Signal filters are electronic circuits designed to stop unwanted signal frequencies from getting through and/or to amplify the wanted frequencies. There are many ways to classify signal filters. They may be low, band or high pass, all pass or notch, according to the frequency ranges that they pass versus those that they block. Filters may be passive or active, depending on whether they include active components like transistors or op amps or just passive components.

Signal filters are also either linear or non-linear and either analog or digital. Digital signal filters, which operate on quantized samples of the original analog waveform, provide superior frequency response to analog filters. Digital filters may be finite impulse response (FIR) or infinite impulse response (IIR), where the response to the input can only last for N + 1 sample periods. FIR digital filters have numerous advantages over IIR filters because they're more stable, there isn't any feedback and therefore no accumulation of rounding errors and they can be designed to have linear phase vs frequency characteristics. When you need a signal filter, remember:

1. Buy linear analog signal filters.

2. Buy signal filters specially designed to eliminate noise.

3. Consider digital signal filters.

Action Steps
The best contacts and resources to help you get it done

Use analog signal filters for lower cost implementation and for high frequencies

Analog filters have the edge over digital filters when it comes to RF and VHF work because the highest frequency sampled must be no more than half of the sampling frequency in order to avoid aliasing errors according to the Nyquist sampling theorem. Plus, analog filters don't incur quantization errors due to storage and limitations in computation and, depending on the filtering requirements, analog filters can be implemented without using active components, but with only resistors, capacitors and inductors.

I recommend: NuWaves will design and manufacture the filters you need and the whole RF system you're developing. It serves commercial, industrial and military markets. Masline Electronics is a wholesale distributor that also carries a complete line of signal filters.

Eliminate noise interference from your electronic circuits with signal filters for noise

One major requirement for filtering is the need to eliminate noise from the signal. In signal processing, in all different frequency ranges, noise is something you must control in order to obtain the highest quality output signal possible.

I recommend: National Semiconductor has the model LMV1088 analog amplifier IC that drastically reduces ambient background noise that enters a microphone, making this company a popular choice in many industries, most especially telecommunications. Spectrum Control has an assortment of EMI power filters available. They even have low-leakage, non-magnetic EMI filters for use in medical equipment.

Use a digital signal filter for versatility and for low frequency filtering

In addition to the aforementioned advantages of digital filters, they also are programmable, operating from a program in memory, thus they can be altered by changing the program and without having to change the circuitry. Digital signal filters are easy to design, test and implement on a computer. They aren't susceptible to drift and temperature variations as are analog filters and they can process low frequency signals much more accurately than analog filters. Digital filters also have a greater degree of versatility in how they adapt to changes in the signal.

I recommend: Alligator Technologies manufactures a wide variety of digital filters designed for test and measurement purposes, i.e. filter boards that plug into a computer, using interfaces such as PCI and ISA. It also carry many rack-mounted models. Frequency Devices has an array of different types of filters, including digital filters, popular in military, telecommunications and medical applications.

Tips & Tactics

Helpful advice for making the most of this Guide

• •  Though the Butterworth signal filter offers the flattest response curve, don't use it for filtering pulsed waveforms, because the Butterworth filter has a phase shift which would result in a ripple in the pass band. Instead, use a Bessel filter, which has very little phase shift and won't distort the pass band.