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Thread: Intermodulation Distortion (IMD) in FPV

  1. #1
    Forum Member
    Join Date
    Dec 2014

    Intermodulation Distortion (IMD) in FPV

    Hi all,

    With our FPV racing group, we have often noticed poor FPV performance when flying in groups. This often occurs even when we spread out channels across the 5.8 range. My father, a retired RF engineer, did some experiments and came to the conclusion that inter-modulation distortion (IMD) is at least part of the problem. As far as I am aware, this is an often ignored issue in the FPV community.

    A group of pilots operating at 5.8 GHz are especially prone to IMD. The close proximity, high power transmitters, even spacing of the channel frequency, and poor receivers contribute to the problem. Per the video, there are some simple steps that can be taken to mitigate the issue. IMD is when two other transmitters combine and produce interference in your receiver at your desired FPV frequency.

    I know the video is not the most exciting (the demo is at 3:45 in for those with short attention spans). I do think that if the FPV community adopted some of these strategies we could have better racing and group flying.

    Last edited by Jon h2; 27th June 2015 at 12:01 PM.

  2. #2
    Forum Member
    Join Date
    Oct 2014
    Very good points, and well presented.
    I don't think it has never been considered, I believe it is simply that when using many systems together you have issues. I believe that would be obvious to most EE's.
    The reasons for the issues are both TX AND RX design - they are basically terrible ;-) ... Great for single use - which is their general purpose.
    Most of the problems could be mitigated by using correct filtering and good design practices on the TX and RX , more importantly on the TX to start with. Band pass or low pass filtering is required and it needs to be of good quality. 2nd harmonics of the main frequency would then be reduced tremendously - problem virtually solved. With correctly filtered harmonics the issue is minimised to be virtually non existent.
    Don't forget, most of these systems are extremely poor design and production quality - no matter what the make is. People tend to use horrible quality transmitters that generate so much spurious garbage. They then add poorly designed and produced antennas.

    Adding attenuators to the rx and any resulting improvement is an issue with lack INPUT Filtering, lack of dynamic range, overloading, and crappy IF filtering - and, POSSIBLY IMD - but IMD would be the last thing I was really concerned about here. The quality / design of the IF filtering is highly questionable! - it is probably as wide as a barn door. Correct bandpass filtering on the rx would also help a little, improved design and dynamic range would help - but, the TX is the main culprit here. To recap, the main issue is extremely poor quality design - especially of the TX units and lack of bandpass / low pass filtering. The resulting harmonics from the poor quality tx units causes issue then in the also poor quality design of the rx units - again, proper TX LOW PASS filtering would help tremendously. This is definitely not an overlooked issue, it is simply one that was inevitable. Try low pass filtering the TX units and quality shielding first.
    A short term goal would be to improve the low pass output / filtering of the TX and quality RF amp design on the TX. Most of them are total spurious generators, also keep power level lower where possible. Bandpass input filtering, Good AGC or manual gain control, and, PROPER IF filtering of the RX units would be a long term goal.
    Until someone designs and sells good quality equipment, it is an issue that will be difficult if not impossible to 'fix'.

    Look to the TX units first. Not the RX.
    Last edited by Captainslarty; 29th June 2015 at 04:58 AM. Reason: clarification

  3. #3
    Forum Member
    Join Date
    Dec 2011
    I'm a little confused. What the video is demonstrating is that the harmonics are being generated in the first stages of the receiver. If they weren't generated in the receiver then padding down the receiver's input wouldn't have had any effect at all as the ratio of signal to harmonics wouldn't change as it's being padded. But if the receiver's input is nonlinear, then the amount of harmonics will be level dependent.

    Does anyone have real spectrum readings of any of these transmitters showing high harmonic output that might cause these problems?

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