Explain frequency and impedance scaling with suitable example.
Answers
From Practical Filters and Couplers: A Collection from Applied Microwave & Wireless
Frequency and Impedance Scaling
The presented schematics are all normalized for ?3 dB cutoff at ?=1 radian/sec. To adjust this cutoff for a specific filter need, the components must be scaled to move the pole frequencies by the appropriate amount. To scale the low-pass filters given in Figures 1, 3 and 5 for a ?3 dB cutoff frequency f c . Choose a resistor value Z, then:
where i=1, 2, 3, and C iN is the normalized value of C i. To scale the filters given in Figures 2, 4 and 6, choose a capacitor value C, then:
where i=1, 2, 3, and R iN is the normalized value of R i.
In Figures 2 and 5, resistors R 4 and R 5 can have any value, as long as they are equal. In Figures 3 and 6, resistor R 5 should be multiplied by the chosen value of R 4 for the final value.