LOW PASS FILTERS Real-world signals contain both wanted and unwanted information. Therefore, some kind of electronic signal filtering technique must separate the two before processing and analysis can begin. Every electronic design project produces signals that require electronic signal filtering, processing, or amplification. The most popular filter is called a low pass filter. The low pass filter will pass signals below a corner frequency and will attenuate signals above that corner frequency. The figure below shows a frequency response chart of one type of low pass filter, the Butterworth. In this chart the corner frequency is a normalized frequency of 1. Any noise or signal above the frequency of 1 will be attenuated and any noise or signal below a frequency of 1 will be passed. In order to pick a low pass filter there are several interrelated questions that you need to answer. What Corner Frequency? The corner frequency of a filter is the point above which signals start to attenuate. In the adjacent chart filter attenuation starts at a normalized frequency (f/fc) of 1. What that means is that you can pick your proposed corner frequency and use the chart to predict attenuation. If, for instance, the corner frequency you think that you need is 500 Hz then you can multiply all of the numbers across the bottom of the chart by 500 to see what happens in that filter. So at 1 times 500 or 500 Hz the attenuation will be about 3 dB, at 2.0 times 500 or 1000 Hz for an 8 pole filter (poles will be explained next) the attenuation will be about 48 dB, etc. How many poles? In simple terms the more the poles the more the attenuation for a given frequency. In the chart above you can see that at 2.0 times the corner frequency the attenuation for a 2 pole filter will be about 12 db, for a 4 pole it will be about 24 dB, a 6 pole will be about 36 dB, etc. For this kind of filter, the Butterworth, for every pole you add the attenuation will increase by about 6 dB. In active filters, ones that use operational amplifiers and capacitors the normal number of poles are 2, 4, 6, or 8. In a passive filter, ones that use coils and capacitors there can be any number of poles. What transfer function? Typical transfer functions are Butterworth, Bessel, Elliptic, Constant Delay, and Chebychev. Each has its advantages and disadvantages depending on what you are filtering. Picking the correct transfer function is complicated but there are some general rules. If your signal is complex and you want to preserve it you should use either the Bessel or the Constant Delay. For maximum attenuation the Elliptic is best. If you are not sure we recommend that you call the Frequency Devices technical support line at 815-434-7800 Extension 236. Active or Passive Filter? Active and passive filters both can accomplish the same thing. The differences are shown in the table. Active Passive Number of poles 2, 4, 6, or 8 normally any number Power required yes no Size usually smaller than passive usually larger Output impedance usually below 10 ohms usually 50 ohms For more information you can consult the Frequency Devices Design Guide Or you can call us for technical support at 815-434-7800 Extension 236. For more information on low pass filters