A 34dB SNDR instantaneously-companding baseband SC filter for 802.11a/g WLAN receivers

In wireless receivers, large dynamic range (DR) input signals necessitate additional power consumption in baseband active filters. A 5
^th
order Chebychev, ladder type, companding SC low-pass filter is described in this paper which provides gain switching based on the instantaneous value of the signal to handle peak-to-average-power ratio (PAPR), thereby reducing power dissipation for a given DR. Companding compresses the high DR input signal, processes it in a lower DR system (the filter in our case) and then expands the signal at the output [1]. Companding is an alternative to AGC, which sets the filter internal gains during the preamble/midamble so that the signal level in the filter is optimal to provide the minimum SNDR required by the specifications. The IEEE 802.11a/g WLAN receiver [2] presents 2 limitations to the use of AGC. First, the standard puts a stringent AGC settling time requirement (≪5.6µs). Since the filter needs extra settling time, AGC is implemented after the filter and before the ADC in the receiver baseband. Secondly, the high PAPR of OFDM signals requires headroom of at least 12dB in the DR of the filter. While dynamic impedance and gain scaling techniques have been proposed in the past as an alternative to AGC [3], this work addresses the PAPR problem.