**Low Pass Filter:**

The prototype T and π low pass filter sections are as shown in the Fig. 9.3.

**Design Impedance (R**_{0}):

_{0}):

Here in low pass filter sections,

Total series arm impedance Z_{1} = jωL

Total shunt arm impedance Z_{2} = -j/ωC

Hence, Z_{1} . Z_{2} = (jωL) (-j/ωC) = L/C which is real and constant. Hence sections are constant K sections so we can write,

**Reactance Curves and Cut-off Frequency Expression:**

As both T and π sections have same cut-off frequency, it is sufficient to calculate f_{c} for the ‘T’ section only.

The reactance curves are as shown in the Fig. 9.4.

From above characteristic it is clear that all the reactance curves have positive slope as all curves slope upward to the right side with increasing ω.

The curves are on opposite sides of the frequency axis upto point A; while on the same side, from point A on wards. Hence all the frequencies upto point A give pass band and above point A give stop band. Thus point A marks cut-off frequency given by ω = ω_{c}.

At point A, ω = ω_{c}, the curve for (X_{1}/4 + X_{2}) crosses the frequency axis, hence we can write,

The algebraic approach to calculate cut-off frequency is as follows.

From above expression it is clear that, Z_{0T }is real if ω^{2}LC/4 < 1 and imaginary if ω^{2}LC/4 > 1. Hence condition ω^{2}LC/4 -1 = 0 gives expression,

Thus, above prototype section passes all frequencies below ω = 2/√LC while attenuates all frequencies above this value. Therefore cut-off frequency of low pass filter is given by

Above frequency comes out to be the same as calculated by reactance sketch method.

**Variation of Z**_{0T} and Z_{0π }with Frequency:

_{0T}and Z

_{0π }with Frequency:

Consider expression

From equation (2), we can write

Similarly we can write,

Hence

From equation (5), it is clear that as frequency increases from 0 to f_{c}, Z_{0T} decreases from R_{0} to 0 in passband. For π section, from equation (6), it is clear that in pass band as frequency increases for 0 to f_{c}, Z_{0π} increases from R_{0} to ∞.

The variation of Z_{0T} and Z_{0π} with frequency is as shown in the Fig. 9.5.

**Variation of Attenuation Constant α with Frequency:**

In pass band attenuation is zero. In stop band attenuation is given by,

In stop band, as frequency f increases above f_{c}, attenuation also increases. The variation of α with frequency is as shown in the Fig. 9.6.

**Variation of Phase Constant β with Frequency:**

In stop band, phase constant β is always equal to π radian. In pass band where α = 0, the phase constant β is given by

As frequency increases from 0 to f_{c}, β also increases from 0 to π radian. The variation of β with frequency is as shown in the Fig. 9.7.

**Design Equations of Prototype Low Pass Filter:**

The design impedance R_{0} and cut-off frequency f_{c} can be given in terms of L and C as follows.

Dividing equation for R_{0} by f_{c}, we get,

Multiplying equation for R_{0} and f_{c} we get,

Equations (9) and (10) are called design equations for prototype low pass filter sections.