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Thoughts On FM Interference

radio8z · 2016-02-04T13:48:20Z

In another thread,there are some suggestions on how to reduce the possibility of adjacent channel interference from Part 15 FM transmitters which may use higher power than is now permissible. Rather than interrupt that thread with a technical topic it seems best to present some information here. Harmonics, splatter, and spurs are words often used to describe interference and filtering is offered as a solution but from the context of the use of these terms there may be some confusion about this. The comments that follow are not textbook or dictionary definitions but represent my attempt to discuss this clearly so the terminology used can have some consistent meaning. Keep in mind that harmonics, splatter, and spurs can cause interference but each has different causes and different solutions. HARMONICS are signals which appear on integer multiples of the carrier frequency and therefore are widely separated and will not cause adjacent channel interference. All transmitters produce harmonics and since they are widely spaced from the operating frequency they can easily be prevented from causing problems with proper and simple filtering. The filtering is simple because low Q filters can be used and often the use of a tuned antenna is sufficient. SPURS, short for spurious emissions, appear at somewhat random frequencies around the operating frequency and my be in or out of band and are caused by such things as improper transmitter component layout, weak power supply decoupling, or improper neutralization or tuning of the transmitter. Though filtering is offered as a solution, the proper solution is to correct the deficiency in the design or operation of the transmitter. SPLATTER happens when a transmitter is overmodulated or has severely distorted audio which causes the bandwidth occupied by the signal to widen beyond the "channel bandwidth" and this will cause adjacent channel interference because the effect is centered around the operating frequency (harmonics and spurs are not). Audio filtering and limiting of the modulation can be used to reduce this problem but filtering the RF signal out of the transmitter is not practical. Such filtering would require such high Q (selectivity) that expensive components such as cavities or crystal arrays would be needed. In addition, such filtering would introduce phase shifts which distort the modulated signal so RF filtering is not a viable solution. Careful control of modulation is required. Interference can also happen even with a perfectly clean signal produced by a transmitter if the signal overloads a receiver or if it mixes with another signal producing heterodyning or "intermod" which is heard at the receiver. There is nothing that can be done at the transmitter to prevent this other than to reduce the transmitted power and this is one of the reasons for the FCC limits on Part 15 operation. Preventing interference to other radio services requires addressing all of these different problems and each requires its own solution. Some are hardware issues and some are operational issues. If higher power transmitters are to be allowed then both hardware design and proper adjustment and operation of the equipment will be needed. Even with good equipment and practice things can go wrong and the operator needs to know how to detect and correct problems. Neil

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Anonymous

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Ignore this post, link won't work in this forum.

Bruce.

Posted : 10/02/2016 12:05 am

Anonymous

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Mr. Bruce said: "I asked, because I wanted to purchase a high quality transmitter and a 25 watt part 73 certified LPFM transmitter is what I was in the process of purchasing, it costed me $4,000 and had the same circuitry the licensed LPFM stations would use, or even the high powered ones, minus the larger amplifier section."

I said: You only mentioned the transmitter.

Posted : 10/02/2016 5:17 am

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wdcx Said:

You only mentioned the transmitter.

MrBruce Said:

Yes, you are correct, I tried to keep a lengthy post from being lengthy. It didn't work out that way did it? LOL! Oh, well.

Bruce.

Posted : 10/02/2016 4:42 pm

Anonymous

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Mr. Bruce said: He's talking to me.

I say: Ok, sorry. I'll butt out. I wasn't being negative to you I was simply pointing out good equipment is expensive.

Posted : 10/02/2016 7:43 pm

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mram1500 Said:

Mr. Bruce said: He's talking to me.

I say: Ok, sorry. I'll butt out. I wasn't being negative to you I was simply pointing out good equipment is expensive.

MrBruce Said:

Never butt out, you misunderstood me.

And....you are correct good equipment is expensive.

Bruce.

Posted : 10/02/2016 9:58 pm

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