Improved line spectral frequency estimation through anti-aliasing filtering

A study of the classic line spectral frequency (LSF) extraction methods along with the assumptions made during their estimation is presented. LSF extraction is investigated from an over-sampling and decimation perspective and LSFs are shown to contain high-frequency variations that led to spectral overlapping problems. An anti-aliasing filter prior to decimation, with a cut-off frequency dependent on the final LSF vector transmission rate, is proposed to alleviate the aliasing problem of the classic extraction methods. The proposed method shows a clear advantage because it produces the same quantisation distortion as the classic methods at a lower bit requirement, with significant reduction in 2 and 4 dB outliers that greatly affect synthesised speech quality. This was also confirmed through a listening test     

Resonant frequencies of open and closed loop frequency selectivesurface arrays

The effects on the transmission responses of loop element frequency selective surface arrays of breaking the conductor at suitable locations are described for simple squares and for highly convoluted geometries based on Peano curves. This simple modification generates a large reduction in the reflection band frequency for a fixed unit cell size     

雷达抗有源干扰试验闭环设计

雷达抗有源干扰试验是雷达试验项目中的一项重要内容,试验的充分与否直接关乎雷达在未来复杂电磁环境中的生存能力。本文通过试验开环设计、试验数据检验与修正所形成的闭环设计模式,达到科学、全面、完备的试验设计目的。     

Current-mode phase-locked loop with constant-Q active inductor CCO and active transformer loop filter

This paper presents a current-mode phase-locked loop (PLL) with a constant-Q CMOS active inductor current-controlled oscillator (CCO) and a CMOS current-mode active-transformer loop filter. The constant-Q active inductor provides a large and swing-independent quality factor such that the phase noise of the CCO utilizing the constant-Q active inductor is comparable to that of CCO with spiral inductors. The current-mode active-transformer loop filter offers the advantage of a large and tunable inductance and low silicon consumption such that the loop bandwidth of the PLL can be made small and tunable. The PLL was designed in TSMC-0.18 μm 6-metal 1.8V CMOS technology and analyzed using SpectreRF from Cadence Design Systems with BSIM3v3 device models. The phase noise of the PLL was analyzed using Cadence’s Verilog-AMS behavioral modeling. The phase noise of the CCO with the constant-Q active inductor is ?123.1 dBc/Hz at 1 MHz frequency offset, over 10 dB better as compared with that of the CCO with conventional active inductors, and is only a few dB higher than that of the CCO with spiral inductors. The phase noise of the PLL with an active-transformer loop filter and a constant-Q CCO is ?116 dBc/Hz at 1 MHz frequency offset, nearly 20 dB lower than that of the PLL with the same active-transformer loop filter and a conventional active-inductor CCO. The lock time, power consumption, and phase noise of the PLL are 60 ns, 34 mW, and ?116 dBc/Hz at 1 MHz frequency offset, respectively. The total silicon consumption of the PLL excluding bond pads is 0.013 mm².     

Multiple converter performance and active filtering

The application of active power filtering to power systems in limited by the low switching rate of available high power inverter switches. In this paper, parallel and series connection of multiple voltage source inverter bridges are examined to increase their effective switching rate. The analysis of the inverter voltage spectrum for double edge modulation shows the modulation desired for the separate bridges in open loop. Intuitively “i” bridges in series or parallel increases the effective switch rate by a factor “i”. However the modulation process for each power converter maintains its pattern for its switching period T which gives a roll off at high frequencies and reduces effective bandwidth. The experimental system of two 10 kVA voltage source inverters demonstrated how the closed loop control strategies for the active filter can be applied for multiple bridges implementing periodic optimized error sawtooth feedback control. The double bridge closed loop system showed that the lowest switch frequency terms were double that of the separate bridges. The higher frequency switching lines that should have been cancelled in theory were still discernible due to the finite precision of the edge timing     

A spectral frequency synthesizer

The spectral digital frequency synthesizer has extremely low battery power consumption. A model, providing two ×460 radio channels in the VHF range, was developed and packaged.     

A frequency steered phase-locked loop

Narrow bandwidth phase-locked loops (PLLs) can have difficulty acquiring lock reliably when there is a significant difference between the input signal and the free run frequency of the PLL's voltage-controlled oscillator (VCO). The new technique presented here incorporates an accurate local reference frequency into the PLL structure. The range of frequencies to which the new PLL structure can lock can be confined to a desired small region around the accurate local reference frequency. The new PLL structure also provides other benefits such as reduction of VCO phase noise. The new technique does not require any monitoring nor any switching of the local frequency reference signal which is always acting. The key parameters of the new PLL structure are identified and the performance characterized     

Noise filtering with the nonlinear optical loop mirror

The operation of a nonlinear optical loop mirror as a passive and signal polarization independent noise filter, is investigated experimentally. Spontaneous emission not within the signal spectrum is rejected leaving a receiver noise limited solely by signal-spontaneous beat noise. The receiver power penalty caused by spontaneous emission (@BER=10^-9 (PRBS)) was improved from 3 dB to 0.4 dB by propagating a signal through the loop. A comparison between this nonlinear filtering: technique and a variable band-pass filter is also made     

Ranging by signal circulation on a closed loop

The first echo signal that is received by a radar device is employed to trigger the emission of a new signal. All signals travel along a closed loop of propagation that includes transmitter, target, receiver, and trigger components. Experiments are described in which the signal circulation rate is measured. This quantity provides accurate access to range data. The closed-loop technique appears to offer better short-range data and increased precision.     

An adaptive filtering approach to spectral estimation and SARimaging

We present an adaptive FIR filtering approach, which is referred to as the amplitude and phase estimation of a sinusoid (APES), for complex spectral estimation. We compare the APES algorithm with other FIR filtering approaches including the Welch (1967) and Capon (1969) methods. We also describe how to apply the FIR filtering approaches to target range signature estimation and synthetic aperture radar (SAR) imaging. We show via both numerical and experimental examples that the adaptive FIR filtering approaches such as Capon and APES can yield more accurate spectral estimates with much lower sidelobes and narrower spectral peaks than the FFT method, which is also a special case of the FIR filtering approach. We show that although the APES algorithm yields somewhat wider spectral peaks than the Capon method, the former gives more accurate overall spectral estimates and SAR images than the latter and the FFT method     

Stabilisation of closed loop systems by time delays

It is generally recognised that time delays will cause closed loop systems to become unstable. The adverse effect on stability caused by time delays is presented. Bounds on time delays for the stabilisation of unstable closed loop systems are also derived.<>     

Comparison of open loop and closed loop schemes for multiple transmit antennas in OFDMA systems

The open loop transmit antenna diversity scheme and the closed loop beamforming technique are compared considering user mobility in OFDMA systems where both frequency distributed subchannelisation and frequency localised subchannelisation are provided. Outage capacity is considered for performance measurement.     

Adaptive filtering in the frequency domain

Adaptive filtering in the frequency domain can be accomplished by Fourier transformation of the input signal and independent weighting of the contents of each frequency bin. The frequency-domain filter performs similarly to a conventional adaptive transversal filter but promises a significant reduction in computation when the number of weights equals or exceeds 16.     

利用锁频环路的快速跳频合成信号发生器

介绍一种高性能快速跳频合成信号发生器,利用锁相环路和锁频环路相结合的原理和跳频校准控制技术而实现的。着重介绍了延迟线鉴频器锁频环路的工作原理、低噪声特性和宽带特性,仪器实现低相噪和快速频率转换的机理。介绍了通过信道、序列设置,每个频率点的环路增益参数和预调电压的校准和存储,实现快速跳频的方法。     

横向压电驱动变形镜的迟滞特性及其闭环校正

横向压电驱动变形镜在自适应光学系统中应用广泛,其利用了压电陶瓷的横向逆压电效应驱动镜片实现变形。在高电场强度下变形镜迟滞曲线存在特殊的蝴蝶形,增加了控制难度,且变形镜无法正常工作。针对这一问题利用压电陶瓷极化及铁电材料的电滞回线理论进行了分析,明确了蝶形曲线产生的原因。通过实验确定了变形镜矫顽场强度在-500~-400 V/mm之间,迟滞曲线回归一般的柳叶形状。根据迟滞曲线的特点设计了静态的PID闭环校正系统,并进行了校正实验。结果表明,闭环校正后线性度得到明显提升,迟滞率可降低至1.8%。     

An active approach to harmonic filtering

Harmonic distortion in electricity supplies can seriously affect the performance of a wide range of electrical equipment. In this paper, the author describes how the use of active power harmonic filters is providing an effective solution to a growing problem     

Maximum entropy spectral analysis in antenna spatial filtering

An eight-element linear receiving array working inGband is simulated. The azimuthal response is formed using a maximum entropy algorithm. This gives the antenna a remarkable ability to detect low-level signals in the presence of high-level signals. The sidelobes encountered when using Fourier analysis do not seem to exist. The resolution of two equal targets can be made to depend on the signal-to-noise ratio only, by averaging the prediction coefficients from many data sets containing random phase difference information from the two targets.     

New wide-range fast-acting frequency tracking loop

A digital frequency tracking loop with wide tracking range and fast acquisition performance is described. Designed with high speed logic, the loop can be used in fast digital frequency synthesisers and FM demodulators.<>     

A digital phase-locked loop for generating frequency discriminating codes and frequency multiplication

A new digital phase-locked loop system realizale by a few off-the shelf digital ICs is described. The system is locked by tracking the input square wave and produces an output binary code whose value is proportional to the input frequency. It is characterized by wide locking range and fast capture time down to very low frequencies (<1 Hz) and suited to low frequency multiplication. Asymmetry in capture time, however, limits its use as an FM demodulator to only slowly varying signals.     

EMG-controlled closed loop electrical stimulation using a digitalsignal processor

A simple but powerful method of removing stimulation artefact from voluntary electromyogram (EMG) is presented. By using a fast digital signal processor the underling voluntary EMG from stimulated muscle can be extracted in real time. This is then processed to form a control signal to effect the stimulation amplitude