HAM Radio 1/HamRadio.cdr/misc/interm/intermod.doc

Text File | 1988-08-09 | 13.0 KB | 397 lines

                              Intermodulation Analysis

     WHAT IS INTERMODULATION?

          Intermodulation (IM) occurs whenever one or more radio frequencies
     are combined in a non-linear device. This produces sum and difference
     frequencies which are known as IM products.  The non-linear device may be
     a transmitter, a receiver, an isolator, a bad connector or any number of
     other items.  Sometimes the condition is desirable, as in the mixer stage
     of a superheterodyne receiver where signals are combined and selected to
     produce the familiar "Intermediate Frequencies" (I.F.).  Most of the time
     however, intermodulation is undesirable and steps must be taken to prevent
     or eliminate the conditions which give rise to intermodulation.  Such
     measures usually include the use of circulators and cavities to prevent
     signals from entering the final amplifier stages of transmitters, and
     the use of band-pass cavities at the inputs of receivers.  These factors
     are of special concern at mountain top repeater sites.

          The number of receivers and transmitters at most repeater sites
     can vary anywhere between a dozen to a few hundred.  The number of
     intermodulation products which can be generated could be in the thousands.
     A site manager may be able to exert total control over the conditions
     at a particular site, or only nominal influence over the activities of
     other users of the site.  In either case, a site manager faced with an
     intermodulation problem would be faced with the formidable problem of
     tracking down the one combination out of possibly tens of thousands
     which might be the source of the problem.

          This program is designed to make that task easier.  Even though the
     number of intermodulation products is theoretically infinite, only a
     limited number of them are significant for a given set of receivers.
     The products which we are mainly interested in are only those which
     will fall on or near a receiver frequency, and are of low enough order
     to be of significant amplitude.  What does "order" mean?  The general
     scalar form of the equation for intermodulation products is:

     For two transmitters, Tx(1) and Tx(2);

                    F = nTx(1) + mTx(2)  and...
                    F = nTx(1) - mTx(2)  | n = 1,2,3... where m = n+1

     The order of the product is defined as m+n.  These are merely the
     coefficients of the terms of the equations.

          As you can see, when n is 1, m is 2 and the order is 3.  The order
     of the product is always odd.  And since the frequencies involved are
     really phasors, the sign of F is related to its phase and not its
     amplitude so for purposes of analysis we can ignore it.*



     *    A phasor is a vector which rotates about its origin with time.  The
     intermod program uses absolute value to eliminate the signs.














          The accompanying program to this document calculates intermodulation
     products for, two and three-transmitter combinations and second and third
     harmonics of single transmitters.  Two-transmitter combinations of 3rd,
     5th and 7th order, and since the coefficients are all 1, 3rd order
     products of three-transmitters.  The equation for three-transmitter
     products is:

                    F = Tx(1) + Tx(2) - Tx(3)

          We artificially limit the order of the products to 7th because as
     the order increases, the amplitude of the product signal decreases and
     therefore becomes insignificant.  The program would take forever
     to finish calculating an infinite number of products.

          Once the products of interest have been calculated its merely
     a matter of sorting through the data to see which products fall on
     which receiver frequencies and print the results.  Since the program
     incorporates a sort function, we can get a list of the "hits" in
     ascending frequency and finding out which transmitters might be
     candidates for "most likely suspect" in our search.












































     ABOUT THE PROGRAM

          This program was originally inspired by a simple intermod program
     written in BASIC for the TRS-80 Model I by Dennis Hanley.  The source
     code for INTERMOD 2.3 is QuickBasic 4.0 and with advanced features like
     recursion, makes it possible to code this program in ways not possible
     with ordinary BASIC.  Believe it or not, there is only one line number
     in the whole program.  The program was developed over a period of several
     months and was capably critiqued by Dennis Fandrich, Abel Montes, and Jon
     Maudal.  Many of the features of the program were suggested and tested by
     several trial runs on their various systems and data.  The program is
     capable of managing up to 1000 transmitters and 1000 receivers
     simultaneously.  It automatically tests for the presence of a math co-
     processor, and uses it if present.  If a co-processor is not present
     it uses the built-in floating-point emulation.

          INTERMOD.EXE runs on any PC/XT or AT compatible running MS-DOS 3.0
     or higher, with or without floating-point processor support.  However,
     if you plan to use large arrays (more than 100 transmitters and receivers)
     or use INTERMOD on an XT, then use of a coprocessor is strongly
     recommended.

          The program has only one menu and commands are all one-letter,
     single keystroke commands based on the first letter of each menu item.
     The program will prompt for other information as each category requires.
     Hitting the ENTER key without the required data terminates the command
     and returns to the main menu.  Where information appears in angled
     brackets, e.g.; <N>, it specifies the default value the program will
     use when ENTER is keyed at the prompt.  In the ADD and DELETE command
     categories, ENTER is used to terminate data entry, and if you are in the
     transmitter data sub-category the command proceeds to the receiver data
     sub-category.


































     THE COMMANDS

          A    Add data to the transmitter/receiver arrays.  Enter each
               frequency one at a time.  As data is added the entire array
               is displayed and the user is prompted for more input.

          C    Change Color.  Toggles the display between white-on-blue and
               white-on-black.  Useful if you have monochrome monitor on
               a color adaptor or a LCD display and want more contrast, or
               simply want a change of scenery.

          D    Delete Frequencies.  Used to delete frequencies from the
               transmitter and receiver arrays, transmitters first, then
               receivers.  Searches the current array for a match to the
               entered data, deletes it if found and re-displays the entire
               array.  Enter the frequencies one at a time.  ENTER terminates
               data entry in the current array.

          E    Exits the program.  If data in the arrays has been altered
               the program asks the user if the data should be saved.

          F    Files.    Display a DOS style directory, with pauses.
               Allows pathnames, wildcards etc.  Prompts before returning
               to main menu.  Useful for checking for data file names
               without exiting INTERMOD.

          G    Gateway.  Executes COMMAND.COM as a child process.  Typical
               DOS gateway.  Type EXIT at the DOS prompt to return
               to INTERMOD.

          L    Load file.  Loads an INTERMOD data file from disk.  Prompts
               user for filename, if none entered, returns to main menu.
               If file not found or other error it returns with description
               of error.

          N    New Data.  The usual way to get new data into the program prior
               to saving or printing it.  Prompts user for the site name
               and date before entering the "add data" mode.

          O    Order.  Performs Quick-Sort of data in transmitter and
               receiver arrays.  Really fast.  Do this before printing
               the products to get consecutive output.  Sorts in ascending
               order.

          P    Print Products.  Prints the products to the screen, line
               printer and disk if selected, the user can direct the output
               to the printer or to a disk file or both.  INTERMOD calculates
               two-transmitter products first, then three-transmitter, then
               harmonics.  Allows user to select which products to calculate,
               default state is all products.  Prompts user for the search
               bandwidth, this tells INTERMOD how wide the hit "window" will be.
               Default window is 15 kHz.  Prompts user for choice of form-feed
               or continuous output if printer or disk is selected.













          S    Save data.  Saves the INTERMOD data file to disk.  Prompts
               user for filename.  Same protocol as Load.

          V    View data.  Allows user to view the data in memory without
               risk of altering it.  Displays 140 frequencies at a time.
               Prompts at end of each page of data.

          F1   Aborts a product printout in progress.


     OPERATING TIPS

          Keep the number of receivers in any one file as small as
     possible, enter only those receivers which you are the most
     interested in.  The number of products increases very rapidly as
     the number of transmitters increases, multiply that by the number of
     receivers and a single run can take several hours.

          Send the output to a disk file.  This allows you to print
     the entire output file using the DOS print command instead of
     sending it to the printer as the program calculates.  If this
     isn't a problem, send it to both printer and disk.  The output is
     identical whether sent to the disk or to the printer.

          The actual receiver frequency is included at the rightmost column
     of the output.  This makes searching for a target receiver fairly easy
     since the receivers are listed numerically in each category.

          If the printer is not ready when the INTERMOD tries to use it
     an error message is displayed.  Pressing ENTER at the prompt causes
     the program to continue from the error.  Pressing F1 before ENTER tells
     INTERMOD to return to the main menu.  If the printer is on-line and
     printing products, pressing F1 causes INTERMOD to return to the main
     menu immediately.

          Use consistent filenames when naming data files and output
     files.  Something like SAMPHILL.DAT for input data files and
     SAMPHILL.OUT or SAMPHILL.PRO for the product files.

          Group the transmitters and receivers by building.  The strongest
     products will most likely be generated nearby.  It's easier trace the
     products outward from the victim as well.

          Remember, the presence of a product in the list is not an
     indication that the product actually exists.  INTERMOD produces a
     list of POTENTIAL products and the most likely sources given the
     proper conditions.  Only careful interpretation and testing can
     prove the correctness of the result.


















     LICENSE AND CONDITIONS OF USE

          You are free to use, copy and distribute this product provided:

               NO FEE IS CHARGED FOR USE, COPYING OR DISTRIBUTION OF
               THE PRODUCT.

          The program and the documentation must not be altered in any way.

     This program is provided AS IS, without warranty, expressed or implied,
     including but not limited to suitability and fitness for a particular use.

          If you find INTERMOD to be a useful and convenient product, a
          contribution would be appreciated, send a contribution of $20.00 to:

               Geoffrey L. Joy
               12964 Smoketree Pl.
               Chino, CA 91710

          Contributors will be placed on a list of "most favored users" and
          will be notified promptly of any upgrades or improvements.