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Design considerations of bandpass LC filters for RF applications
Abstract
This paper presents different design considerations of active Q-enhanced LC bandpass filters. An architecture for implementing high order filters is proposed. It uses electric coupling to emulate the effect of the transformer, thus providing bandwidth tuning with small passband ripple.
... A Q-enhancement technique can be employed in order resolve this problem [3]. Nevertheless, non linearity and power consumption of the modulator will be increased [4], [5]. On the other hand, it is known that electro-mechanical resonator such as MEMS (MicroElectroMechanical System), SAW (Surface Acoustice Wave) and BAW (Bulk Acoustic Bave) have a high Q-factor [6]. ...
... A International Journal of Computer Theory and Engineering, Vol. 4, No. 4, August 2012 Sample and Hold (S/H) block is used in order to subsampling the signal at the output of the resonator with a sampling frequency F s . As detailed in [9], the relation between Fs and the center frequency f p is given by: 4 ,0 21 ...
... Thus, the common-mode gain can become zero by keeping , which can be satisfied by controlling the sizes of transistors. Fig. 10(c) shows the complementary negative resistance circuit based on the positive feedback crossing structure [13], which is used to increase the output impedance of the transconductors in order to achieve higher Q factor for the filter. The transistor sizes of the negative resistance circuit are tuned to ensure that the DC operation point of the output is the same to the desired common-mode output level. ...
... From (13) and (14), the ac response of the filter structure in Fig. 12(a) can be expressed as (15) where Since the two center frequencies in (15) are determined by the two separate LC resonators, the bandwidth and the passband flatness are sensitive to process and temperature variations. Fig. 12(b) shows the filter structure in which two identical resonators are coupled by a negative feedback path . ...
In this paper, a linearity improvement technique is proposed for a low-distortion Gm-C bandpass filter operating in high IF ranges. The proposed transconductor eliminates Gm" value at the output by superposing the opposite non-linear behaviors of two differential structures in parallel. For the bandpass filter, instead of conventional biquad structure, a resonant-coupling structure is adopted for a flat frequency response which is insensitive to process and temperature variations. Fabricated in 65 nm CMOS process, the implemented 80 MHz bandpass filter shows a flat bandpass characteristic with 0.1 dB ripple, third-order harmonic rejection of 27 dB, IIP3 of -2 dBm, and NF of 21.5 dB, while consuming 11 mA from 1.2-V supply. The filter occupies the chip size of 0.5 × 0.5 mm2.
... But passive filtering [5] is not of much interest since passives are hard to tune for wide ranges which makes this approach adequate for narrowband non-tunable receivers. Monolithic implementations of active LC filters have been proposed in low cost CMOS technology [5][6][7]. Due to the limited quality factor of on-chip inductors, Q-enhancement has been used to design narrowband RF filters. However, this has been plagued with high power consumption, poor selectivity, linearity and noise performance. ...
In this paper, an active filtering technique is presented which is capable of filtering the out-of-band blockers in wireless receivers. The concept is based on the feedforward cancellation technique where a blocker replica is subtracted at the output of the low-noise amplifier (LNA). In contrast to the previously reported feedforward cancellation methods, exact gain and phase matching are easily obtained in the proposed architecture to produce a highly selective narrowband frequency response at the output of the LNA with wide rejection bandwidth. For the proof of concept, the system is implemented in a 65 nm CMOS technology. It occupies a total area of 0.8 mm2 and the current consumption is 24 mA from a 1.2 V supply. The system post-layout simulations showed a blocker rejection of more than 33 dB for blocker signals 100 MHz away from the desired signal when the feedforward path is activated. The noise figure (NF) of the entire system is 3.8 dB that degrades to 5.8 dB when the feedforward path is activated.
Vast attention has been paid to active continuous-time filters over the years. Thus, as cheap, readily available integrated circuit OpAmps replaced their discrete circuit versions; it became feasible to consider active-RC filter circuits using large numbers of OpAmps. Similarly, the development of integrated operational transconductance amplifier (OTA) led to new filter configurations. This gives rise to OTA-C filters, using only active devices and capacitors, making it more suitable for integration. The demands on filter circuits have become ever more stringent as the world of electronics and communications has advanced. In addition, the continuing increase in the operating
frequencies of modern circuits and systems increases the need for active filters that can perform at these higher frequencies; an area where the LC active filter emerges. What mainly limit the performance of an analog circuit are the non-idealities of the used building blocks and the circuit architecture. The research is concentrating on the design issues of high frequency continuous-time integrated filters.
Several novel circuit building blocks are introduced. A novel pseudo-differential fully balanced fully symmetric CMOS OTA architecture with inherent common-mode detection is proposed. Through judicious arrangement, the common-mode feedback circuit can be economically implemented.
On the level of system architectures, a novel filter low-voltage 4th order RF bandpass filter structure based on emulation of two magnetically coupled resonators is presented. A unique feature of the proposed architecture is using electric coupling to emulate the effect of the coupled inductors, thus providing bandwidth tuning with small passband ripple.
As part of a direct conversion dual-mode 802.11b/Bluetooth receiver, a BiCMOS 5th order low-pass channel selection filter is designed. The filter operated from single 2.5V supply and achieves 76dB of out-of-band SFDR. A digital automatic tuning system is also implemented to account for process and temperature variations.
As part of a Bluetooth transmitter, a low-power quadrature direct digital frequency synthesizer (DDFS) is presented. Piecewise linear approximation is used to avoid using ROM look-up table to store the sine values in a conventional DDFS. Significant saving in power consumption, due to the elimination of the ROM, renders the design more suitable for portable wireless communication applications.
In this paper an integrated SAW-less narrowband RF front-end for direct conversion wireless receivers is presented. The analysis of the feedback system shows a shift of the center frequency f<sub>RX</sub> for the overall RF bandpass filter from its nominal value f<sub>LO</sub>. The proposed architecture incorporates a notch filter at 2f<sub>LO</sub> to insure that there is no shift in f<sub>RX</sub>. The design has been implemented in 65nm CMOS process. It consumes 44mA from a single 1.2V supply. Simulation results show a rejection of more than 15dB in a bandwidth of +/-500MHz around 2GHz due to the additional feedback loop. The theoretical and simulation results are in close agreement.
Vast attention has been paid to active continuous-time filters over the years. Thus as the cheap, readily available integrated circuit OpAmps replaced their discrete circuit versions, it became feasible to consider active-RC filter circuits using large numbers of OpAmps. Similarly the development of integrated operational transconductance amplifier (OTA) led to new filter configurations. This gave rise to OTA-C filters, using only active devices and capacitors, making it more suitable for integration. The demands on filter circuits have become ever more stringent as the world of electronics and communications has advanced. In addition, the continuing increase in the operating frequencies of modern circuits and systems increases the need for active filters that can perform at these higher frequencies; an area where the LC active filter emerges. What mainly limits the performance of an analog circuit are the non-idealities of the used building blocks and the circuit architecture. This research concentrates on the design issues of high frequency continuous-time integrated filters. Several novel circuit building blocks are introduced. A novel pseudo-differential fully balanced fully symmetric CMOS OTA architecture with inherent common-mode detection is proposed. Through judicious arrangement, the common-mode feedback circuit can be economically implemented. On the level of system architectures, a novel filter low-voltage 4th order RF bandpass filter structure based on emulation of two magnetically coupled resonators is presented. A unique feature of the proposed architecture is using electric coupling to emulate the effect of the coupled-inductors, thus providing bandwidth tuning with small passband ripple. As part of a direct conversion dual-mode 802.11b/Bluetooth receiver, a BiCMOS 5th order low-pass channel selection filter is designed. The filter operated from a single 2.5V supply and achieves a 76dB of out-of-band SFDR. A digital automatic tuning system is also implemented to account for process and temperature variations. As part of a Bluetooth transmitter, a low-power quadrature direct digital frequency synthesizer (DDFS) is presented. Piecewise linear approximation is used to avoid using a ROM look-up table to store the sine values in a conventional DDFS. Significant saving in power consumption, due to the elimination of the ROM, renders the design more suitable for portable wireless communication applications.
A novel CMOS circuit for obtaining a bandpass response from a triple-coupled-inductor arrangement is presented, featuring Q-enhancement and center frequency tuning by means of vector-modulating a current flowing through one of the coupled inductors. A 0.35-μm CMOS LC filter prototype employing the technique has been fabricated and exhibits a center frequency tuning range of 11% around 1 GHz and Q values up to 180. The input 1-dB compression point is -13 dBm with Q set to 20 and a power consumption of 12.2 mW. Additionally, an input impedance matching scheme around a spiral transformer is presented, which tracks the center frequency of the filter. The active-LC approach can be applied to higher order filter responses and find applications in tunable building blocks for agile RF front ends and multistandard radios.
The capacitive gate structures available in digital-oriented CMOS
processes are reviewed, with emphasis on their use as linear capacitors.
It is shown that the voltage harmonic distortion in MOS gate capacitors
biased in either accumulation or strong inversion is almost technology
independent. Experimental and analytical results indicate that the total
harmonic distortion in an adequately biased (2.5 V) gate capacitor can
be kept low (THD <-40 dB for a 3-V voltage swing)
The achievable quality factors (Q) of on-chip inductors fabricated
with standard Si technology are limited. Inductor Q factors can be
enhanced by active means, resulting in GHz-range filters with adequate
dynamic range for certain applications. One such application is the
replacement of off-chip filters in the transmit part of a wireless
transceiver, following a mixer. The limits of the technique are tested
here by using it in a filter for the entire band used in the PCS 1.9 GHz
standard. A tuning-friendly filter can be built with magnetically
coupled LC resonators. Two identical Q-enhanced LC resonators, together
with the loading impedances, are used form a balanced 4th-order bandpass
filter. The filter and its tuning system are implemented in a 0.25 μm
BiCMOS technology
A novel loss-control loop for use in voltage-controlled oscillator
indirect tuning systems of radio-frequency continuous-time filters is
presented. In contrast to conventional schemes, this loop can achieve
robust amplitude regulation
A technique is proposed in this paper for linearizing active
Q-enhanced monolithic LC filters. This technique employs capacitive
dividers and tunable linear transconductors. Various multi-tanh
translinear circuits can be used to implement the linear
transconductors, taking into consideration the need for linear operating
range and the low supply voltage. A linear Q-enhanced LC filter using
the proposed technique has been implemented in a 0.5 μm bipolar
process. The demonstrated filter chip operates at a center frequency
around 1 GHz with the quality factor tunable up to 400 without spurious
oscillation. For a Q of 40, the input intercept point and spurious-free
dynamic range are -6.67 dBm and 36 dB, respectively, at a power
consumption of 68 mW and with 7 dB of gain
A fully differential high-Q bandpass filter that uses lossy
integrated inductors is presented. The circuit is implemented in a 0.8
μm BiCMOS technology and realizes a center frequency of 750 MHz with
a Q-factor that is tunable from 10 to 490 while dissipating 80-100 mW
from a single 5 V supply. Since the objective of the prototype was to
explore the proposed Q-enhancement technique, the dynamic range is
limited to 25 dB for Q=20
Q-enhanced LC filter technology offers an alternative to the use
of direct conversion techniques for implementing fully integrated
receivers. Design and performance issues for QE LC filters are discussed
and a fully integrated 850 MHz, two-pole, bandpass filter with an 18 MHz
3 dB bandwidth is reported. The prototype design is implemented in a
standard 0.8 μm CMOS process and achieves a rejection of over 50 dB
at 100 MHz offset, an in-band dynamic range of 75 (90) dB when used in a
system with a 1 MHz (30 kHz) final IF bandwidth, and a third-order
intercept point that exceeds +25 dBm at an 80 MHz offset from the
passband center,
A second-order active bandpass filter using integrated inductors
was implemented in Si bipolar technology. The filter uses special
techniques to make the quality factor and the center frequency tunable.
For a nominal center frequency of 1.8 GHz and a quality factor of 35,
the filter has 1 dB compression dynamic range of 40 dB, and draws 8.7 mA
from a 2.8 V supply