FM Noise

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I could not agree more with Steve Johnston (“Johnston Laments FM Noise,” June 15).

I regularly connect a spectrum analyzer to OTA TV or FM antennas and witness enormous RFI. When the trigger is set to “Internal,” the noise peaks appear to be random. But if you set the trigger to “Line,” you’ll instantly see noise peaks sync up at about 8.3 ms time intervals, or 1/120 Hz. That is, the noise is synchronous with 60 Hz, perhaps associated with zero crossings.

Forty years ago the FCC protected the broadcast spectrum with a pretty heavy hand. But what’s evolved over time are a plethora of external switching power supplies (“wall warts”), imported and domestic, and numerous other AC powered devices that seem to be substantially unregulated, much less policed.

So while I agree with Mr. Johnston’s conclusion that better control of RF noise is needed by the manufacturing industry, I lament that there’s probably little motivation for them to do that. Competitive pricing pressures encourage shortcuts, and often the EEs who contribute designs to production are inexperienced and unaware.

What is needed are more rigorous requirements for compliance testing.

Tom Ballister
RF/Audio Consultant Truckee, Calif.


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