# Frequency intermodulaton



## Prof (Oct 22, 2003)

Can someone refresh me on the principles of radion frequency intermodulation. I know some channels will interact with others. I have been out of RC for a while but recall certain channels that do interact and can cause glitching. I tried searching the forum but intermodulation was a word that the browser ignored.


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## Mayhem (May 5, 2002)

Its been a while but if I can remember channels can have trouble with others that are 3 channels ABOVE the one being used. Channel 74 and 77, 77 and 80 etc. Channel 90 on 75 band is the safest and not succeptable to intermodulation as I understand it. Part of the reason odd frequency's are not allowed at some tracks although most don't know it. I did extensive research on the subject when I first started racing there was one racer I always had glitching trouble with when on the track, and sure enough the channels fit. Dirty, or out of tune radio's can also contribute to this phenomenon. Havn't witnessed it it a while, modern radio's may have addressed the problem I suspect. In a nutshell what I found out about the subject...


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## tfrahm (Nov 14, 1998)

"3rd Order Intermodulation" is when you have 3 channels that are equally spaced, such as 70, 72, 74... The problem is that the upper two combine to generate harmonics which clobber the low one... In this case, 72 and 74 would combine to produce harmonics that would glitch channel 70... The gap between channels doesn't matter, as long as the three are equally spaced... (80, 75, 70 or 70, 71, 72 -- each case would glitch the lower of the three...)


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## Mayhem (May 5, 2002)

Yes I seem to remember that as being accurate.....also known as 3rd order distortion if doing a search.


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## Prof (Oct 22, 2003)

Thanks guys. Thanks again for the refresher. 

As background, I have a KO PROPO MARS EX-1 radio (early model) that came with Channel 88. I was getting glitching when another racer with a late model MARS was on channel 83 and I was on channel 79. I switched back to channel 88 and had no more problems.


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## tfrahm (Nov 14, 1998)

Yep -- at least if 3rd order intermodulation was a possibility, higher channels are always "safer" than lower ones... Locally, we have also found that out of tune transmitters (such as ones that have been "thrown" a time or two) will "spill" over other nearby channels...


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