RF Filters Theory

Low-Pass Filters

• Low-pass filters are filters that block high frequencies and let lower frequencies pass. RF filters theory for low-pass filters calls for filter circuits that have a capacitor connected across the load. High frequencies pass through the capacitor while lower frequencies pass on to the load. Adding resistors and additional capacitors tunes the circuit to approximate the theoretical curve that the function of the filter requires. Adding an inductance in series with the load further blocks high frequencies.

High-Pass Filters

• Filters that pass high frequencies on to the load use the same characteristics of capacitors and inductances as low-pass filters. A capacitor in series with the load blocks low frequencies while an inductance across the load passes the low frequencies to ground. RF filters theory predicts that the load sees only high frequencies and defines a transfer function or characteristic that describes how much of each frequency can pass through the filter.

Band-Pass Filters

• Many RF applications require filters that pass a particular frequency "F" but not the frequencies higher or lower than "F." These are called band-pass filters and consist of a high-pass and a low-pass filter in series. RF filters theory specifies a low-pass filter that blocks frequencies higher than "F" and a high-pass filter that blocks frequencies lower than "F." Only frequencies equal to "F" can pass through both filters.

Band-Stop Filters

• Some applications need filters that block a particular frequency "F." These are called band-stop filters, and they have high-pass and low-pass filters in parallel. The high-pass filter passes all frequencies above "F" in its branch while the low-pass filter passes frequencies below "F." The frequency equal to "F" can't pass in either branch while frequencies above and below pass through one of the two filters..

Transfer Functions

• In addition to describing how RF filters block certain frequencies, RF filters theory describes how certain named filters transfer frequencies. The Butterworth filter transitions slowly but smoothly from passing to blocking. The Bessel filter transitions even more slowly and smoothly but shows an increasing pass-through characteristic for frequencies in its pass band. The elliptic filter changes from passing to blocking very quickly but has ripples in both the passing and blocking characteristics while the Chebyshev filter transitions less quickly than the elliptic filter and has ripples only in the pass-band characteristic.