In message <[email protected]>, dated Thu, 17 Aug 2006, [email protected] writes
>Let's say that you have a high resolution imaging system. The system >has a focal plane array that detects IR radiation...the signals from >this array are read out as a clocked analog data stream. i.e. each >pixel gives an analog value when it is read. > >This clocked analog stream goes through an A/D converter. > >The sampling/clock rates are all set so that the eventual output of >this imager is a completely digital data stream that is sent out as >NSTC video. But NTSC is analogue. You must D/A the digits and then add them to the NTSC syncs. What else have you left out of your description? > >The video is piped out of the imager to a display via coax cable. > >Here's the problem. The display shows what appears to be a "light >snow-like" noise. Could that be background IR emission from the surroundings of the imager? > >I'm thinking about some analytical and measurement methods that could >be used to track down the noise source. > >What would happen if I fed the video signal directly to a Spectrum >Analyzer??? I'm assuming that I would have to install a "pad" on the >spectrum analyzer input to attenuate the signal and help to impedance >match the 75 Ohm coax with the 50 Ohm spectrum analyzer input. Your spectrum analyser will surely handle a volt of input signal? And I wouldn't worry too much about the mismatch, because I doubt that the spectrum analyser will tell you much about the noise. You may well be surprised at what you DO see (see below). > >It seems to me that we may be able to detect some frequency peaks that >persist even if we change the scene that the camera is looking >at...maybe these peaks would point us toward the noise source in our >system? 'Snow-like' noise probably won't have any frequency peaks other than are generated by the NTSC waveform construction. You do know that the video signal is, in the frequency domain, a sequence of harmonics of the field and line frequencies, with the video information as sidebands? You are NEVER going to see quasi-random signals in amongst all that. > > >What about an FFT? What would be the feasibility of grabbing a subset >of the data and performing an FFT on it? Since the noise is all over; >it doesn't seem to me that we would have to perform the FFT on all >65,000 points of pixel data. Maybe we could use a 100x100 subset near >the center of the image? I really don't advise going down that route! > >Of course, I've also instructed my friend who is working on this to run >a wet finger over the circuits while the video is running to see if he >can crudely locate the noise source by looking for a change in the snow >on the screen. How about pointing the imager at the interior of your freezer and seeing if the noise goes away then? > >Here's another question for those of you who are more familiar with >video than myself. Is there an intuitive method of homing in on the >noise freqencies by looking at how the noise appears? For instance, >our noise is a sort of granular "snow" that drifts on the screen...it >doesn't stay still. There are intuitive methods, but the only way I can see to pass on the intuition would be to actually show you the various effects and explain what causes them. Descriptions in words just don't work. -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk 2006 is YMMVI- Your mileage may vary immensely. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________
