Elektor_Mag_-_January-February_2021

lektor January & February 2021 77

other filters, but the point where the curve drops below the defined

amount of ripple. If you would like to normalise the curves to the

-3dB point, in order to compare them better, you will have to adjust

the corner frequency accordingly.

the stop band. How much ripple we can allow depends on the

application.

In Figure 8 we have enlarged the pass-band from Figure 7. Here

we can see that the corner frequency is not the -3dB point, as with

L4

R 1 C 1

L5 R 2

C 2

C 4 C 5

7. Order

BS Shunt First BS Series First

L1 L2

L6

C 3

C 6

L3

R 1 C 4

L2 R 2

C 5

C 2

L4 L5

L3

C 6

C 3

L6

L1

C 1

5. Order

L4

R 1 C 1

R 2

C 4

L1 L2 C 2 L3 C 3

L1

R 1

C 1 R 2

C 4

L4

C 5

L5

C 2 L2

C 6

L6

C 3 L3

7. Order


8. Order
   BP Shunt First BP Series First

C 7

L7

L5

C 5

L6

C 6

L7

C 7

200522-004a

L3

R 1 C 1

R 2

C 3

L1 L2 C 2

L1

R 1

C 1 R 2

C 3

L3

C 4

L4

C 2 L2

L4

C 4

C 7

L7

L7

C 7

200522-004b

L3

R 1 C 1

L4 R 2

C 2

C 3 C 4

L1 L2

R 1 C 3

L2 R 2

C 4

C 2

L3 L4

L1

C 1

Figure 4: Basic schematics for band-pass and band-stop filters of fifth and seventh order with Cauer or inverse Chebyshev characteristics in PI and T
configurations.

L1

1287

L2

1287

C 1

1n97

C 2

6n36

C 3

1n97

R 2

50

R 1

50

200522-005

Figure 5: Butterworth fifth-order low-pass filter with PI structure and with a

corner frequency of 1 MHz.

Figure 6: Frequency response of a fifth-order Butterworth

low-pass filter in the pass-band. The green curve is the correct

response and the red line shows what happens when a 100-Ω

filter is connected to a source and load of 50 Ω.