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Radio
Broadcast Technical Consulting and Sales
10032 Island Drive, Brainerd, MN
56401
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Radio
Broadcast Technical Consulting and Sales |
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Technical Tips from Mark W. Persons |
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Unwanted Spurious Radiation on an AM Station |
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This is a spectrum analyzer view of an AM radio station on 960 KHz. I found this while doing the annual FCC required occupied bandwidth and RF harmonic measurements on a station. This would not pass the FCC test and needed to be attended to. My IFR A-7550 Spectrum Analyzer really told the story. Unwanted spurious radiation peaked at about 50 KHz from the carrier. There was also plenty of noise going out to 100 KHz. Now...what to do about this mess??? |
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The transmitter is a 24-year-old Continental 315R-1. It is a 5 KW Power Rock. The name plate is just above the card cage which houses the logic, audio, PDM (Pulse Duration Modulation), and low voltage power supply cards. |
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Since it was unlikely that a 50 KHz spur would occur in the high-level stages of the transmitter, I turned my attention to the low voltage power supply. It furnishes +5 VDC, +12 VDC, -12 VDC, and -6 VDC to low-level parts in the transmitter. Because the transmitter was that old, it was likely that electrolytic capacitors in the power supply had opened and were no longer preventing the voltage regulators from going into oscillation. Two of the larger capacitors, in the module, had been replaced previously. I replaced all of the electrolytic and tantalum capacitors just for good engineering practice and to save the customer time in troubleshooting.
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Success!!! The noise went away returning the station to a normal signal. It is a good thing that we continue to do these NRSC (National Radio Systems Committee) measurements so that problems are found and repaired.
The stories go on and on.
Stop in again sometime. I'll leave the soldering iron on for you.
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page last edited 02/02/2007
