Skip to main content

Scaling a ladder to 1000 rad/s and 75 Ω load

Short answer

With kf=1000 rad/s and km=75 Ω, scaled elements: C1=1.3/(75×1000)=17.33 μF, L2=(75×1.5)/1000=112.5 mH, C3=1.2/(75×1000)=16 μF, L4=(75×0.5)/1000=37.5 mH, RL=75 Ω. Passband edge 1000 rad/s.

What is the significance of normalization and de-normalization in filter design? The following is a low pass filter with ωp = 1 rad/sec. Modify the circuit so that it becomes a low pass filter with a pass band of 1000 rad/sec and a load resistance of 75 Ω. [Figure: source V1, shunt 1.3 F capacitor, series 1.5 H inductor, shunt 1.2 F capacitor, series 0.5 H inductor, 1 Ω load (output V2)]

Significance of normalization and de-normalization

Filter tables and prototypes are given for ωp\omega_p = 1 rad/s and 1 Ω. Normalization lets one table serve every filter and keeps calculations simple; de-normalization (scaling) converts the prototype to the actual frequency and impedance level with practical element values, without changing the response shape:

R′=kmR,L′=kmLkf,C′=CkmkfR' = k_m R, \qquad L' = \frac{k_m L}{k_f}, \qquad C' = \frac{C}{k_m k_f}

Numerical

Given prototype (ωp\omega_p = 1 rad/s): shunt C1C_1 = 1.3 F, series L2L_2 = 1.5 H, shunt C3C_3 = 1.2 F, series L4L_4 = 0.5 H, load RLR_L = 1 Ω.

Required: ωp\omega_p = 1000 rad/s and RLR_L = 75 Ω.

kf=10001=1000,km=751=75k_f = \frac{1000}{1} = 1000, \qquad k_m = \frac{75}{1} = 75 C1′=1.375×1000=17.33 μFL2′=75×1.51000=0.1125 H=112.5 mHC3′=1.275×1000=16 μFL4′=75×0.51000=0.0375 H=37.5 mHRL′=75×1=75 Ω\begin{aligned} C_1' &= \frac{1.3}{75 \times 1000} = 17.33\ \mu\text{F} \\ L_2' &= \frac{75 \times 1.5}{1000} = 0.1125\ \text{H} = 112.5\ \text{mH} \\ C_3' &= \frac{1.2}{75 \times 1000} = 16\ \mu\text{F} \\ L_4' &= \frac{75 \times 0.5}{1000} = 0.0375\ \text{H} = 37.5\ \text{mH} \\ R_L' &= 75 \times 1 = 75\ \Omega \end{aligned}
        112.5 mH      37.5 mH
 o---+---UUUU----+----UUUU----+
 V1  |           |            |
  17.33uF      16 uF        75R   V2
     |           |            |
 o---+-----------+------------+

Answer: C1C_1 = 17.33 μF, L2L_2 = 112.5 mH, C3C_3 = 16 μF, L4L_4 = 37.5 mH, RLR_L = 75 Ω; passband edge 1000 rad/s. (If the source has a 1 Ω resistance it also becomes 75 Ω.)

✓ Answer checked by Claude Sonnet 5.5

All Filter Design chapter 1 questions →