When co > 1/~ then co2r 2 is much greater than 1 and so we can neglect the 1 and the magnitude is 20 lg coz. This is a straight line of slope +20 dB per decade which intersects with the zero decibel line when co3 = 1. Figure 5.13 shows these lines, the asymptotic approximation to the true plot, for the low
and high frequencies, their intersection being the break point or corner frequency of 09 = 1/3. The true plot has a maximum error which is 3 dB at the break point. The errors are the same as in Table 5.1.