Low-pass LC filters are recognized by having the inductor (or inductors) in series with the signal path and the capacitors shunted across the signal path. The lowpass filter (LPF) attenuates all signal frequencies above its cutoff and passes all frequencies below the cutoff.
(A) Three-element, T-section low-pass filter and
(B) three-element, pi-section low-pass filter.
Figure 1 shows two basic single-section LPFs. The t-filter configuration is shown in Fig.1A, and the pi-filter configuration is shown in Fig. 1B. The values of the capacitors and inductors are calculated using the equations:
(1)
(2)
These equations will also be used to calculate the values of the components in the other filters as well, although the numbering of the constants (K) will be different. The values of constants K1 and K2 are found from Table 1.
Table 1. Filter design constants for Fig. 1
Example: Calculate the component values for both t-filter and pi-filter configurations for a low-pass filter with a cut-off frequency of 35 MHz.
The inductors should be single components, wound, or selected and set for the specific inductance required. The capacitors, on the other hand, can be made up from several capacitors in series and parallel in order to obtain the correct value. Remember when doing this, however, that tolerances can make the whole thing less than useful. Most capacitors have tolerances of 5 or 10%, unless otherwise noted. It is best to use as close a value capacitor as possible, and that could involve handselecting capacitors, according to actual capacitance using a meter or bridge.
The inductors can be either homewound or purchased. Although it’s possible to use adjustable inductances and capacitances in these filter circuits, it is not recommended that they be adjusted in the circuit. The adjustable components can allow one to obtain the specific values called for in the equations, but they should be preset to the value prior to being connected into the circuit. This job can be done using either a LC bridge or a digital LC meter, such as are found on some digital multimeters.
Each section of the filter provides a certain degree of attenuation, as indicated by the steepness of the roll-off slope beyond the cutoff frequency. Cascading sections will increase the roll-off slope, so they will also increase the attenuation obtained at any given frequency in the stopband.
