Muriel Bittencourt de Liz wrote: >> In another room (next room) I have turned on a TV. In channel 2 (VHF) I could see two horizontal lines, about 17cm apart from each other. They keep circulating at the TV screen in a low (subjetive) velocity.<<
This is a common symptom for (US) power-frequency related noise. Such radio noise usually occurs every 8 1/3 milliseconds -- one burst per voltage peak -- while our 512 line interlaced NTSC video sends each 256 line half screen every 16.675 milliseconds. The noise bursts thus create _two_ lines on the screen, 8 1/3 milliseconds apart. but because the television frame scans a bit slower than the 60 Hz 16 2/3 millisecond period, the lines on successive frames fall earlier in each one, and appear to be moving. When measuring EMI outside a shielded chamber, we often need to distinguish between television ambients, and EMI modulated at 60 or 20 Hz. A common method is to change analyzer video synch to "line;" if the noise bursts hold still, they're power related, while if they move slowly across the screen, they're from a TV signal. >> Is there a relation/pattern in: * the distance (between the lines) * orientation of the lines (horizontal, vertical, diagonal) * velocity of the lines in the screen (slow, steady, fast) << Diagonal lines may be similarly caused by a source pulsed or modulated at frequencies of the same order as the horizontal sweep frequency of 15.75 KHz. A 35 KHz SPS might produce two noise pulses about every 30 microseconds. Because these would fall later on each successive horizontal sweep, which is about every 28.6 microseconds, you might see diagonal lines on the screen moving from left to right. But lines ion te screen, especially wide ones, may also be associated with synch loss due to overload of the TV tuner. Cortland This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
