Butterworth Filters

Butterworth Filter Order

In the previous sections we looked at simple First-order type Low and High Pass filters that contain only a single resistor and a single reactive component (a capacitor) within their circuit design. In applications that use filters to shape the frequency spectrum of a signal such as in communications or control systems, the shape or width of the roll-off also called the "Transition Band", for a simple first-order filter may be too long or wide and so active filters designed with more than one "order" are required. These types of filters are commonly known as "High-order" or "Nth-Order" filters.

The complexity or filter type is defined by the filters "order", and which is dependant upon the number of reactive components such as capacitors or inductors within its design. We also know that the rate of roll-off and therefore the width of the transition band, depends upon the order number of the filter and that for a simple 1st-order filter it has a standard roll-off rate of 20dB/decade or 6dB/octave.

Then, for a filter that has an "Nth" number order, it will have a subsequent roll-off rate of 20n dB/decade or 6n dB/octave. So a first-order filter has a roll-off rate of 20dB/decade (6dB/octave), a second-order filter has a roll-off rate of 40dB/decade (12dB/octave), and a fourth-order filter has a roll-off rate of 80dB/decade (24dB/octave), etc, etc. High-order filters, such as third, fourth, and fifth-order are usually formed by cascading together single first-order and second-order filters. For example, two second-order low pass filters can be cascaded together to produce a fourth-order low pass filter, and so on. Although there is no limit to the order of the filter that can be formed, as the order increases so does its size and cost, also its accuracy declines.

Decades and Octaves

One final comment about decades and octaves, a Decade is a tenfold (factor of 10) increase or tenfold decrease on the frequency scale for example, 2 to 20Hz is 1 decade or 50 to 5000Hz is 2 decades (50 to 500 and then 500 to 5000hz). An Octave is a doubling (factor of 2) or halving (divide by 2) of the frequency scale for example, 10 to 20Hz is 1 octave and 2 to 16Hz is 3 octaves (2 to 4, 4 to 8 and finally 8 to 16Hz). Either way Logarithmic scales are used to denote a frequency value.

Logarithmic Frequency Scale

Since the frequency determining resistors are all equal, and as are the frequency determining capacitors, the cut-off or corner frequency (ƒc) for either a first, second, third or even a fourth-order filter must also be equal and is found by using our now old familiar equation:

As with the first and second-order filters, the third and fourth-order high pass filters are formed by simply interchanging the positions of the frequency determining components (resistors and capacitors) in the equivalent low pass filter. High-order filters can be designed by following the procedures we saw previously in the Low Pass and High Pass filter tutorials. However, the overall gain of high-order filters is fixed because all the frequency determining components are equal.

Filter Approximations

So far we have looked at a Low and High Pass 1st-Order filter circuits, their resultant frequency and phase responses. An ideal filter would give us specifications of maximum pass band gain, minimum stop band attenuation and steep pass band and stop band edge frequencies and it is apparent that a large number of network responses would satisfy these requirements.

Not surprisingly then that there are a number of "approximation functions" in linear analogue filter design that use a mathematical approach to best approximate the transfer function we require for the filters design. Such designs are known as Elliptical, Butterworth, Chebyshev, Bessel, Cauer as well as many others. Of these five "classic" linear analogue filter approximation functions only Butterworth Filters and especially the Butterworth Low Pass filter design will be considered here as its the most commonly used function.

Low Pass Butterworth Filter

The frequency response of the Butterworth Filter approximation function is also often referred to as "maximally flat" (it has no ripples) because the pass band is as flat as possible at 0Hz (DC) with no ripples until the cut-off frequency at -3dB, and then rolls-off down to zero in the stop band. This is because it has a Quality Factor, Q of just 0.707. However, one main disadvantage of the Butterworth filter is that it achieves this passband flatness at the expense of a wide transition band as the filter changes from the passband to the stopband. It also has poor phase characteristics as well. The ideal frequency response, referred to as a "brick wall" filter and the standard Butterworth approximations, for different filter orders are shown below.

Note that the higher the order and number of cascaded stages the closer the filter is to the ideal "brick wall" response. However, in practice this "ideal" response is unattainable.

Where the general equation for a Butterworth filters frequency response is given as:

Where: n represents the filter order and epsilon ε is the maximum passband gain, (A_max). If A_max is defined at a frequency equal to the cut-off -3dB corner point (ƒc), ε will then be equal to 1 and therefore ε² will also be 1. However, if you now wish to define A_max at a different voltage gain value, for example 1dB, or 1.1220 (1dB = 20logA_max) then the new value of epsilon ε is found by:

	Where:   H0 = the Maximum Passband Gain, Amax.   H1 = the Minimum Passband Gain.
Transpose the equation to give:

The Frequency Response of a filter can be defined mathematically by its Transfer Function with the standard Voltage Transfer Function H(jω) written as:

Where:   Vout = the Output signal voltage.   Vin  = the Input signal voltage.      j   = to the square root of -1 (√-1)     ω  = the Radian Frequency (2πf)

Note: (jω) can also be written as (s) to denote the S-domain. and the resultant transfer function for a Second-order Low Pass Filter is given as:

Normalised Low Pass Butterworth Filter Polynomials

Butterworth produced standard tables of normalised second-order low pass polynomials given the values of coefficient that correspond to a cut-off corner frequency of 1 radian/sec.

Filter Design - Butterworth Low Pass

Find the order of an active low pass Butterworth filter whose specifications are A_max = 0.5dB at a passband frequency (ωp) of 200 radian/sec, A_min = 20dB at a stopband frequency (ωs) of 800 radian/sec. Also design a suitable Butterworth filter circuit to match these requirements.

Firstly, the maximum passband gain A_max = 0.5dB which is equal to a gain of 1.0593 (0.5dB = 20log A) at a frequency (ωp) of 200 rads/s, so the value of epsilon ε is found by:

Secondly, the minimum stopband gain A_min = 20dB which is equal to a gain of 10 (20dB = 20log A) at a stopband frequency (ωs) of 800 rads/s.

Substituting the values into the general equation for a Butterworth filters frequency response gives us the following:

Since n must be an integer (whole number) then the next highest value is n = 3 ie, a 3rd order filter is required. Then to produce a 3rd order filter a 2nd order and a 1st order circuit are required.

From the Normalised Low Pass Butterworth Polynomials table above, the coefficient for a 3rd order filter is given as (1+s)(1+s+s²) and this gives a gain of 3-A = 1, or A = 2. As A = 1 + (Rf/R1), choosing a value for both the feedback resistor Rf and resistor R1 gives us values of 1kΩ and 1kΩ respectively, (1kΩ/1kΩ + 1 = 2).

We know that the cut-off corner frequency -3dB point (ω_o) can be found using the formula 1/CR, but we need to find ω_o from the passband frequency ω_p then,

So, the cut-off corner frequency is given as 284 rads/s or 45.2Hz, (284/2π) and using the familiar formula 1/CR we can find the values of the resistors and capacitors for our 3rd order circuit.

3rd-order Butterworth Low Pass Filter

and finally our circuit of a 3rd-order low pass Butterworth Filter with a cut-off corner frequency of 284 rads/s or 45.2Hz, a maximum passband gain of 0.5dB and a minimum stopband gain of 20dB is constructed as follows.