OTA-C Filters with Finite Zeros

In this paper we present a novel technique for OTA-C filter realizations with finite zeros based on doubly loaded passive ladder networks. Only grounded capacitors are needed; all floating capacitors are replaced with active simulations eliminating bottom-plate parasitic capacitors and non-observable poles. Bandpass, highpass and bandstop filters are easily obtained from a lowpass OTA-C prototype applying standard frequency transformations that preserve the active simulation of floating capacitors, i.e. the finite zeros realization. Also, we address the high frequency stability problem of highpass and bandstop OTA-C filters simulating doubly loaded passive ladder networks. These filters usually have floating nodes where the OTA excess-phase acting over the nodal parasitic capacitance can introduce unstable poles at high frequencies. The stability problem is fairly the same for highpass and bandstop OTA-C filters based on approximations with and without finite zeros; only the most complex case (filters with finite zeros) will be addressed here.     

Log-domain high-order low-pass and band-pass filters

Continuous time current-mode high-order low-pass and band-pass filters based on the log-domain concept are presented in this paper. The passive RLC ladder networks are used as the prototype to achieve the proposed filter by simulating the RLC network synthesis method. The achieved filters have inherited the good sensitivity performance from the RLC passive prototype. Fifth-order RLC ladder low-pass filter and sixth-order RLC ladder band-pass filter are used as prototypes and the signal flow graph (SFG) technique is used for the synthesis. The SFG can identify group of integrators and several signal paths. Log-domain lossy and lossless integrators based on BJT technology are deployed to achieve the integrators for realization of proposed filters. The simulations were carried out and the results exhibited several features which are in agreement with the RLC prototype. The frequency response of filters along 100 kHz to 10 MHz can be electronically tuned through 5–500 µA of bias currents. The THD lower than 1% of LP and BP filters were measured at 10 MHz input. The multi-tone tested was included in the paper for verifying the performance of proposed LP and BP filters. The intermodulation distortions around −50 dB and −60 dB were also investigated for the proposed LP and BP filters.     

Exact design of stray-insensitive switched-capacitor lowpass ladder filters from unit-element prototypes

A new technique is described in which stray-insensitive SC lowpass ladder filters of the LDI type are designed without the need for network synthesis. These filters exactly simulate the response of a unit element cascade prototype filter and may be designed with a usable frequency range up to one half the sampling frequency. This design procedure requires no more than simple hand calculation.     

Design of Variable Two-Dimensional FIR Digital Filters by McClellan Transformation

In this paper, the technique of McClellan transformation is applied to design variable 2-D FIR digital filters. Compared with the conventional transformation, the 2-D transformation subfilter and the 1-D prototype filter are designed such that their frequency characteristics are adjustable. Moreover, they are tunable by the same variable parameter, so the variable characteristics of 1-D prototype filters are coincident with those of 2-D subfilters. Several examples, including variable fan filters, variable circularly symmetric filters, and variable elliptically symmetric filters with arbitrary orientation, are presented to demonstrate the effectiveness and the flexibility of the presented method.      

On the Elimination of Reactive Element Loops and the Use of Special Approximations in the Design of OTA-C Filters

Loops of reactive elements in passive filters cause a series of problems in OTA-C simulations of these structures. These loops can be eliminated in a systematic way by the procedures described in the paper. The use of special approximations in the design of the passive prototype filters allows the elimination of multiple loops of reactive elements that appear in usual LC ladder filters with finite transmission zeros. This leads to more efficient active simulations, where problems with floating capacitors, DC or high-frequency instability, or limited operating frequency are avoided.     

Synthesis of highpass switched-capacitor LDI filters

Exact techniques are given for the design of highpass LDI switched-capacitor ladder filters. The filters have no lumped counterparts and cannot be obtained from a lumped prototype by a transformation. Only stray-insensitive circuits are employed in the realisation, and the filters retain the lowsensitivity properties of a passive (distributed) prototype function.     

低复杂度的可变分数时延滤波器设计

为实现低复杂度、高精度的可变分数时延滤波器设计,该文提出一种截止频率可控的高效设计法。该方法将全相位滤波器的解析设计与三次样条插值和泰勒级数展开相结合,既可以通过设置时延参数精确地调整滤波器的分数时延,又可以通过设置截止频率参数快速配置Farrow结构中各子滤波器的抽头系数,从而灵活地调整滤波器的截止频率。仿真实验表明,所提方法适用于设计具有中、低截止频率的可变分数时延滤波器,其设计复杂度相比于现有的加权最小二乘设计法低1个数量级。     

Exact design of highpass switched-capacitor filters of the LDI type

The letter presents a technique by means of which stray-insensitive SC highpass ladder filters may be designed without the need for network synthesis. The starting point is a unit-element cascade (i.e. lowpass prototype from which the corresponding highpass filter is obtained by direct frequency transformation.     

基于多输出端电流模式全差分积分器的梯形滤波器实现

本文提出了利用低电压多输出端电流模式全差分积分器（MCDI）综合连续时间电流模式滤波器的方法。首先对所设计的无源梯形滤波器用信号流图形（SFG）进行综合，然后用MCDI实现该SFG，分析并模拟了MCDI及所提出的滤波器的特性，应用3.3V、0.5μm、CMOS工艺参数仿真得到的1dB波纹四阶Chebyshev低通滤波器功耗仅为1mW左右，且其截止频率可在很宽的范围内调控，此外，这种滤波器还具有结构简单，对称性好，失真小等优点，适于全集成。     

A Unified Discussion of High-Q Waveguide Filter Design Theory

For the general design of conventional, high-Q, direct-coupled waveguide filters to be based on the frequency behavior of a classical ladder network prototype, it is necessary and sufficient that the reflecting elements of the filter be replaceable by admittance inverters and that the lengths of transmission line be replaceable by resonant elements. The error due to the latter assumption is of the order of twice the square of the percentage bandwidth measured in guide wavelengths, and the classical synthesis problem is a limiting case of a solvable transmission line problem. In this limit, an exact equivalence is established between the design of a direct-coupled filter and the design of a quarter-wave-coupled filter based on the same ladder network prototype. Design formulas for equal ripple and maximally flat performance are given for the VSWR's of the reflecting elements in terms of dimensionless quantities. Detailed comparison of previous formulas is made.     

Efficient implementation of tunable ladder filters using multi-output current controlled conveyors

A methodology to realize continuous-time current-mode tunable ladder filters of any order has been presented. This proposed technique individually simulates signal flow graph (SFG) of each branch element from passive filter prototype using only multi-output second generation current controlled conveyors (MCCCIIs) and grounded capacitors. This leads to simple structure, ease of design and suitability for IC fabrication. A third-order Butterworth low-pass filter, a third-order elliptic low-pass filter and a sixth-order Chebyshev band-pass filter are employed to demonstrate the proposed realization scheme. These simulated filters retain minimum requirement of passive and active elements and provide the filter corner frequency tunability. Moreover, the method allows an implementation of the elliptic filters by simply adding floating capacitors to all-pole filter structures.     

低功耗、低灵敏度有源带通滤波器设计的新方法

提出运用低通到带通的频率变换法与阻容细化法相结合的思想,直接利用2阶的低通(LP)滤波器原型电路实现4阶低功耗、低灵敏度的带通(BP)滤波电路的设计新方法。具体而言,通过对低通原型滤波电路进行阻抗变换以及时其对应的RC电路实现所谓“有损的LP-BP变换”,就可以实现带通(BP)滤波电路的设计。选用2阶切比雪夫(Chebyshev)低通滤波器电路为原型,给出4阶带通滤波器的具体设计过程,并通过PSpice软件来验证此种方法的可行性和实用性。     

Three-channel variable filter-bank for digital hearing aids

This study proposes a three-channel (3-channel) variable filter-bank (VFB) that consists of variable lowpass, variable bandpass and variable highpass digital filters. The three variable digital filters are obtained from a normalised analog prototype Chebyshev type-I lowpass filter using analog frequency transformations along with a modified bilinear transformation. Since both the magnitudes (gains) and band edge frequencies of the three variable digital filters are independently adjustable, the 3-channel VFB is considerably flexible, and can be successfully applied for compensating various hearing loss patterns in digital hearing aids. Various audiograms have been used to verify that high-accuracy fittings can be achieved with low order variable filters. Moreover, the authors reveal and theoretically prove the numerator coefficient-symmetries of the variable lowpass, variable bandpass and variable highpass filters, and show that each variable filter requires only one multiplication for its numerator filtering operations, so the total number of multiplications can be significantly reduced. More specifically, only 11 multiplications and 14 additions are required in the whole 3-channel VFB. Therefore the 3-channel VFB has extremely simple structure and high tuning flexibility for hearing aids.     

Linear phase variable digital bandpass filters

A transformation for obtaining a linear phase variable cut-off low-pass digital filter has been recently described in the literature [1]. In this letter, this transformation is worked out for the case of linear phase bandpass digital filters with variable bandwidth and center frequency.     

Modular log-domain filters realized using wave port terminators

Log-domain wave filters that simulate the passive LC ladder prototype filters are introduced in this paper. The proposed circuits are constructed from the wave equivalents of the reactive elements corresponding to those in the prototype circuit. The wave equivalent circuits are obtained by using a novel log-domain wave port terminator. The wave equivalent of a capacitor in a shunt branch was chosen as an elementary building block for creating high-order filters. The wave equivalent circuits of all other reactive elements in a shunt or in a series-branch connection can be readily obtained using the elementary building block plus some inverters. This way the derived high-order filter configurations are modular. A design example of a third-order elliptic low-pass filter is given, and the performance of the filter was verified by simulation.     

Experimental validation of exact design of switched-capacitor ladder filters

Experimental results validating a recently introduced exact theory for switched-capacitor lowpass ladder filters are presented. The prototype filter is a third-order Butterworth design using a low sampling/filter-cutoff ratio of 8.28 and has been fabricated in monolithic form.     

Accurate switched-current ladder filters based on active current mirrors

Describes the design of switched-current ladder filters based on active current mirrors, and the dependency of the filter characteristic on current mirror implementation from both simulations and prototype measurements. Experiments show that the proposed current mirror improvements are necessary for meeting a given filter specification.<>     

用于宽带接收的近似完全重构滤波器组的改进设计

本文提出了一种可用于宽带接收信号近似完全重构的余弦调制滤波器组。首先将滤波器组的设计转化为原型滤波器的设计,利用代价函数保证其通带平稳和阻带衰减,针对已有算法中幅度失真较大的问题,文中利用代价函数保证过渡带具有平方根余弦滚降特性,并且为了避免加强目标函数约束所带来的误差,对通带截止频率进行调整使得原型滤波器的3dB通带截止频率等于理想滤波器的通带截止频率。文中还推导了以滤波器系数为变量,代价函数的闭合解表达式,并针对滤波器设计复杂度高的问题,采用了基于迭代求解原型滤波器的方法。实验仿真表明,该方法得到的调制滤波器组较已有设计方法具有更好的性能。      

Parasitics-insensitive switched-capacitor ladder filters

A simple method for designing parasitics-insensitive switched-capacitor (s.c.) ladder filters is described. The response of the s.c. filter is related to that of a continuous-time prototype circuit by the bilinear z-transform.     

IC Voltage to Current Transducers with Very Small Transconductance

This paper deals with the design of very small ac transconductance voltage to current transducers intended for the design of low frequency continuous-time filters, very large resistors and other applications. The first type of Operational Transconductance Amplifiers (OTA) is based on a triode biased transistor and a current division technique. The second one uses partial positive feedback which allows to reduce transistor dimensions but the sensitivity to transistor mismatches increases. The proposed techniques can be used for the design of high-order low frequency IC filters, ladder or based on biquads, with moderated transistor dimensions while the dynamic range-cutoff frequency performance is comparable to previously reported structures. A 10 Hz third order lowpass ladder filter has been designed with these techniques, and it shows a dynamic range of 62 dB. Besides, a novel biasing technique for capacitive sources coupled preamplifiers is proposed. Experimental results for a prototype, fabricated in a 1.2 m 1 level below 15 _RMS and dynamic range of 63 dB. The power consumption is only 10 watts and the supply voltages are ± 1.5 volts.