Signal Filters

Signal filters block or decrease (attenuate) unwanted frequencies or signal wave characteristics. There are several basic types of signal filters. Low pass filters block or attenuate signals at frequencies above a specified cutoff frequency. Conversely, high pass filters block or attenuate signals at frequencies below a specified cutoff frequency. Band pass filters block or attenuate signals at frequencies outside of the specified low pass and high pass cutoff frequencies. Often, these filters combine or cascade low pass and high pass filters. Band pass filters, which block a portion of the frequency spectrum, allow both lower and higher frequencies to pass. All pass filters, which can be used to introduce desired phase shifts in signals, allow all frequencies to pass. 

Signal filters are often programmable and provide anti-aliasing features that prevent the misrepresentation of signals during improperly slow sampling. They can be either analog or digital. Analog signal filters are designed with resistors and capacitors and process analog signals in applications with low-noise requirements. Digital filters are designed with solid-state components and can process both digital and quantized signals. There are two basic categories of digital filters: finite impulse response (FIR) and infinite impulse response (IIR). FIR filters are suitable for multi-rate applications and can be implemented with fractional arithmetic. IIR filters require less memory and fewer calculations than comparable FIR filters, but are more susceptible to problems of finite-length arithmetic. 

There are several frequency responses for signal filters. Bessell filters feature a relatively flat pass band, slow roll-off, and complex circuitry. The stopband is fairly monotonic or ripple-free. Butterworth filters provide a very flat response and almost no attenuation in the pass band. The roll-off rate is somewhat slower than other filters. Cauer or elliptic filters provide the fastest roll-off for a given order. Typically, both the pass band and the stop band have ripples. Chebyshev filters also provide a very fast roll-off, but at the expense of a rippled pass band. Linear phase response filters preserve the phase characteristics of signals by time delaying signal components equally. 

Device specifications for signal filters include the number of analog channels and the number of differential channels. When single-ended outputs are available, suppliers often specify the maximum number of analog channel outputs as twice the number of differential outputs. Differential channels, which have two inputs, define the voltage as the signal to process between the two inputs. Other specifications for signal filters include the number of poles, the high pass cutoff frequency, and the low pass cutoff frequency. Most signal filters are available with 2, 4, 6, or 8 poles. The high pass frequency is the nominal frequency above which input signals are passed and below which signals are blocked. Low pass cutoff frequency is the nominal frequency below which input signals are passed and above which signals are blocked.

There are several form factors for signal filters. Some devices mount on integrated circuits (ICs), standard DIN rails, or printed circuit boards (PCBs) that attach to enclosures or plug into computer backplanes. Others bolt into walls, cabinets, enclosures, or panels. Rack-mounted units fit inside a standard 19” telecommunications rack. Modular styles include stackable units that dock in bays, slots, or boxes. Benchtop or freestanding signal filters often feature full casings or cabinets and integral interfaces.