19.7-MHz Chebyshev LPF Targets IEEE 802.11n

The IEEE 802.11n wireless-local-area-networking (WLAN) standard employs a high-throughput extension option. As a result, a WLAN system's bandwidth can be 40 MHz in support of higher data rates. When the direct-conversion architecture is employed for these systems, a lowpass filter (LPF) with a bandwidth of 10 and/or 20 MHz is required. The design of a wideband LPF involves the selection of both the filter inductor-capacitorresistor (LCR) prototype (i.e., Butterworth or Chebyshev) and G_m C and active-RC. Shouhei Kousai and Mototsugu Hamada from Toshiba Corp.'s Center for Semiconductor Research and Development and Fui Ito and Tetsuro Itakura from Toshiba's Mobile Communication Laboratory have presented a quality-factor (Q) -tuning scheme that realizes wideband Chebyshev active-RC filters with reduced biasing current requirements.

This fifth-order LPF with a Q-tuning circuit has been implemented for draft IEEE 802.11n in 0.13-m CMOS technology. According to the researchers, the proposed Q-tuning technique results in a low-power, 19.7-MHz, active-RC Chebyshev LPF. The filter boasts 2 dB gain, 30 nV/Hz input-referred noise, and -113 dBV input power at 1-dB compression. With an area of just 0.2 mm², it draws 7.5 mA current from a 1.5-V supply.

When the pole frequency is comparable to an amplifier's gain-bandwidth (GBW) product, one should take into account the pole of a variable resistance to tune the bandwidth. In addition, the insertion of a variable resistor into a lossy integrator is critical to the compensation of poles. See "A 19.7-MHz, Fifth-Order Active-RC Chebyshev LPF for Draft IEEE 802.11n with Automatic Quality-Factor Tuning Scheme," IEEE Journal of Solid-State Circuits, November 2007, p. 2326.