Patent Application
20150256148
1. Field of the Invention
The present invention relates to integrated circuit filters, and particularly to a reconfigurable Nth-order integrated circuit filter.
2. Description of the Related Art
The second generation current conveyor (CCII) was used to realize several high-order filters. Some of these filters, however, have a single output, and hence modifying the filter type would require changes in the hardware. In addition, the absence of a programmability feature of the classical CCII hinders the use of most of these filters in integrated circuit (IC) applications. Available filters adopting electronically programmed CCII are associated with limited tuning range.
Thus, a reconfigurable Nth-order filter solving the aforementioned problems is desired.
The present reconfigurable Nth-order filter includes a second generation current conveyor (CCII) adopting active current division networks (CDNs) for implementing the present filter. A second generation current conveyer leads to wide control of filter coefficients for reconfiguration of the filter. Programmability characteristics are demonstrated through experimental results obtained from integrated circuit chips fabricated in a 0.18 μm CMOS process.
These and other features of the present invention will become readily apparent upon further review of the following specification and drawings.
Similar reference characters denote corresponding features consistently throughout the attached drawings.
The reconfigurable Nth-order filter includes a second generation current conveyor (CCII) adopting active current division networks for implementing the present filter. The CCII leads to wide control of filter coefficients for reconfiguration of the filter. Programmability characteristics are demonstrated through experimental results obtained from integrated circuit chips fabricated in a 0.18 μm CMOS process. The terminal characteristics of a CCII can be described as follows: Iy=0, Vx=Vy and Iz=±Ix where the positive and negative signs denote CCII+(Ix and Iz have same direction) and CCII−, respectively. The following points should be observed in order to realize filters with independent coefficients. First, it can be seen that distributing the inputs or summing desired outputs using active elements with programmable gains a0 through an inherently result in filters with independent coefficients of N(s). Second, it can be seen that these topologies result in independent tuning of D(s) coefficients if programmable active elements are used to realize the feedback factors (b0 through bn−1). With the help of known filter topologies, voltage mode (VM) and current mode (CM) reconfigurable Nth-order filters (RNOFs) based on CCII can be developed. Dual- or multi-output CM active devices are often utilized to reduce the number of active elements.
A single CCII can be utilized to efficiently develop the integrator such that the feedback factor and output signals can be realized using the same device as the CM RNOF based on multi-output CCII 200, shown in
This topology is adopted for CM signals since it is more efficient. A similar topology based on OTAs was developed in prior art current-mode high-order operational transconductance amplifier-capacitor (OTA-C) filters.
The transfer function (TF) of a general Nth-order filter (NOF) response can be expressed as:
where a0 through an, are real numbers, and b0 through bn-1 are positive real numbers.
The second-generation current conveyor (CCII) is an essential building block for current mode (CM) processing. A reconfigurable Nth-order filter (RNOF) is a versatile filter that can be flexibly used to realize any Nth-order filter function without hardware changes. Hence, the RNOF serves a wide range of applications. Such filters are core parts of systems utilizing reconfigurable, e.g., field-programmable analog arrays. The present invention uses CCII in the design of RNOF.
Integrated circuit applications require filters to be associated with programmable parameters to adjust the filter frequency response compensating for components, process and temperature variations. The nature of RNOF demands even broader programmability features to accommodate wide range of selectivity requirements. In this regard, NOF filters can be classified based on their TFs coefficients into programmable or not programmable designs. Also, it is important to further classify the programmable filters as those with orthogonally or independently tuned parameters. Independent tuning of the RNOF coefficients allows adjusting one parameter (e.g. accommodating different bandwidths) but also changing the filter type (e.g. low-to-bandpass), and modifying the overall response (e.g. Butterworth (BT) to Chebyshev).
Traditional CCII, however, does not exhibit programmability feature obstructing its utilization in integrated VLSI applications involving complete systems on chip.
Integrator based filter topologies provide all functions simultaneously with possible independent tuning characteristics. They adopt either distributed inputs (DIs) or summed outputs (SOs) topologies to generate various N(s) terms. In practice, there could be two different circuit realizations these filters based on the availability of the sn function of N(s) from the core circuit. The circuits without sn term are often canonic in the sense that n devices are required to realize the n integrators. In such cases, sn is often obtained through subtracting various outputs from the input signal results in a TF with several matching conditions for realizing the sn function. While there are various known combinations utilizing CCII none show the advantages gained from the configuration developed in the present widely programmable multi-outputs CCII 100 shown in
for m=0 to n−1,
Clearly, the coefficients of D(s) and N(s) (from equation (1)) can be independently adjusted through various βbαb and βaαa, respectively, of equations (2) and (3).
The 4th-order filter 200 obtained from the topology of
The corresponding total power consumption of the filter is approximately 1.55 mW. The input- and output currents are converted to voltage signals using passive resistors of 1kΩ. A VB is also used to drive the output terminal. Equal resistors (R0=R1=R2=R3) of 159kΩ and equal capacitors (C0=C1=C2=C3) of 1 pF are used. These values lead to nominal pole frequency (fo) of approximately 1 MHz. With eight-bit CDN, β can be adjusted from 0 to 0.996 whereas a can be changed from 1.004 to 256. First, CDNs are programmed to realize lowpass BT response with minimum pole frequency. This is achieved by selecting the lowpass output through setting βa1 through βa4 to zero. The lowest pole frequency of approximately 250 kHz is obtained by setting βb0αb0 to 0.0039 while the other corresponding values of βbαb and βa0αa0 are given in Table 1. The table shows the required theoretical values and the closest available digital word.
Then, the pole frequency was varied to its maximum value of 4 MHz by setting βb0αb0 to 256. The corresponding values for other βbαb and βa0αa0 preserving the BT response and same gain are given in Table 1. Next, the pole frequency was set to an arbitrary value of 1 MHz through programming βbαb and βa0αa0 according to the values given in Table 1. The obtained measurement results demonstrating pole frequency adjustments are shown in plot 400 of
The bandpass response is obtained by setting βa0, βa1, βa3, and βa4 to zero while the highpass response is realized by making βa0, βa1, βa2, and βa3 equal to zero. Also, gain adjustments are demonstrated by increasing the gain to 12 dB as shown in plot 500 of
The linearity of the filter was determined by finding the input third-order intercept point (IIP3) determined by performing several intermodulation (IM3) tests using 800 and 900 kHz signals. IIP3 estimation for in-band signals measured at the LPF output is found to be approximately 28 dBm (referenced to 50Ω). The output noise root spectral density for the LPF at gain of 12 dB was approximately 365 nV/Hz1/2 over the passband. Thus, the total in-band noise for the LPF is calculated to be less than −68 dBm over bandwidth of 1 MHz. These values correspond with in-band spurious-free dynamic range of 64 dB.
The number of active devices can be reduced by using multi-output devices. But that approach traditionally leads to losing the programmability feature of N(s) and/or D(s) coefficients. In order to promote programmability feature, devices having different current gains would be required. The present reconfigurable Nth-order filter adopts two CDNs in the design of digitally programmable CCII to provide wide tuning range suitable for a field-programmable analog array (FPAA). In general, the available solutions provide simulation results. It is clear that the present filter comprised of CCII circuitry 100 cascaded in the topology of CM RNOF 200 would provide improved performances due to significant reduction in the number of active devices and passive resistors. The present reconfigurable Nth-order filter would require n+1 CCM and 2n+1 CAs (DCCFs) compared with 3n−2 CCIIs and CAs for prior art designs. The present reconfigurable Nth-order filter exhibits a wide tuning range and offers advantages which are crucial for FPAA applications. The advantages of the present reconfigurable Nth-order filter are supported by experimental results obtained from 0.18 μm CMOS process.
It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.
