ET's FPV IMD Tools
IMD (Intermodulation Distortion) is when two
transmitters combine to produce interference at a third
frequency. For instance, for 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.)
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:
IMDTabler
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 5840 5760 5800 5880
FreqSetGen
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 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).
So far the best 6-frequency set I've found is this one:
ETBest6: 5645 5685 5760 5805 5905 5945 IMD rating: 87 view
Removing 5805 yields a very good 5-frequency set:
ET6minus1: 5645 5685 5760 5905 5945 IMD rating: 100 view
Distribution
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:
FreqSetGen numFreq "possibleFreqs" "mandatoryFreqs" [minFreqSep] [maxRunSecs]
Example:
FreqSetGen 4 "5740 5760 5780 5800 5820 5840 5860 5880" "5740" 37 300
Source Code
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