据过渡管道传递矩阵,以随机白噪声为激励源输出的管口噪声作为输出声压信号,得到组合声源系统的输入声压信号,实现了对组合声源系统声音信号的补偿。实验结果表明,与传统均衡器调节方法相比,该方法能够在较宽的测试频率范围内输出平稳的声压信号;其次,利用修正前后的声学信号对扩张腔的传递损失进行测量,修正后得到的测试曲线与理论值吻合度较高,证明了该方法的可行性。