Akp

23-4 (A) Three-element, T-section low-pa.ss filter and (B) three-element, pi-section low-pass filter.

These equations will also be used to calculate Die values of the components in the other filters as well, although the numbering of the constants (K) will be different. The values of constants K\ and K-> are found from T&ble 23-1.

Table 23-1. Filter design constants Eor Fig.

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.

S180

t filter:

The inductors can be either homewound or purchased, Although it's jKissible to use adjustable inductances and capacitances in these fitter circuits, it is not recommended that they be adjusted in the circuit. The adjustable components can allow one to obtain the specific values catted for in the equations, but thpy 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.

The inductors should be single components, wound, or selected and set for the specific inductance required. TTie capacitors, on the other hand, can be made up from several capacitors in series and paraJlei in order to obtain the correct value. Remember when doing this, however, that tolerances can make the whole tiling 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 hand-selecting capacitors, according to actual capacitance using a meter or bridge.

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. Figure 23-5 shows t-filter and pi-filter circuit in the two-section version. The calculation constants are shown in Table 23-2.

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