R.F. Burns said:
I am not here to fight with you but I do have a simple question, which you as a licensed station owner might be able to answer. Say a listener complains of interference from a distant IBOC station to their local radio station. We know that all consumer radios have filtering whose selectivity varies with the strength of a specific signal. As an example, if you are .5 miles from a 50 KW array, your radio might overload with the high level of RF it has to deal with and that station might appear to be operating over 2 or three adjacent channels away from its fundamental. Now take the same radio and try listening at a distance of 10 miles and low and behold, there is no apparent interference to the stations neighbors. So is the interference really there? Only a properly calibrated spectrum analyzer can say for sure. Who amongst us owns a spectrum analyzer? With that said, will all of these so called nuisance complaints be acted upon by the FCC or will they end up in the commissions working folder which never gets attended to?
Over a year ago, I investigated a case of interference from a non-directional 50 kW station (that had recently turned on IBOC) to a smaller 2.5 kW station operating on the 4th-adjacent channel in a neighboring county about 30 miles away. The hiss was audible on my Blaupunkt car radio within several miles of the 50 kW and affected some area within the smaller station's 2 mV/m contour. Some regular listeners had complained to the owner of the smaller station about the noise increase -- it was a significant rise in the noise floor, about 10 to 15 dB. A retired FCC monitoring-station employee living in the area also verified the problem on his receiver.
I do own a recent-vintage R&S spectrum analyzer, so I drove to the area and took some measurements on the IBOC station using a shielded loop, and their transmitter appeared to comply with the NRSC-5 limit at 40 kHz from the analog channel.
After thinking this over and running some calculations, I concluded that the problem stemmed from a third-order "2A-B" intermod product being generated in the receiver. The strong 50 kW analog carrier was able to pass through the first tuned RF stage and drive the front end into non-linearity, allowing the lower primary group of digital sidebands to mix with the upper group, creating a noise product that could then get through the IF stage and fall atop the desired audio of the 2.5 kW station.
So, although the offending station had a compliant transmitter plant, some listeners within the smaller station's protected contour were experiencing a noise problem -- which is, in fact, new interference -- and suggests the AM IBOC system is not as "compatible" as we were told. As Barry McLarnon has pointed out, AM receivers lack the ability to discriminate against undesired amplitude-modulated noise that falls within the IF passband, which is why we have all these reported problems with IBOC in the AM band.
Fortunately, the system has worked out better in the FM band because those receivers take advantage of the WBFM "capture effect".
