04.06.2007 12:00 AM
FCC DTV Receiver Tests Confirm Concerns About Intermodulation Interference
Last Friday, just before the FCC shut down its Web site for maintenance, the Office of Engineering and Technology released its report
on Interference Rejection Thresholds of Consumer Digital Television Receivers Available in 2005 and 2006.
The OET laboratory tested 10 DTV receivers--seven that exhibited the characteristics of a fifth-generation receiver in the FCC's SHERVA testing, and one purchased in 2006 and two purchased in 2005 that exhibited multipath performance typical of a fourth-generation receiver. Most of the tests used only the first eight receivers, which were clearly identified as having single-conversion tuners.
The testing found many of the receivers did not fully comply with the ATSC Recommended Practice for rejection of interfering signals on channels two to 15 channels away from the desired signal. Adjacent-channel performance was better and some receivers did come close to the recommend practice after the difference between the white Gaussian noise used in the tests and an 8-VSB signal was taken into account.
The FCC laboratory conducted several tests on the receivers at various signal levels, ranging from weak to strong. One surprising result was an increase in susceptibility to interference from sources located seven channels above the desired channel. The analog n+7 taboo existed to protect receivers from interference from the local oscillator of nearby receivers tuned to a channel seven channels (the IF offset) above the desired channel. The report hypothesizes the interference is created by the interfering signal beating with the local oscillator in the TV receiver, creating interference on-channel.
One finding of concern to broadcasters (but no surprise to readers of Charlie Rhodes' TV Technology column), is that intermodulation between two signals can be a significant source of interference to TV receivers. The study did not include any tests showing the susceptibility of amplified antennas to intermodulation distortion. I hope this will be considered for future tests, as many indoor antennas sold today include a preamplifier.
There is far too much data in the report to cover in this weekly newsletter. Take time to at least skim the report. While the large amount of test data is not always easy to follow and there is no editorial comment on the results, the tables and summaries provide a good overview of how much protection today's DTV receivers will need from other transmitters in the TV bands.