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1987-03-04
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Power Mountain Software Systems
P.O.Box 243
Provo, Utah 84603
Copyright 1987
all rights reserved
Filter 1.1 Documentation
By Larry Ashworth
2/09/87
** What's New! **
There have been some big changes in the program since version 1.0. It is
now possible to design 75 different filters instead of 7, and the program is
now Shareware. We hope you like it, and will support its' further development.
All rights to this program are reserved, and no modification to this
program will be allowed, whatsoever. Feel free to write us if you feel a
modification to this program might be warranted. We will make any modifications
we think desirable. We intend to continue to develop it into a VERY major
program, and we would like your input.
You may distribute this program freely, and in fact are encouraged to do
so. If you are seeing this program for the very first time, remember this is
not the latest version more than likely. Registering will provide you with the
current one, and also get you the next update. It is virtually impossible to
keep you informed as to all the most recent additions. So if you really like
what you see, by registering you will get to see what's happening.
I'm not sure at this time how many versions will be released to the public
as shareware, it will depend on the support of the public. We'll release some
more things I'm sure, but we don't know how well the Shareware concept will
work with this type of software, so I don't want to promise more than I'm
prepared to deliver.
==============================================================================
!! YOU MAY DISTRIBUTE ONLY THE SHAREWARE VERSION, NOT THE REGISTERED VERSION !!
==============================================================================
The purpose of this program is to:
A.) Provide some basic background for those who are investigating the
principles of electronics, and filters in particular, for the first
time, or to provide a review for those who studied it sometime ago.
Potential problems are covered, to make it easier to get what you want.
B.) To aid those who need a good bandpass filter design for some audio
application, and would like the computer to generate a schematic, and
compute values.
C.) To help those who need a filter, to choose components that will
perform as desired, or at least to help make a sensible compromise.
This includes guidelines and suggestions as to the operational
amplifier choices available.
Page 1
** BACKGROUND AND BASICS **
Since we have to start somewhere, and since we don't have room on this
disk for a complete course in electronics, we are going to assume that you
already have some background in the basics.
To begin remember that all of electronics begins with the study of a few
basic components, known as resistors, capacitors, and inductors. These basic
components can be used in an infinite variety of circuits and are the basic
building blocks of all electronics. Combined with semiconductor technology and
the invention of the vacuum tube, virtually all of the developments of modern
technology have been created. These fundamental elements form the function
blocks upon which we have built everything.
There are many different function blocks that are often used in the design
of communications and audio circuits. One of these is the filter. As a matter
of fact virtually all electronic equipment uses some sort of filtering. A
filter is a circuit that electronically determines which frequency, or range
of frequencies, will be allowed through.
There are essentially two classes of filters known as R.F., and Audio.
R.F. filters are so called because these are used for Radio Frequencies,
consisting of high frequency ranges used in communications and in navigation.
These are much higher than the range of the human ear. Audio frequencies are
those which are within the range of human hearing. In this case from 10 hz to
25 khz or so.
For our purposes we will discuss audio filters, but the principles are
the same for R.F. types.
All filters basically consist of four types, they are LOWPASS, BANDPASS,
NOTCH, BAND REJECT, and HIGHPASS. I say four types because notch and band
reject are both the same thing, differing only in the width, or range, of
frequencies that are rejected.
A wide bandpass filter design can be formed from the use of a highpass in
front of a lowpass filter, in fact this is the way a speech filter for
communications is designed. In the case of speech, only the range of
frequencies extending from 300 hz to 3000 hz are required to understanding what
is said. By concentrating all the power of a transmitter over a narrower range
of frequencies better communicating under adverse conditions is possible. In
this situation we would use a high pass filter of 300 hz which would pass
frequencies above 300 hz to the input of a low pass filter at 3000 hz, thus
forming a 300 - 3000 hz bandpass filter system.
A bandpass filter can be very narrow, and there are many applications
where only a very small band of frequencies are desired. As in the case of
morse code communications in amateur radio. The Butterworth bandpass filter has
the property of being able to pass a small band of frequencies with nearly the
same amplitude throughout the pass band, while allowing very steep skirts for
maximum attenuation of unwanted signals or noise. (The number of poles
determines just how steep these skirts are.)
Notch, or band reject filters are used to eliminate or attenuate a
frequency or band of frequencies. This particular version ("Filter 1.1") does
not create notch filters, though we intend to add them to the program in the
future. Band reject filters can be created through the use of high and low pass
filters, simply by selecting the appropriate filters to reject the frequencies
you consider undesireable.
Page 2
** COMPONENT CHOICES **
This program is designed to allow you to very quickly design and document
a filter design. A great deal of effort and research has gone into creating a
program that will make your life easier. However, there are hidden 'gotchas'
just waiting in the wings to ruin an otherwise excellent design.
Before we get into a few of these here are a few pointers:
A. The resistor and component values given are calculated values and
need to be rounded out to the nearest standard value. The tolerance of
resistors and capacitors is suggested in the program, and as you use it you
will see that exotic filter shapes require very tight tolerances. Simpler
filter requirements yeild less stringent component accuracies. In general, when
choosing components remember, the closer you can come to the calculated values
the more likely you are to get the predicted results.
One of the really nice features of this program, is the inherent
ability to experiment with values, until the right combination is found that
meets your requirements. Capacitor values should be kept below 1 uf., while
resistor values should be kept at around 2k and above. Juggle capacitor values
around until you come up with the results that meet your requirements. Remember
also that resistance values above a Megohm may not be a good idea, try a larger
capacitor. Some very low frequency filters require a large capacitor, but
they're expensive.
B. There are a great many good op-amps to choose from, and if you feel
a little insecure about picking one for the job you're doing, I would like to
suggest the TL071 (single), TL072 (dual), or TL074 (quad) packages. They are
high performance, low noise, high slew rate devices that work very very well in
this type of circuit. Their pin outs are industry standard, and the price is
reasonable. If you need extremely low power consumption the TL061 series has
the same pin out and can be substituted. These devices can be found in a Texas
Instruments Linear Data Manual. The TL061, '62, and '64 devices are not quite
as good in critical applications, however, due to the sacrifice in slew rate
for lower power consumption.
C. If you have noticed there are potentiometers in the Butterworth
filter schematics, these can be 1/4 watt pc-mount types, and the reason they
are there is to make it possible to make the fine tuning adjustments necessary
to get the system to work. The tuning procedure requires that you have a
frequency generator, and a frequency counter. Using the values given, all that
you need to do is to make sure that each section is tuned to the correct
frequency, and then to make sure with your ac voltmeter that the gain for that
section is correct. Start with the first section, and continue through to the
last.
Page 3
** LOW & HIGH PASS FILTER TYPES **
When you run the low pass and high pass filter design options, you are
asked to choose a slope option. In case you aren't sure what we're talking
about, I'll define them for you. This new feature we have put in the program,
greatly enhances it's power and flexibility. You can now tailor each section to
give you the exact slope you need for your application.
These slope options are created by varying the DAMPING, of the section. As
the damping is reduced from 2 toward 0, the initial rolloff of the filter
becomes steeper. This allows higher rejection of unwanted signals than would
otherwise be the case, but the thing that I want to bring out is trade offs
involved.
Changing the damping has an affect on the passband of the filter, reducing
the damping causes ripples to occur. You will notice in fact, slope options
called 1db, 2db, and 3db dip. This refers to the passband ripple inherent to
these filters. Flattest amp refers to the fact that the amplitude in the
passband is flat. Very desirable in audio applications such as
bi-amplification.
'Highly damped' and 'best delay' are the slowest to rolloff initially,
'compromise' a little faster, and so on....
** POTENTIAL PROBLEMS **
This program works very well, but the results can be unrealistic. This is
due to the fact that no matter how rediculous your specifications are the
program will try to come up with values. Values that can be impossible. In a
case such as this a little common sense is in order.
When you design a Butterworth bandpass filter, keep the bandwidth as wide
as you can to get what you need accomplished. Excessively narrow filters are
extremely difficult to tune, the requirements for the op-amp get unreasonable,
and very often even if you get it running the ringing of the filter is bad
enough to cause more troubles than you gain. There are some important physical
limits that there just is no way around.
A one hertz wide filter will take 1 second to reach approximately 90
percent of the maximum amplitude. The higher the Q, the closer to oscillation
you are, and the greater the problems. Trust me. Keep it as wide as you can.
You'll be happier. Use more poles if you need it really narrow, but don't cut
those sidebands, 'cause you need 'em. There have been a great many good
articles on filters in the various hobby electronic magazines, and of course in
many good textbooks on communications, which can give you more information on
why narrower isn't always better.
We've tried to trap out unreasonable inputs, but I suppose there is a
chance we missed something.....
R E M E M B E R !
The ultimate rolloff of a filter is governed by the number of poles, only
the initial rolloff is changed.
More poles, as a general rule, is more desirable than trying to make a two
pole filter act like a six pole.
Components become more critical as you lower the damping, and it gets a
litle tricky to build a practical filter. Use discretion.
On a final note, remember that if you want to print out the schematics
generated by the program, you must run the GRAPHICS program from DOS or the
equivalent or you will not get a proper print-out of the screen.
Page 4
** THANK-YOU **
I would like to take this opportunity to thank Dale Spanheimer, for his
help in putting this program together. A lot of effort has gone into this
thing, and I really appreciate his help. The updates are coming a lot faster
due to his assistance.
Thanks are also in order to Thayne Harmon, who adapted the source code for
use on the Atari ST. He has made, and helped to implement, some very nice
changes in the program.
Now let me say thank-you to YOU. It's people like you that make the
Share-ware software concept work. We have faith in you, and we want to provide
a program that is professional, and as high a quality as possible. I feel that
this way our future will be assured, and that people such as yourself will
support this kind of effort. Please don't let us down, and we will work to be
of service to you.
Larry Ashworth
Power Mountain Software Systems
P.O.Box 243
Provo, Utah 84601
Page 5