From: bcoleman@hayes.com (Bill Coleman)
Newsgroups: rec.models.rc
Subject: Re: Radio interference - 1 more case
Date: 14 Oct 92 13:40:52 EDT
Distribution: na
Organization: Hayes Microcomputer Products, Norcross, GA

In article <1992Oct13.145406.14689@bmerh85.bnr.ca>, mkfeil@bcrki9.bnr.ca (Max Feil) writes:
> In article <6181.2ad984f9@hayes.com> bcoleman@hayes.com (Bill Coleman) writes:
>>I fail to understand all the mathematical complexity of RC radio interference
>>problems. While it is true that intermodulation products could cause a problem,
>>and that certain mathematical relationships could cause unwanted signals to
>>pop up in the IF, there are much simpler explanations that are not considered.
>>
>>You experience above may have been simply that the transmitter you flew too
>>close to overloaded your receiver. It really wouldn't matter what frequency
>>he was on, just as long as the signal saturated the front end (or the
>>intermediate stages) of the receiver. Perhaps the signal got into the
>>servo wiring. Who knows? 
> 
> I know what you are saying, and it is possible. However in my case the ground
> tests showed that there was a definite incompatibility between the two radios.
> The other guy's channel 50 transmitter affected my channel 16 receiver MUCH
> more than it should have. 

My point was that the interference may have had nothing to do with the channel
used. Perhaps if he had been on channel 48 or 52, you'd have the same results.

Did this happen with ANY channel 50 and 16 radio, or just your two?

> My original thought was that something must be out
> of tune or not working right for this to happen. If everybody's radio worked
> this way we would have a real problem! In my previous article I was merely
> suggesting that perhaps the radios were working to the best of their ability,
> but that an artifact of the known problems with single conversion receivers
> could explain this particular interference.

If you were using a single conversion rx, then you most likely experienced
receiver overload. Single conversion designs are more susceptable to this.

>>He indicated that if you stand closer than 20 feet, it causes intermod.
>>Well, I KNOW that intermod is a RECEIVER phenominon.
> 
> Well, perhaps what you KNOW is not all there is to it. I suggest you read
> Call Orr's "Radio Spectrum" article in the Sept 1990 issue of RCM. He does
> a fairly extensive test of "transmitter generated 3IM", complete with 
> spectrum analyzer pictures. I think that Cal Orr KNOWS more in this case...

I don't have the article in question to compare. My experience working with
radios indicates that rx-generated intermod is an extremely common phenomenon.
Tx-generated intermod is extremely rare, and results from recticification
of two extremely strong signals through some non-linear element.

This usually requires signals of exceptionally strong strength. The little
1 watt transmitters used in RC aren't powerful enough to cause this.

I'm not saying that interference doesn't happen. It obviously does. What I'm
saying is that searching for these complex mathematical channel relationships
is moot. There are simpler explanations, such as receiver overload, insufficient
sheilding in the rx or tx, insufficient RF bypassing of servo leads, and
insufficient filtering of transmitter byproducts (perhaps out of tune). 

-- 
Bill Coleman, AA4LR                ! CIS: 76067,2327    AppleLink: D1958
Principal Software Engineer        ! Packet Radio: AA4LR @ W4QO
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