Passive filters use only passive components such as resistors, capacitors and inductors. They do not produce any amplification of the input signal. In passive filters, the size and arrangement of these electrical and electronic components determines which frequencies are allowed to pass. Typically, passive filters use resistor-capacitor (RC), resistor-inductor (RL), inductor-capacitor (LC), or resistor-inductor-capacitor (RLC) circuits. Resistors are two-terminal devices which resist an electric current by producing a voltage drop across its terminals. Inductors block high-frequency signals and conduct low-frequency signals. By contrast, capacitors block low-frequency signals and conduct high-frequency signals.
Types of Passive Filters
There are many types of passive filters, but basic categories include low pass, high pass, band pass, and universal or programmable. Low pass filters attenuate frequencies below a frequency called the cut-off frequency. High pass filters attenuate frequencies above the cut-off frequency. Bandpass filters are used to attenuate frequencies above and below a range of frequencies (i.e., the bandwidth or passband of the filter). Any signal with a frequency within the bandpass range passes through these passive filters. Conversely, any signal with a frequency outside the bandpass is reduced. Universal passive filters can be configured by the user to function as a low pass, high pass, or band pass filter. Coupled resonant band pass filters are also available.
Specifications for passive filters include response type, filter interface, and environment. Passive filters have two response types: transmission and reflection. Transmission responses are given as insertion loss, phase nonlinearity, or differential group delay. Generally, reflection response is specified as return loss. Filter input/output interfaces for passive filters include coaxial connectors, solder lugs, and tabs. Environmental factors include operating and storage temperature, as well as humidity, altitude, shock and vibration.
Filter characteristic is an important specification to consider when selecting passive filters. Choices include Bessel, Butterworth, Chebyshev, and elliptic. Bessel filters are passive filters with a passband which maximizes the group delay at zero frequency, thus showing a constant group delay in the passband. Butterworth filters are designed so that the frequency response is flat in the passband. Chebyshev filters feature a very steep roll-off, but have ripples in the passband. Elliptic filters (Cauer filters) exhibit equalized ripple in both the passband and the stopband.