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Kirt Blattenberger - RF Cafe WebmasterCopyright
1996 - 2016
Webmaster:
Kirt Blattenberger,
 BSEE - KB3UON

RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The Internet was still largely an unknown entity at the time and not much was available in the form of WYSIWYG ...

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Butterworth Lowpass Filter Poles

Butterworth poles lie along a circle and are spaced at equal angular distances around a circle. It is designed to have a frequency response which is as flat as mathematically possible in the passband, and is often referred to as a 'maximally flat magnitude' filter. Prototype value real and imaginary pole locations (ω=1 at the 3 dB cutoff point) for Butterworth filters are presented in the table below.

The Butterworth type filter was first described by the British engineer Stephen Butterworth in his paper "On the Theory of Filter Amplifiers", Wireless Engineer (also called Experimental Wireless and the Wireless Engineer), vol. 7, 1930, pp. 536-541.

See my online filter calculators and plotters here.

Butterworth filter prototype element values are here.

Pole locations are calculated as follows, where K=1,2,...,n.   n is the filter order.

Butterworth filter poles equation - RF Cafe

The Butterworth polynomials may be written in complex form as above, but are usually written with real coefficients by multiplying pole pairs which are complex conjugates, such as s1 and sn. The polynomials are normalized by setting ωc = 1.

The normalized Butterworth polynomial equations have the general form:

RF Cafe: Butterworth filter equation - n even

RF Cafe: Butterworth filter equation - n odd

n Factors of Polynomial Bn(s)
1 (s + 1)
2 (s2 + 1.4142s + 1)
3 (s + 1)(s2 + s + 1)
4 (s2 + 0.7654s + 1)(s2 + 1.8478s + 1)
5 (s + 1)(s2 + 0.6180s + 1)(s2 + 1.6180s + 1)
6 (s2 + 0.5176s + 1)(s2 + 1.4142s + 1)(s2 + 1.9319s + 1)
7 (s + 1)(s2 + 0.4450s + 1)(s2 + 1.2470s + 1)(s2 + 1.8019s + 1)
8 (s2 + 0.3902s + 1)(s2 + 1.1111s + 1)(s2 + 1.6629s + 1)(s2 + 1.9616s + 1)
Order (n) Re Part (-σ) Im Part (±jω)
1 1.0000  
2 0.7071 0.7071
3 0.5000
1.0000
0.8660
4 0.9239
0.3827
0.3827
0.9239
5 0.8090
0.3090
1.0000
0.5878
0.9511
6 0.9659
0.7071
0.2588
0.2588
0.7071
0.9659
7 0.9010
0.6235
0.2225
1.0000
0.4339
0.7818
0.9749
8 0.9808
0.8315
0.5556
0.1951
0.1951
0.5556
0.8315
0.9808
9 0.9397
0.7660
0.5000
0.1737
1.0000
0.3420
0.6428
0.8660
0.9848
10 0.9877
0.8910
0.7071
0.4540
0.1564
0.1564
0.4540
0.7071
0.8910
0.9877

Filter Design by Steve Winder - RF CafeData taken from "Filter Design," by Steve Winder, Newnes Press, 1998. This is a great filter design book, and I recommend you purchase a copy of it.

 

Related Pages on RF Cafe
- Filter Transfer Functions
- Filter Equivalent Noise Bandwidth
- Filter Prototype Denormalization
- Filter Design Resources
- Bessel Filter Poles
- Bessel Filter Prototype Element Values
- Butterworth Lowpass Filter Poles
- Butterworth Filter Prototype Element Values
- Chebyshev Lowpass Filter Poles
- Chebyshev Filter Prototype Element Values
- Monolithic Ceramic Block Combline Bandpass
  Filters Design
- Coupled Microstrip Filters: Simple Methodologies for
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