IMD (Intermodulation Distortion) is when two
transmitters combine to produce interference at a third
frequency. For instance, if using the frequencies 5760, 5800 and
5840,
any two of these will work well together, but when all three
frequencies are in use IMD interferece is generated at 5760 and 5840
Mhz. If the 5840 frequency is changed to 5880 then IMD
interference is not seen at any of the three frequencies in use.
IMD is calculated with this formula: F3 = (F1*2) - F2
See here for an rcgroups discussion thread on IMD.
(Also here and here.) There's a video
that demonstrates the effect.
Folks on the thread have generated spreadsheets to tabulate IMD
values. This looked like a good place for some software, so I've
created the tools below:
The IMDTabler
tool
generates a table of IMD values for a given set of frequencies.
The values are generated using the IMD formula F3 = (F1*2) - F2, where
the left column shows the F1 values, the top row shows the F2 values,
and in the table are the F3 values. Table cells are colored red
to indicate where IMD frequencies would
cause interference with the given frequencies, and hovering the mouse
cursor over a cell will show more information. Each generated
table is given an IMD rating, with a score of 100 corresponding to
minimal IMD interference.
Clicking on one of the links at the top of the IMDTabler
page will generate a table with the given frequencies. To
generate tables for the examples described above: 5760 5800 58405760 5800 5880
The FreqSetGen
tool
attempts to generate an optimal set of frequencies given a larger set
of possible frequencies. The tool has the following entry fields:
Number of frequencies: The number of frequencies in each
generated set.
Possible frequencies: A list of all the possible
frequencies to choose from. Note that the larger this list is,
the more time the generator processor will need to complete.
Mandatory frequencies: An optional list of frequencies
that mush appear in each generated set.
Minimum separation: The minimum separation (in MHz)
between values in the generated frequency sets. Separation less
than 37 MHz is likely to result in more interference between adjacent
frequencies.
Maximum run time: The maximum time allowed (in seconds)
for a run of the generator process. The largest value allowed is
600 (10 minutes).
This IMD 6C frequency set has become popular for FPV racing. (See here for an
annotated frequency chart.)
When using Raceband frequencies, there's one 4-frequency set with good
IMD and very good separation between values (>40MHz):
5658 5732 5843 5917 IMD rating:
100 view
So far the best 6-frequency set I've found is this one:
ETBest6: 5645 5685 5760 5805 5905 5945 IMD
rating: 67 view
Removing 5805 yields a very good 5-frequency set:
ET6minus1: 5645 5685 5760 5905 5945 IMD rating:
98 view
Removing 5645, 5945 and 5925 makes it a lot harder to find sets of
frequency values that are good for IMD interference. These sets
have better IMD ratings than IMD5, but feature more frequencies on the
less-popular transmitter bands:
ET5A: 5665 5752 5800 5866 5905 IMD rating: 88 view
ET5B: 5665 5752 5800 5865 5905 IMD rating: 84 view
ET5C: 5665 5760 5800 5865 5905 IMD rating: 64 view
The FreqSetGen tool can be run as a local application for faster
processing and unlimited maximum-run-time. (Java version 7 or later
is required.) The latest version of ET's IMD Tools may be
downloaded from here. FreqSetGen application usage:
The Java source is available in the distribution, or on GitHub here.
The web content is via Java servlets running on a Tomcat
server. If reusing the code, please provide attribution:
"ET's IMD Tools - http://www.etheli.com"