A 27 MHz programmable bipolar 0.05 degrees equiripple linear-phase lowpass filter

A continuous-time transconductance-C filter which meets the requirement of the read channel of disk drives based on conventional peak detection pulse qualification is presented. To accommodate constant-density recording with variable data rates tip to 48 Mb/s, the cut-off frequency is tunable between 9 and 27 MHz. The filter is placed inside the AGC (automatic gain control) loop and its differential output signal amplitude is typically held at 1 V/sub PP/. Its primary role is to lower the achievable error rates by bandlimiting the noise originating in the magnetic media and the preamplifier. Its second objective is to equalize the bit stream. i.e. to slim the data pulses, allowing higher densities. To minimize pulse peak shifts in time, an accurate linear phase (or constant group delay) response over the signal bandwidth is essential. The transfer function implemented is seventh-order 0.05 degrees equiripple linear-phase. By allowing negligibly small ripple, the region of constant delay can be extended to about twice the cut-off frequency f/sub c/, compared to 1.5 f/sub c/ for a Bessel filter of equal order, traditionally used in this application. The tradeoff, however, is a nearly twofold increase in sensitivity to transconductance excess phase.<
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