New command circuit design for tuning high-Q pseudo 8-path switched-capacitor filter

Abstract

This paper describes a new association of a high-Q pseudo 8-path switched-capacitor (S-C) bandpass filter and its command circuit made up by a ring VCO with 'XOR' gates. The proposed circuit presents the possibility of tuning over a radio-frequency (RF) broadband allowing to sweep different channels with a high quality factor. Designed with CMOS process (0.35 µm, for instance), this circuit is intended to replace the surface acoustic wave (SAW) filters in modern wireless receivers. Measurements carried out on a circuit demonstrator show a tunable center frequency range of 200 MHz [300-500 MHz], with a quality factor higher than 300.

Novel LC pseudo switched capacitor filter suited for wireless RF applications

“…Simulations realized with the novel architecture (tuned LC pseudo switched capacitor filter), show the same transfer characteristic behavior compared with the classical tuned RC switched capacitor filter [8]. For these global circuit simulations, all the parasitic effects are taken into account.…”

“…This solution which required a clock frequency equal to the filter center frequency multiplied by the number of branches to commutate can be exclusively used at low frequencies. An original solution was proposed recently in [8] which consists in using a command circuit made up by a ring voltage controlled oscillator (VCO) with "XOR" gates.…”

“…The original association of such command circuit with switched capacitor filters present significant advantages in particular the reduction of command signal frequency (switching frequency) compared to a low frequency solution by a shift register [8]. Moreover, this association can make this type of filters more attractive for designers.…”

“…Figure 2 (b) shows the obtained output constellation compared with the ideal one in case of correlated command sources generated by the ring VCO. These constellations are obtained at a switching frequency equal to 500 MHz, using a π/4-DQPSK digital modulation with a symbol rate equal to 24.3 KHz, by applying the same simulated phase noise value on command signals (−111 dBc/Hz at 1-MHz of the carrier frequency) [8].…”

“…Indeed, if a significant number of branches would permit to improve the filter performances, on the other hand it would lead quickly to an excessive complexity of the design [8]. The compromise retained for this study is eight branches; this number seems to be an acceptable limit of the complexity in the realization on-chip.…”