An ideal filter has two types of bands: the stop band defines the region of frequencies that are eliminated by the filter, while the pass band defines the region of frequencies that the filter allows through. Practical filters add one more, the transition band. This is the area where the filter is moving between the stop band and pass band.

In our look at filter design specifications, we will use the example of a lowpass filter. The extension to the other kinds of filters should be fairly straightforward. Figure 2 shows the parameters for a lowpass filter in the frequency domain.

Figure 2

From this image, the passband is the region from − ω p ω p to ω p ω p , the transition bands are the regions from − ω s ω s to − ω p ω p and from ω p ω p to ω s ω s while the stop band is the region less than − ω s ω s or greater than ω s ω s .

In the figure above, e p e p and e s e s represent the acceptable tolerance (or error) around the desired level that the passband and stopband respectively may vary. The behavior within the transition band is not specified, allowing anything to occur there, as long as the width is within specifications.

Example 1

This example will look at a moving average system.

Figure 3

(a) Time domain representation of the moving average. (b) The frequency domain representation of the moving average system is a sinc function. H⁢i⁢ω=e−(i⁢ω⁢T2)⁢sinω⁢T2ω⁢T2 H ω ω T 2 ω T 2 ω T 2

Some notes about this system:

• It is lowpass
• It has linear phase with jumps of π radians when the sinc function changes sign
• The duration of the filter is inversely proportional to its bandwidth
• This filter is finite impulse response (FIR)
• It cannot be built with passive R, L, C circuits
• We do not have independent control over all four design specifications

We are going to design a moving average filter with the following design specs: ω p =100⁢π ω p 100 , e s =0.1 e s 0.1 , e p =0.1 e p 0.1

Figure 4

With this specification, we are allowing ω s ω s to be a dependant variable (since we need one). We can now find the equation for this moving average system.

We begin with

|H⁢i⁢ω|=1.1⁢sinω⁢T2ω⁢T2 H ω 1.1 ω T 2 ω T 2

(1)

We will now solve for TT with

|H⁢i⁢100⁢π|=1.1⁢sin50⁢πT50⁢π⁢T=0.9 H 100 1.1 50 T 50 T 0.9

(2)

For these specs, T≃0.007 T 0.007 . This means that |H⁢i⁢ω| H ω does not stay below e s =0.1 e s 0.1 until ω s ≃771⁢π ω s 771 .

Figure 5: A graphical look at the transfer function of this lowpass filter with the passband and stopband noted.

It is very clear from this representation that the transition band is huge ( 671⁢π 671 ). This is a very bad filter, especially when you consider that it cannot be implemented with passive circuitry. Fortunately better filters (e.g. Butterworth, Chebyshev and Elliptical) do exist.

In the discussion of the different filters (Butterworth, Chebyshev and Elliptical) is common to see explanations based on lowpass filters. This explanation is very nice when first learning about them, because it is sufficient to understand the fundamentals of each of them. It is acceptable, because there exist fairly straightforward techniques to convert these lowpass filters into highpass, bandpass or bandstop filters. These techniques are the lowpass to highpass transformation, lowpass to bandpass transformation and lowpass to bandstop transformation.