Newnes Circuit Calculations Pocket Book

Filters and attenuators 315

For stage 2

W

3

19.9

antilog - 0.5 = 0.316

19.9 x 0.316 = 6.29 W

For stage 3

6.29

antilog 0.8 = 6.3

W

4

= 6.29 x 6.3 = 39.69 W

Note that

39.69

Overall gain = 10 ^g

-

^

-

= -1.0 dB

which agrees with

-4 - 5 + 8 = -1 dB

(See program 271, page 311.)

18.3

Low pass filters

A low pass filter transmits low frequencies and attenuates high

frequencies. A typical frequency response is shown in Figure 18.5.

At low frequencies the output voltage is constant, but as the

frequency is increased a point will be reached when the output will

begin to fall. The cut-off frequency, /

c

, is where the output voltage

VQVT

1.0

3dB

down

0.707

0 ...