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.
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).
Removing 5805 yields a very good 5-frequency set:
ET6minus1: 5645 5685 5760 5905 5945 IMD rating: 100 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"
Here's a video demonstrating IMD effects on 5.8GHz FPV
frequencies. One thing that's shown is that 5685 and 5925 do not
interfere with each other directly, but the trouble comes when the
third frequency of 5800 is added to the mix. Also shown is that
you can have F1 (5740) and F5 (5820) running happily together, but then
when F3 (5780) comes in it steps on the other two (while the person on
F3-5780 may not see much interference at all).