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In the text of this Note, important terms are highlighted and can be found in the Basic Glossary in the Resource Library where they are explained. A few also have dedicated In-Depth Filter Information Notes available.

This section poses the basic questions, which apply to any filter requirement. But there is no single filter solution to any application: it's always a question of balancing objectives and constraints to find the best filter for your application. We like to address the following questions when recommending a filter:

  • What does the basic filter need to achieve?
  • How many Poles are needed (and what are poles anyway?)
  • What other capabilities and features?

What does the basic filter need to achieve?

The more difficult the task, the more sophisticated the filter needs to be in its basic signal response. There are four important criteria for selecting the right filter; do you need to:

1. Separate wanted frequency components from 'nearby' unwanted ones. Here the difficulty is set by the frequency ratio between wanted and unwanted components.

As this number gets closer to 1 (from either side) the job gets harder, and you need a filter with more and more discrimination.

2. Separate wanted frequency components from unwanted ones, which are much 'louder', i.e. higher in level.

The difficulty in this case is determined by how much you need to reduce the level of the unwanted signals. The bigger the reduction needed (expressed in decibels), the more rejection you require.

3. Preserve the accuracy of waveform reproduction of the signal made up of the wanted frequency components.

All filters change the signal passing through them in some way. The more accurate you need your waveform reproduction needs to be; the less vector error can be tolerated from the filter.

4. Control the disturbance caused to the output signal by a sudden 'step' change in the input signal.

All filters take some time to recover after being 'shocked' by a sudden change in the input signal. The more cleanly you want the system to settle down, the lower the overshoot required in the filter response.

We will refer to these four criteria in the rest of this Note.

What are poles and how many are needed?

Briefly, a filter with more poles needs more 'active ingredient' components, making it larger and more costly to build. The increased complexity allows the filter response to offer improved performance in one or more of the categories above. Generally, the more 'difficult' your application is, the more poles will be needed in the filter response.

We have many filter responses designed to give a spread of capabilities in terms of the four criteria overleaf. We give those 'Option' numbers; some of them also have 'familiar' names. Examples of these families are the Butterworth and Bessel filters, which are available in four, six and eight pole versions on many of our products. The chief application for Bessel filters is where the fourth criterion is important; for such tasks they excel, although they are really rather poor at the other three.

We believe that, in many signal conditioning systems, the potential for waveform distortion from the filtering is underestimated. To alleviate this, we recommend the use of filters which offer good performance in the third criterion, wherever this can be done and still meet the requirements imposed by the other criteria. Using the latest computer techniques, we have designed a range of responses which excel in this respect; our generic name for these filters is Low Vector Error filters.

The eight pole filter Option range offers several excellent choices. For alias protection applications (the classic application requiring excellent figures in both the first and second criteria), our Option 01 filter (a member of the class of elliptic filters) has become the industry standard response shape. A very good Low Vector Error eight pole response is offered; Option 41 provides 82dB rejection at three times the cutoff frequency.

We cannot do justice in this guide to the full range of Kemo filter responses (over 90 designs); not only are there many more lowpass responses than the ones highlighted here, but there are a large number of highpass, band pass and bandstop responses. Advice on how to assemble the best filtering for your signal conditioning needs is always available, so please do contact us with any questions you may have -we'll do our best to help.

What other capabilities and features?

The Products section of our website describes key products, separated into various groups according to common themes of usage which have cropped up repeatedly over many years. Products range from benchtop filtering instruments of great versatility, through complete signal conditioning front-ends for data acquisition systems, to basic, but state-of-the-art, filter modules. Most of our products have gain, ranging from simple, fixed gain to units that include attenuation and a wide range of gain settings. Most of products include the ability to provide excitation to an IEPE type transducer. And for those needing real portability we offer several products with DC power supply. Setting of frequency cut-offs ranges from fixed to resistor adjustable thru DIP switch set to front panel switches of one sort or another all the way to units which feature both front panel controls and displays plus computer interface.

[Kemo products are made in the UK by Kemo, Ltd.]