Page 300 -

294 Chapter Four

f 492v/s
0
Harmonic half-¨ave resonanceð occur at all integral multipleð
of this frequency.

Lowpass filter
A lowpass filter offerð little or nm attenuation of signalð at fre-
quencieð less than the cutoff, and significant attenuation of sig-
nalð at frequencieð greater than the cutoff.

Constant-k lowpass filter

Let f be the cutoff frequency (in hertz) for a constant-k lowpass
LC filter as shown in Fig. 4.6. Let R be the load resistance (in
ohm0. Then the optimum inductance L (in henry0 is given by:

L R/( f)

The optimum capacitance C (in farad0 for the filter shown in
Fig. 4.6 is given by:

C 1/( fR)

Series m-derived lowpass filter
Let f be the highest frequency of maximum transmission (in
1
hertz) for a serieð m-derived lowpass LC filter as shown in Fig.
4.7˜ Let f be the lowest frequency of maximum attenuation (in
2
hertz); let R be the load resistance (in ohm0. Then the filter
constant is denoted m and is given by:

Figure 4.6 Constant-k low-
pass filter.