HAM Radio 1/HamRadio.cdr/tech/efilter/filter11.doc

Text File | 1987-03-04 | 15KB | 294 lines

                        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




























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