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III. Gm-C Filtering - Epublications - Université de Limoges

III. Gm-C Filtering - Epublications - Université de Limoges

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Figure 55. Rejections of a constant Q (Q=10) and constant BW (BW=20MHz) second or<strong>de</strong>r bandpass<br />

filter<br />

II.1.d Double Notch Topologies<br />

Another possible topology has been studied, consisting in two casca<strong>de</strong>d notch filters<br />

creating a selective bandwidth as illustrated in Figure 58. Once again, the transfer function of<br />

a single notch filter can be obtained from the low-pass transfer function by means of the<br />

following transformation:<br />

s.<br />

BWω<br />

s → , 2 2<br />

(II.22)<br />

s + ω0<br />

where BWω is the filter stopband bandwidth.<br />

This leads to the following transfer function of a double notch, consi<strong>de</strong>ring their<br />

quality factor:<br />

H DNF<br />

( s)<br />

2 2<br />

2 2<br />

s + ω1<br />

s + ω2<br />

=<br />

.<br />

.<br />

2 sω1<br />

2 2 sω2<br />

2<br />

ω1<br />

+ + s ω2<br />

+ + s<br />

Q<br />

Q<br />

1<br />

- 46 -<br />

2<br />

(II.23)<br />

The low-pass to notch transformation [II.3] corresponds to the transformation of the<br />

passive elements of the circuit given in Figure 56:

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