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Introduction
Frequency Intermodulation
Frequency Intermodulation (IM) occurs when 2 or more frequencies
create new and usually unwanted frequencies that can cause interference.
The general equation describing intermodulation frequencies is:
where
IM is the newly created intermodulation frequency,
n1,2,3.. are integer (+ or -) coefficients
A, B, C are the existing radio transmit frequencies
If IM falls within the bandwidth of a receiver, it may cause interference in
that channel.
The intermod order is the sum of the absolute values of the intermodulation
coefficients:
For example, A + 2B - 2C will give an intermod order of 5 (|1| + |2| +
|-2| = 5).
F-Intermod
calculates intermodulation products in a given bandwidth, equal to or below a
selected intermodulation order.
Intermod Frequency Power
Power of each intermod
frequency is calculated using the below equations.
v0 = a0
+ a1v1 + a2v2 2 + a3v3
3 + ... + arvir
vi = EAcos2πAt
+ EBcos2πBt + ECcos2πCt + ...
Pi = vi2
/ z
where
v0 is
the output voltage
vi is the input voltage
ar is the nonlinear coefficient
EA, EB, EC ... are the voltages of
the carrier frequencies at A, B, C
A, B, C are carrier frequencies
The following assumptions
are made for these calculations:
a. all intermods are created at one location, for example in one amplifier.
b. nonlinear coefficient for each intermod order are determined by field
measurements
c. losses due to propagation and/or transmission are not considered in these
calculations
See below section Calibrating Intermod Power
for more details.
Note, accurate values of nonlinear coefficient need
to be used in order to achieve accurate intermod power levels. Do
not use default nonlinear coefficients provided with this program to
determine absolute intermod levels.
Why do I need to calculate intermodulation products?
Intermodulation products should be calculated before installing two or
more radio transmitters at a site in order to determine what new frequencies
may be created by intermodulation. These new frequencies can cause serious
interference with the site radio receivers and other nearby receivers. It is
also a requirement by many frequency spectrum regulating agencies to ensure
interference is not created for other existing radio stations.
Optical Four Wave Mixing
This program also calculates fiber optic system four wave mixing (FWM)
products that can occur in DWDM links. Four wave mixing is the fiber
optic equivalent to radio frequency intermodulation interference and can
cause interference over DWDM channels. This software calculates the
possible interference frequencies that may occur in DWDM system. Enter the DWDM channels in THz.
Additional Intermod information can be found at these sites:
Wikipedia -
Intermodulation
www.tpg.com
- Good explanation
ARRL explanation
Installation
Download the FIntermodSetup.exe file onto a temporary directory in your
computer then run FIntermodSetup.exe. You may also run it directly from a CD.
Follow on screen instructions as the program loads. FIntermodSetup.exe can be
deleted after installation is complete.
This restricted version of F-Intermod will only calculate the Intermod
frequencies for up to 3 channels, and will expire after 20 days. After the
trial period expires, you will be prompted to purchase a license key. If you
have purchased a license, enter the unlock code when prompted. The registered
version will calculate intermods for up to 10000
channels.
Program Operation
INPUT
First, choose a frequency unit for F-Intermod to use. The default
unit is MHz. To change it to another unit such as kHz or GHz, click
Options -> Base Frequency Unit and select the desired unit. This must be
done when the input table is empty.
Channel Name
Enter the channel name or label to a maximum of 12 characters. An entry in
this field is not required.
Tx Frequency (Wavelength)
Enter a transmit frequency (or wavelength if nm unit is selected) for
the channel. The maximum and minimum value that can be entered are
999,999.99999 and 0.00001 respectively.
Rx Frequency (Wavelength)
Enter a receive frequency (or wavelength if nm unit is selected) for
the channel. The maximum and minimum value that can be entered is
999,999.99999 and 0.00001 respectively.
Half Bandwidth
Enter the half bandwidth for the communications channel. For example,
if the bandwidth is 30 MHz then enter 15 MHz. Maximum value is
500,000.00000 and minimum value is 0.00000. If zero bandwidth is
entered, only intermod frequencies that fall exactly on the Tx or Rx values
will be displayed. All output and display bandwidths are half bandwidth
values.
Duplex, Half Duplex or Simplex
Select whether this channel is Full Duplex, Half Duplex or
Simplex. Note that if Simplex is chosen you must enter a transmit OR
receive frequency, not both.
For duplex setting, the
program assumes that the channel transmitter and receiver are on all the
time.
For half duplex setting, the program
assumes that the channel transmitter or receiver are on, but not at the same
time. So when the transmitter is on the receiver is off and vice
versa. Therefore intermod created by the channel's transmit frequency
will never appear in that channel's receiver.
For simplex setting, the
program assumes that there is only one way communications. That is for
that channel only the transmitter is on always (100%) or the receiver is
on always (100%). But not both at any time.
Run
Click to run the calculations for the input data seen in the input
table.
Clear
Clears and resets all the data input and output except for units.
Add
Click "Add" to enter data (Channel Tx Frequency, Rx
Frequency, Half Bandwidth) into the input table.
Delete Row
To delete a channel, click on the row in the input table to select it and
then click "Delete".
List Intermod Frequencies
If "For all channels"
is selected, the program calculates all intermod frequencies normally.
If "For planned
channels" is selected, the program will only display
the intermod frequencies that the planned channels contribute to. This is
useful for calculating the intermod frequencies for new
transmitters/receivers being added to an existing site. A channel can
be designated as planned by double clicking on the channel's "Status"
entry in the input table. A planned channel row will be green.
If "For existing
channels" is selected, the program will only use the
existing channels in the calculation. Planned channels will not be included
and will not be displayed in the output results.
Channels are labeled "Existing" by default. To change
the status to "Planned", double click on that cell in the
input table. Planned channels are highlighted in green
Intermod Order
Enter the intermod order, an integer from 3 to 31 inclusive. Typically
intermod orders of less than 7 are used. For odd intermod order use the
pull down selection. For even intermod order, type value into the selection
box.
Single Intermod Order
Select this check box if you want the output to display only intermod
order products of order equal to the order selected in the "Intermod
Order" box. If it is not checked, the output will display
intermod products up to and including the order entered in the "Intermod
Order" box.
Include Tx Frequencies
Select this check box if you want intermod frequencies to be calculated
that fall in the transmitter frequencies bandwidth (transmitter intermod) and
receiver frequencies bandwidth. The program assumes the transmitter
bandwidth is the same as the receiver bandwidth. If the is not selected
only intermod frequencies that fall in the receiver bandwidths will be
calculated.
Display Channel Name in Results
Select this check box if you want to see the channel name in the
output table rather than its frequency. Channel name size is maximum 12
characters.
List All Intermod
Select this check box to display all intermod without any receiver or
transmitter channel bandwidth filtering. Output intermod listings can
still be filtered into a frequency range by selecting "Specify Output
Frequency Range".
Specify Output Frequency Range
Select this check box to specify a filtered output frequency range and
then enter the lower and upper frequency range in the two boxes below it.
Input Table Copy, Paste, Delete Editing
Right click while the mouse is over a input table cell will
produce an additional menu that will allow you to Copy, Paste, Select Column,
Select All and Delete. You can copy one or more entries to and from
Excel using this method.
To copy columns of data from an Excel spread sheet, highlight the column
of frequencies to be copied in Excel and select copy. Then in
F-Intermod input table click on the first frequency cell that has been
copied, right click and select paste.
Individual input table cells can be edited directly by double left clicking
on the cell.
Any input or output table column can be sorted, ascending or
descending, by double right clicking on the header of the column to be
sorted.
Channel Database (click on << button)
Click on the << button to access the Channel Data form. This is another
method to view, edit and add information to the Channel Database.
To edit a channel, make changes on the form and then click "Save Data".
To add a new channel, click "Add New Channel", enter the information then
click "Save Data".
It is possible to export the Channel Database (Input Table) to an MS Excel
compatible CSV file or to a text file as records. Click on File, then Export
Channel Database. The text file will display each channel
data as separate records.
When all your channels have been correctly entered, press RUN to
execute the program. Depending on factors such as the number of channels,
intermod order, bandwidth, and speed of your computer, the run process can
take a second or hours. You may stop the calculations by pressing
"CANCEL" at any time.
Output
The output is displayed in chart
form sorted by intermodulation order (Order) and intermodulation frequency (I.Mod Freq.).
Sorting by other columns is possible by double clicking on the header
of the column to be sorted. Note sorting the I.Mod Freq, Power,
Channel and Difference columns will
take much longer than other columns since a special sorting routine is used
for these columns.
If no intermodulation (IM) listings are found, then a red “No IM
Found” label will be displayed in the “Total Listings
Found” (called Intermod Total in new versions) box.
Columns
The first column "Order" contains the intermodulation
order of calculated intermodulation frequencies.
The second column "I.Mod.Freq."
contains the calculated intermodulation frequencies in units selected.
The third column "Channel R/T" contains the channel
frequency (or name if "Display Channel Name" selected) that
is being interfered. The "R" or "T"
designates if the intermod is receiver or transmitter intermod.
The fourth column contains the difference between the calculated
Intermod frequency "I.Mod.Freq."
and the channel frequency being interfered. Note, if "List All
Intermod" option is selected, then values in this column have no
meaning.
The rest of the columns contain transmit frequency coefficients that
produced the listed intermodulation frequency "I.Mod.Freq.".
Note that the speed of the program depends on the number of Tx and Rx
frequencies, intermod level, and bandwidth entered. To speed up the
calculations, use a lower intermod level, do not select Include Tx Freq
option, or use a faster computer. Also select Single IM Order to reduce
output.
Printing and Exporting Output
To print the output, click File ->
Print or press Ctrl P. To view the print preview, click File -> Print
Preview. Note that if there are more than 9 transmit frequencies for the
input, not all of the output table will be printed due to insufficient paper
size. In this case, the output can be to exported to a text file, MS Excel compatible
CSV file, or text file which displays the intermod equations rather than the
output grid. For example, if the intermod frequency is 138.365 MHz, the file
will display the equation for this frequency as
138.36500=(-1)(154.01000)+(1)(149.71000)+(1)(142.66500)
where (-1), (1) and (1), are the intermod coefficients, and 154.01, 149.71
and 142.665 are sample transmit frequencies.
The CSV file can be opened with any spreadsheet program. It will show
both the input and output tables along with the channel database data.
Output Too Large for Grid
If there is a large amount of data to display, the display table
may have insufficient size to show all results. Before the limit is reached,
F-Intermod will prompt the user to save the calculated intemod
listings to a file. The data is then saved to one of 3 file formats (.txt, .csv and .txt displaying intermod equations) that is
selectable by the user in the File/Save output to Overflow file... menu.
Note: Some programs will be incapable of displaying large files.
The text files can be viewed with Microsoft WordPad or any standard text word
processor.
For very large text files that WordPad cannot open, use other programs
available from the Internet that can open large files such as Textpad available at www.textpad.com.
Copying
Right click while the mouse is over the output table will produce an
additional menu that will allow you to Copy, Paste, Select Column, and Select
All. You can copy one or more entries to the clipboard. However
you cannot paste to the output table.
T.Pwr
This button is only available if the Power feature is purchased. It
automatically sums all intermod powers that fall onto the same frequency or
same channel. If "List All Intermod" is selected, the result is the
sum of all intermod powers that fall onto the same frequency. If "List
All Intermod" is not selected the result is the sum of all intermod powers
that fall onto the same channel.
If the number of intermod listings overflows into an overflow file, then
this feature will not be able to sum any of the overflow intermods. It
only sums intermods shown in Output Table.
Menu Options
File
New Channel Database
- Clears the input and output tables.
Load Channel Database - Opens a saved input file
with ".cdb" extension. Note, it will also load older version ".txt" database
files. Use
this option if you have an input file saved from a previous version of
F-Intermod, starting from R3.3.
Load Channel Database from R3.2, 3.1 or
3.0 - Opens a saved input file from R3.0 to R3.2. You must use this
option if you have an input file from these versions. Using the previous Open
Program File option will not work.
Save Channel Database - Saves the input table data and options
to the current selected file name with extension ".cdb" (for future loading with "Load
Channel Database"). Note this is a change from earlier versions that
saved the database as ".txt" file.
Save
Channel Database as...
- Saves the input table data and options to a file name with extension
".cdb" for future loading
with "Load Channel Database". It is saved with the a new filename
selected by the user.
Load Template
- Opens a saved template, just F-Intermod configuration settings does not
affect loaded channel data. Has ".tpl"
extension.
Save
Template - Saves template file for future use with ".tpl"
extension. This template file stores all F-Intermod configuration options that are
currently set. Does not save channel data info.
Print preview - Display how
the output will be printed.
Print... (Ctrl+P)
- Print the current input and output.
Export channel database...
MSExcel - Exports the
input table data to an MS Excel compatible CSV file.
Text file - Exports the input table to a text file displaying records
rather than a table.
Export Intermod output ...
Text file - Exports the output table to a text
file.
MS Excel compatible CSV file - Exports the output table to a CSV file
which can be opened with any spreadsheet program, including MS Excel.
Text file showing intermod equations - Exports the output to a text
file showing the Intermod Equations rather than a table.
Save overflow output to file... Select one file format that will be
used by the program to save the output when screen output table is too small to
display all intermod listings.
Exit -
Ends the program.
Options
View Channel Info... - Displays the extra channel information dialog
box.
Units - Set the frequency unit to kHz, MHz, GHz, THz or
nm. The default unit is MHz. This option can only
be set when the input table is empty.
Auto Rx Freq - If checked, then receive frequency will be
automatically calculated from the difference between the last transmit and
receive frequency pair. Default difference (on startup) is zero.
Nonlinear Coefficients - Enter proper nonlinear coefficients for each
intermod order to obtain accurate power output levels for that order.
Industry
Selecting the appropriate industry will hide non appropriate options for that
industry. Selecting "Radio" industry will show all
options.
Help
Guide - Opens this document in your default internet browser.
About - Displays program version and a link to our website, www.telecomengineering.com.
Check for Updates... - Checks for a newer version of F-Intermod at www.telecomengineering.com.
If a newer version exists, you will be prompted to download the new
installation file. Note: If you have any type of software
firewall running on your computer, be sure to allow F-Intermod to access the
internet for this option to work.
Calibrating Intermod Power
In order to achieve any accuracy in the calculated intermod output powers, each
intermod order's nonlinear coefficient needs to be calibrated and adjusted
to your project's specific environment.
Note, the default
nonlinear coefficients loaded when the program starts will not provide any
degree of accuracy of intermod power outputs and should not be used as
absolute values. However the default non linear coefficients can be
used to generate intermod frequency powers that can be viewed as relative
values. These relative values will provide worst and least offending intermod frequencies within
each intermod order.
To calibrate and determine the nonlinear coefficients, follow the below
procedure.
If the nonlinear coefficient for each intermod order is known for the
environment then select menu Option and click Nonlinear Coefficients.
Enter the nonlinear coefficient directly for each intermod order. When
a nonlinear coefficient value is entered it is immediately stored, do not
click Calculate.
If the nonlinear coefficient is not know then:
a. Find one of the fundamental carrier frequencies that is generating the intermod
products and measure its power and the power of all of its harmonics to the highest order
needed in your calculations. For best accuracy use an unmodulated
carrier. For example if you need to determine
intermod products up to and including 5th order, then measure all harmonics
of a carrier up to and including the 5th harmonic. For best accuracy
be sure that there are no other interfering carriers at the same frequency
as the carrier fundamental or the harmonics which will skew measured results.
b. Click the menu Options and then Nonlinear Coefficients and enter for each
intermod order the carrier power and each harmonic power. Click
Calculate for each intermod order for the nonlinear coefficient to be
calculated.
If a carrier frequency harmonic cannot be found or is not available, it is
possible to calibrate using intermod frequencies as follows:
a. Find an intermod frequency for each intermod order
that can be measured. It is important to ensure that there is only
one intermod existing on the frequency (also no other interference at
this frequency). To do this run F-Intermod and sort by Intermod
frequency to determine if more than intermod could occur on the
frequency.
b. Measure the power of each intermod frequency for
each intermod order.
c. Run F-Intermod and identify the frequency for each
order and the associated power level. The power level shown
will not be the same as the power you
measured.
d. Click the menu Options and Nonlinear
Coefficients and adjust the nonlinear coefficient for the intermod
order associated with the measured intermod frequency. Run
F-intermod again to recalculate the power levels and check the new
recalculated intermod frequency power. Keep adjusting the
nonlinear coefficient until the recalculated power level for the
frequency is the same as the measured level. Once it is the same
the nonlinear coefficient is known for that intermod order and continue
to do the same for the next intermod order and frequency.
This completes the nonlinear coefficient procedure.
Intermodulation Examples
The following examples use the default nonlinear coefficients that load with
the program.
Example 1:
Five channels are inputted, 2 duplex, 2 half duplex and 1 simplex receive only.
If the nonlinear coefficient is measured and is accurate then the worst
offending intermod is 3rd order on channel 5, the simplex channel. Power of
-6.9 dBW Channels 1 and 2 contribute to this intermod.
If the nonlinear coefficient is not calibrated, then we can say that there
will be one 3rd order hit on channel 5 with unknown strength.
For 5th order intermod the worst offenders (strongest intermod) will be on
channels 1, 3 and 5. Intermod power levels are unknown.
For 7th order intermod the worst offenders (strongest intermod) will be on
channel 3. Intermod power levels are unknown.
To see the total interfering intermod power for each channel, uncheck "Show
channel name in output", run again and then click T.Pwr.
The Channel R/T column will be sorted and total power value for each channel
will be shown.
As can be seen above the worst intermod is on channel 157.5 MHz.
Computer Platform Requirements and
Specifications
General:
Required computer :Windows 98, NT, XP, ME, 2000, Vista
Minimum RAM memory : 128 Mbytes
Hard Drive Space : 10 MB
Resolution : 800x600 VGA Monitor or better
Other peripherals : one printer (not necessary)
Input:
Transmit frequency : 0.00001 to 999,999.99999 for any units kHz, MHz, GHz, THz
Number of transmit freq. : 1 to 10000
Intermodulation order : 2 to 31
Receive frequency : 0.00001 to 999,999.99999 for any units kHz, MHz, GHz, THz
Number of receive freq. : 1 to 10000
Frequency bandwidth : 0.00000 to 999,999.99999 for any units kHz, MHz, GHz,
THz
Output:
Intermodulation frequency range : 0.00000 to 999,999.99999 for any units kHz,
MHz, GHz, THz
Maximum number of intermodulation output frequency listings : unlimited
Computing time : dependent on intermodulation order, number of transmit
frequencies, receiver bandwidth, sorting time and number of listings
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